ORIGINAL RESEARCH ARTICLE
published: 07 January 2015
doi: 10.3389/fnana.2014.00163
NEUROANATOMY
Evolution of mammalian sensorimotor cortex: thalamic
projections to parietal cortical areas in Monodelphis
domestica
James C. Dooley 1 , João G. Franca 2 , Adele M. H. Seelke 1,3 , Dylan F. Cooke 1,3 and Leah A. Krubitzer 1,3*
1
2
3
Center for Neuroscience, University of California, Davis, Davis, CA, USA
Instituto de Biofísica Carlos Chagas Filho, Universidade Federal do Rio de Janeiro, Rio de Janeiro, Brazil
Department of Psychology, University of California, Davis, Davis, CA, USA
Edited by:
Jose L. Lanciego, University of
Navarra, Spain
Reviewed by:
Francisco Clasca, Autonoma
University, Spain
Marcello Rosa, Monash University,
Australia
*Correspondence:
Leah A. Krubitzer, Center for
Neuroscience, University of
California, Davis 1544 Newton Ct.,
Davis, CA 95616, USA
e-mail: lakrubitzer@ucdavis.edu
The current experiments build upon previous studies designed to reveal the network of
parietal cortical areas present in the common mammalian ancestor. Understanding this
ancestral network is essential for highlighting the basic somatosensory circuitry present
in all mammals, and how this basic plan was modified to generate species specific
behaviors. Our animal model, the short-tailed opossum (Monodelphis domestica), is a
South American marsupial that has been proposed to have a similar ecological niche and
morphology to the earliest common mammalian ancestor. In this investigation, we injected
retrograde neuroanatomical tracers into the face and body representations of primary
somatosensory cortex (S1), the rostral and caudal somatosensory fields (SR and SC),
as well as a multimodal region (MM). Projections from different architectonically defined
thalamic nuclei were then quantified. Our results provide further evidence to support
the hypothesized basic mammalian plan of thalamic projections to S1, with the lateral
and medial ventral posterior thalamic nuclei (VPl and VPm) projecting to S1 body and S1
face, respectively. Additional strong projections are from the medial division of posterior
nucleus (Pom). SR receives projections from several midline nuclei, including the medial
dorsal, ventral medial nucleus, and Pom. SC and MM show similar patterns of connectivity,
with projections from the ventral anterior and ventral lateral nuclei, VPm and VPl, and the
entire posterior nucleus (medial and lateral). Notably, MM is distinguished from SC by
relatively dense projections from the dorsal division of the lateral geniculate nucleus and
pulvinar. We discuss the finding that S1 of the short-tailed opossum has a similar pattern
of projections as other marsupials and mammals, but also some distinct projections not
present in other mammals. Further we provide additional support for a primitive posterior
parietal cortex which receives input from multiple modalities.
Keywords: marsupial, cortical evolution, multimodal cortex, 3a, S1, posterior parietal, thalamocortical projections,
comparative neuroanatomy
INTRODUCTION
The emergence of a six-layered neocortex and its diversification
from 10 to 12 areas to hundreds of cortical areas is the hallmark
of mammalian evolution (Krubitzer, 2007; Kaas, 2011). Our laboratory is interested in how the shared features of all mammalian
brains inform the likely characteristics of the ancestral mammalian brain, how phenotypic transformations such as additional
cortical areas arise, and the constraints imposed on evolving and
developing brains (Krubitzer and Dooley, 2013). There are a
number of cortical areas that are present in all mammalian species
investigated, including primary sensory areas, second sensory
areas, and a few additional cortical areas not exclusively related
to unimodal sensory processing (for review, see Kaas, 2011).
Interestingly, the organization and number of motor areas in
the cortex is quite different in different mammals, and whether
the common ancestor of all mammals actually possessed even
Frontiers in Neuroanatomy
a primary motor area (M1) is unclear (Kaas, 2011). While a
separate motor area, rostral to the primary somatosensory area
(S1) and containing a complete and mirrored body representation is a feature shared among eutherian mammals, this group
represents just one of three major clades of mammals. Further,
even in placental mammals the relationship between M1 and S1
is variable (e.g., Donoghue et al., 1979; Donoghue and Wise,
1982). While only three extant species of Monotremes are available for study, the diversity and relative availability of different
marsupial species has revealed motor representations that are surprisingly divergent from those seen in placental mammals (Karlen
and Krubitzer, 2007). Among marsupials, American opossums
(order Didelphimorphia) are both the largest order (with over
100 species) and form the earliest radiation of marsupials (Kemp,
2005), which occurred approximately 150 million years ago
(Nilsson et al., 2010). Within this order is a small South American
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Dooley et al.
Thalamocortical projections to parietal cortex
marsupial, the short-tailed opossum (Monodelphis domestica).
Short-tailed opossums appear to share many characteristics with
the ancestral mammal, including skull morphology, body size,
brain size and the ecological niche that they occupy (Rowe et al.,
2011), suggesting that these animals could serve as good models
for the brain organization of our earliest ancestors.
It is not surprising then, that among marsupials studied,
opossums are the only order which have been consistently and
convincingly demonstrated to lack a separate motor cortex in
which microstimulation evokes movements (Magalhaes-Castro
and Saraiva, 1971; Beck et al., 1996; Frost et al., 2000); for
review see (Karlen and Krubitzer, 2007). In all opossum species in
which microstimulation techniques have been used, movements
could only be initiated by stimulating S1. Specifically, in both
the big-eared opossum (Didelphis aurita) and the Virginia opossum (Didelphis virginiana), movements could be elicited when
stimulating separate locations throughout the entire extent of S1
(Magalhaes-Castro and Saraiva, 1971; Beck et al., 1996), while in
the short-tailed opossum (Monodelphis domestica), movements
could only be elicited when stimulating the face representation of
S1 (Frost et al., 2000). This partial motor map (face but not body
representation) of the short-tailed opossum is in direct contrast
to the results observed in the big-eared opossum and the Virginia
opossum. While thalamic connectivity of S1 has been investigated previously in Virginia opossums, adding further support
for the existence of an entirely overlapping sensorimotor amalgam (Killackey and Ebner, 1973; Donoghue and Ebner, 1981a,b;
Foster et al., 1981); to date there has been no study of the thalamic
projections to S1 and the surrounding areas in the short-tailed
opossum. Thus, the primary aim of the current investigation is
to characterize the thalamic projections to numerous somatosensory and surrounding cortical areas of the short-tailed opossum
neocortex.
In previous studies, marsupials (including the short-tailed
opossum) have been shown to have at least two complete somatotopic representations within the neocortex: S1 and the second
somatosensory area, S2 (Beck et al., 1996; Huffman et al., 1999b;
Catania et al., 2000; Frost et al., 2000; for review see Karlen and
Krubitzer, 2007). Further analysis of electrophysiological recordings, architecture, and connectional patterns by our own and
other laboratories has supported the presence of additional fields
associated with somatosensory processing, including a rostral
field (SR) as well as a caudal field (SC; Beck et al., 1996; Elston
and Manger, 1999; Huffman et al., 1999b; Wong and Kaas, 2009;
Anomal et al., 2011; Dooley et al., 2013). A third complete somatotopic representation (the parietal ventral area, PV) has been
identified in numerous marsupials investigated (Beck et al., 1996;
Elston and Manger, 1999; Huffman et al., 1999b), however despite
careful exploration, a separate, third somatotopic representation
has not been identified in the short-tailed opossum (Catania et al.,
2000; Frost et al., 2000). However, the region which has been
identified shows characteristics of both S2 and PV, thus we have
elected to refer to this area as S2/PV throughout the text. Further,
a previous study conducted by our laboratory in the short-tailed
opossum identified a multimodal region (MM), caudal to SC and
rostral to V2, which had connections with both somatosensory
and visual areas of the neocortex (Dooley et al., 2013).
Frontiers in Neuroanatomy
In the present investigation we examined and quantified the
thalamic projections to three somatosensory fields in the shorttailed opossum (SR, S1 and SC), as well as the multimodal region
(MM). Further, in order to investigate whether different body part
representations within S1 were associated with the motor system,
thalamic projections restricted to the body or the face representation of S1 were investigated separately and in some cases directly
compared. To determine patterns of thalamocortical connectivity,
several retrograde anatomical tracers were injected into architectonically and/or electrophysiologically defined locations within
these fields and were related to the patterns of projections from
architectonically defined thalamic nuclei.
METHODS
Neuroanatomical tracer injections were combined with architectonic analysis in eight adult short-tailed opossums (4 males, 4
females, 76–136 grams) to determine the thalamocortical connections of parietal cortical areas, including both the lateral and
medial portions of primary somatosensory cortex (S1, for abbreviations see Table 1), the rostral and caudal somatosensory fields
(SR and SC respectively), and the cortical region just caudal to SC
(multimodal cortex, or MM). In several cases, electrophysiological recordings were performed to identify the receptive fields for
neurons at or surrounding the injection site (for table of injection
sites and cases see Table 2). All animals were housed in standard
laboratory conditions, with food and water available ad libitum.
Animals were maintained on either a 12-h light/dark cycle or
a 14/10 h light/dark cycle. All protocols were approved by the
Institutional Animal Care and Use Committee of the University of
California, Davis, and all experiments were performed under the
National Institutes of Health’s guidelines for the care of animals
in research.
NEUROANATOMICAL TRACER INJECTIONS
Animals were placed in an induction chamber and anesthetized
with isoflurane (1–5% per liter O2 ). Following induction, the animal’s head was shaved and a specially fitted mask was placed over
the animal’s nose. Throughout the procedure, the surgical plane
of anesthesia was maintained with 1–3% isoflurane. A 2% lidocaine solution was injected subcutaneously at the midline of the
scalp and around the ears, and animals were paced in a stereotaxic
apparatus. Animals were given dexamethasone (0.4–2.0 mg/kg,
IM) and atropine (0.04 mg/kg, IM) at the start of the surgery.
Respiration and body temperature were monitored throughout
the procedure.
Under standard sterile conditions, an incision was made at
the midline of the scalp, the temporal muscle was unilaterally
retracted, and a small craniotomy was performed over parietal
cortex. In some cases, a photograph was taken over the exposed
cortex to record the position of the tracer injections relative
to blood vessels. In several cases, electrophysiological recordings were performed to confirm the placement of the injections
(Table 2). A small hole was made in the dura over the area of
the injection site, and either a custom-beveled Hamilton syringe
(Hamilton Co., Reno, NV) or a Hamilton syringe fitted with a
glass pipette beveled to a fine tip was lowered ∼300–400 µm
into the cortex. Between 0.15 and 0.3 µL of 10% Fluoro-emerald
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Thalamocortical projections to parietal cortex
Table 1 | List of abbreviations used throughout the text.
CORTICAL FIELDS
3a
Somatosensory area (deep)
A1
Primary auditory cortex
EC
Entorhinal cortex
FM
Frontal myelinated area
M1
Primary motor cortex
MM
Multimodal cortex
PP/PPC
Posterior parietal cortex
S1
Primary somatosensory area
S1 face
Primary somatosensory area, face division
S1 body
Primary somatosensory area, body division
S2
Second somatosensory area
SC
Somatosensory caudal area
SR
Somatosensory rostral area
V1
Primary visual area
V2
Second visual area
SUBCORTICAL STRUCTURES
APT
Anterior pretectum
CeM
Central medial nucleus
CL
Central lateral nucleus
CP
Cerebral peduncle
ELECTROPHYSIOLOGICAL RECORDINGS
eml
External medullary lamina
Hb
Habenula
iml
Internal medullary lamina
LGNd
Lateral geniculate nucleus, dorsal division
LGNv
Lateral geniculate nucleus, ventral division
MD
Medial dorsal nucleus
MGNd
Medial geniculate nucleus, dorsal division
MGNm
Medial geniculate nucleus, magnocellular division
MGNv
Medial geniculate nucleus, ventral division
mt
Mammillothalamic tract
MV
Medioventral nucleus
ot
Optic Tract
PAG
Periaqueductal gray
pc
Posterior commissure
Pol
Posterior nucleus (lateral)
Pom
Posterior nucleus (medial)
Pul
Pulvinar
PV
Paraventricular nucleus
Rt
Reticular nucleus
ST
Subthalamic nucleus
VA
Ventral anterior nucleus
VB
Ventral basal nucleus
VL
Ventrolateral complex
VM
Ventromedial nucleus
VMb
Ventral medial basal nucleus
VP
Ventral posterior nucleus
VPl
Ventral posterolateral nucleus
VPm
Ventral posteromedial nucleus
(FE, Invitrogen, Carlsbad, CA), Fluoro-ruby (FR, Invitrogen,
Carlsbad, CA), or cholera toxin subunit-B (CTB; Sigma-Aldrich,
St. Louis, MO) was pressure-injected into the cortex. A total of
12 tracer injections were made in 8 animals with several animals
receiving multiple injections (Table 2). Following the injection,
the surface of the brain was flushed with sterile saline to remove
any remaining tracer on the cortical surface, the craniotomy was
covered with either bone wax or an acrylic skull cap (depending
on the size of the opening), and the temporal muscle and skin
were sutured. Antibiotics (Baytril, 5 mg/kg, IM) and analgesics
(buprenorphine, 0.03 mg/kg, IM) were given. Animals recovered for 5–7 days to allow for the transport of the tracer, after
which they were euthanized with an overdose of sodium pentobarbital (Beuthanasia; 250 mg/kg, IP), transcardially perfused
with 0.9% saline, followed by 2–4% paraformaldehyde in phosphate buffer (pH 7.4), and finally 2–4% paraformaldehyde in
10% phosphate-buffered sucrose. These procedures have been
described previously (Dooley et al., 2013).
In three cases (08-80, 09-32, and 12-18) electrophysiological
recording experiments were performed following 5–7 days of
recovery. When possible, these experiments helped confirm the
S1/S2 boundary, the boundary of the face/body representations within S1, as well as the rostral and caudal extent of
S1 (Figures 1B,C). For terminal electrophysiological recording
experiments, two animals were anesthetized with 1–3% isoflurane
and one was anesthetized using 30% urethane in propylene glycol (1.25 g/kg, IP). No differences were noted between maps using
different anesthesia methods. Surgical procedures were the same
as those described previously, except a larger craniotomy was performed over the entire neocortex and the dura was retracted.
Digital images were taken so that electrophysiological recording
sites could be related to injection sites and cortical vasculature.
Multiunit electrophysiological recordings of parietal cortex
were performed using tungsten microelectrodes (0.010 inches,
0.5–5 M; A-M Systems, Sequim, WA). The electrode was lowered ∼400 µm below the pial surface, in layer IV of cortex.
Multiunit activity was amplified and filtered (100–5,000 Hz; A-M
Systems Model 1800 Microelectrode AC Amplifier; A-M Systems,
Carlsborg, WA), monitored through a loudspeaker, and viewed
on a computer monitor. At each recording site neural responses
to somatosensory stimulation (consisting of light taps, displacement of vibrissae, brushing of skin, hard taps and manipulation of
muscles and joints) were recorded and receptive fields were both
drawn on illustrations of the body and documented in surgical
notes. These methods have been previously described (Dooley
et al., 2013). Following electrophysiological recording, animals
were euthanized and perfused as described above.
OTHER ABBREVIATIONS
HISTOLOGY
CO
Cytochrome oxidase
CTB
Cholera toxin subunit-B
Once perfused, the brain was extracted, weighed, and photographed and the cortex was separated from the subcortical
structures. In some cases, the hippocampus and basal ganglia
were dissected from the cortical hemispheres. All dissected cortices were then manually flattened between glass slides, briefly
post-fixed in 4% paraformaldehyde in 10% phosphate-buffered
FE
Fluoro-emerald
fl
forelimb
FR
Fluoro-ruby
vGluT2
Vesicular glutamate transporter 2
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Thalamocortical projections to parietal cortex
Table 2 | Asterisks next to case numbers indicate cases for which corticocortical connections were reported previously (Dooley et al., 2013).
Volume injected (µl)
Injection area (mm2 )
FR
0.2
0.07
FR
0.3
0.11
FR
0.2
0.07
Case
Area injected + halo
Hemisphere injected
Tracer
13-114
S1 body
Left
12-13*
S1 body
Right
13-126
S1 face
Left
09-18*
S1 face
Left
FR
0.3
0.07
12-18*
S1
Right
FR
0.3
0.06
13-73
S1
Right
FE
0.3
0.07
08-29*
SC
Left
FE
0.3
0.08
09-18*
SC
Left
FE
0.3
0.14
Injection RF
Figure number
6
7
Face/Vib
Vib/Forepaw
2, 8
9
10
13-73
SC
Right
FR
0.2
0.12
08-80*
MM
Left
FE
0.3
0.07
Unresponsive
12
11
08-80*
SR
Left
FR
0.3
0.07
Lips
13
13-126
S1 body, SR
Left
CTB
0.15
1.03
Forepaw
2
Receptive field of injection site is shown, when available.
sucrose, then allowed to soak for 12–36 h in 30% phosphatebuffered sucrose. The flattened cortex was then sectioned at
30 µm using a freezing microtome. Alternating cortical sections
were stained for myelin (Gallyas, 1979) or mounted immediately
for fluorescent microscopy. In cases in which CTB was injected,
cortical sections were divided into three series, one of which was
reacted for CTB.
Subcortical structures were post-fixed as described above and
then allowed to soak in 30% phosphate-buffered sucrose until
they sunk in solution (24–48 h). Following this, they were sectioned coronally at 30–40 µm. Tissue was divided into 3 or 4
series. One series in all cases was stained for cytochrome oxidase
(CO; Wong-Riley, 1979) while a second was mounted immediately for fluorescent microscopy. When applicable, immunohistochemistry was performed for CTB. Additionally, all cases
were stained for either Nissl or processed for vGluT2 expression
(mouse monoclonal anti-vGluT2 from Millipore, Billerica, MA;
1:5000).
DATA ANALYSIS
Injections sites and cortical field boundaries were reconstructed
as described previously (Campi et al., 2010; Dooley et al., 2013).
Briefly, reconstructions of anatomical boundaries from photographs of individual myelin sections were made (Figure 1A),
and the boundaries from the entire series were examined and
combined into a single reconstruction (Figure 2A; for details,
see Seelke et al., 2012). Each reconstructed section contained
an outline of the section, blood vessels, tissue artifacts, probes,
and architectonic borders of cortical fields. These landmarks
were then directly related to sections mounted for fluorescence or processed for CTB, and a comprehensive reconstruction
of the neocortex was generated that contained the injection
site, injection halo, labeled cells and architectonic boundaries.
Corticocortical connections for these cases have been described
previously (Dooley et al., 2013). Several injections were found
to have spread into surrounding cortical areas, and thus these
cases were not included in quantitative analysis and Table 2;
however the data were qualitatively described and illustrated as
figures (e.g., Figure 2). In all cases included in the study, cortical
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injections spanned all cortical layers and did not extend into the
underlying white matter (Figures 1D–H).
For thalamic sections, labeled cells were plotted using an
X/Y stage encoding system (MD Plot, Minnesota Datametrics,
St. Paul, MN) mounted to a fluorescent microscope and connected to a computer. Additionally, the tissue outline, blood
vessels, and artifacts were plotted, which allowed these sections
to be aligned to the anatomical boundaries determined from the
Nissl, CO or vGluT2 stained tissue (Figures 2C,D). The location of labeled cells in the plotted sections was combined with
adjacent sections to form a single comprehensive reconstruction (Figure 2B). When necessary, brightness and contrast were
adjusted for the digital images using Photoshop CS5 (Adobe,
San Jose, CA). Additionally, in several instances, multiple photographs of a single cortical or thalamic section were turned into a
single composite image using Microsoft Image Composite Editor
(Microsoft, Redmond, WA).
Thalamocortical connections were quantified by summing the
total number of retrogradely labeled cells in the thalamus and
calculating the percentage of labeled cells in a given thalamic
nucleus. This allowed us to normalize the data for injections of
different sizes. This quantification allowed us to determine connection strength for each thalamic nucleus as follows: Strong: >
10%; Moderate: 9% to 3%; Weak: 3% to 1%; Intermittent: < 3%
and not present in all cases.
RESULTS
The goal of these studies was to determine the thalamocortical
connections of parietal somatosensory areas as well as the multimodal region in the short-tailed opossum; with the ultimate
goal of comparing these connections with other marsupials as
well as with eutherian mammals. In the following results we first
describe how cortical field boundaries were determined and the
extent to which our injection sites were restricted to the different areas of interest. Next, we define the different nuclei in the
thalamus by their appearance in histologically processed tissue.
Finally, we describe the patterns of thalamocortical connections
of S1, SC, SR, and MM and quantify the density of projections
from different thalamic nuclei.
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FIGURE 1 | Cortical organization in Monodelphis domestica. (A) A
tangential section of cortex that has been stained for myelin. The medial wall,
caudal wall and piriform cortex have been removed. The arrow points to an
injection located in SC. (B) An illustration of the body of the short-tailed
opossum colored according to the head representation (dark green) and the
body representation (light green). (C) Schematic of cortical fields and body
representations in a brain oriented as in A. Different cortical fields are depicted
in different colors; these colors are used in later figures showing the
anatomical connections (labeled cell bodies in the thalamus) to indicate the
cortical field location of the injection site. Thick lines delineate borders
between different cortical areas, while thin lines separate different body part
DETERMINATION OF CORTICAL FIELD BOUNDARIES
A subset of the cases used in this study were also used in our
previous study of corticocortical connections of parietal cortex
in Monodelphis domestica (Table 2; Dooley et al., 2013). In addition to our previous publication, more extensive descriptions of
the organization of different cortical fields in this species can
be found in other studies by our own and other laboratories
(Huffman et al., 1999b; Catania et al., 2000; Frost et al., 2000;
Kahn et al., 2000a; Karlen et al., 2006; Karlen and Krubitzer,
2009; Wong and Kaas, 2009). In many of these studies, myelin
stains were directly compared to electrophysiologically identified
boundaries (Catania et al., 2000). Additionally, the neocortex of
short-tailed opossums has been assessed in sagittal and coronal
sections, demonstrating the laminar distribution of myelin and
Frontiers in Neuroanatomy
representations within S1 and S2/PV determined in previous
electrophysiological recording studies. Boundaries of cortical areas are based
on architectural, connectional, and electrophysiological data, while
topographic boundaries within S1 are based on electrophysiological
experiments. The functional organization of S1 and S2/PV is adapted from
Catania et al. (2000). (D–H) Superficial to deep sections of the cortical
injection in A. Other than being slightly larger in the most superficial section
(D), the injection halo remains similar in size through intermediate sections
(E–G), before it finally begins to shrink in the final section (H). As with all other
cases included in this study, the injection halo included all cortical layers but
did not extend into the underlying white matter. See Table 1 for abbreviations.
CO (Wong and Kaas, 2009). As in the present study, the primary
sensory areas of the short-tailed opossum, including S1, V1, and
A1, were densely myelinated and their borders could be readily
determined (Figure 1A, see Table 1 for abbreviations). SR and
SC were lightly myelinated strips of cortex approximately 0.5 mm
wide and were located directly rostral and caudal to S1 respectively (Figure 1). V2 was identified as a lightly myelinated strip
approximately 0.5 mm at the rostral boundary of V1. Multimodal
cortex (MM) was a very lightly myelinated region caudal to SC
and rostral to V2. S1 contains a representation of the entire
contralateral body surface, with the hindpaw represented most
medially and the rhinarium and oral structures represented most
laterally (Figures 1B,C; Catania et al., 2000). Thus, S1 can be further divided functionally into body (S1 body) and face (S1 face)
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FIGURE 2 | VPm and VPl project to different body representations within
S1. (A) Reconstruction of the flattened cortical hemisphere from case 13-126
in which one injection of CTB (light green circle) was placed in the S1 body
representation and one injection of FR was placed in the S1 face
representation (dark green circle). (B) Reconstructed thalamic section showing
the boundaries of different subcortical nuclei drawn from histologically
processed tissue. Dorsal is up, lateral is to the left. Colored dots and squares
correspond to labeled neuronal nuclei resulting from each injection, with light
green circles projecting to the light green S1 body injection and the dark green
divisions, generally separated by a small band of lightly myelinated cortex. These techniques were utilized in S1 injections in the
present study in order to determine the location of our injection
sites relative to the body or face representations.
ARCHITECTONIC SUBDIVISIONS OF THALAMUS
Subdivisions of the thalamus were delineated using sections
stained for CO and either Nissl substance or vGluT2, and many
of the nuclei described here have been previously distinguished
by our own and other laboratories (see Turlejski et al., 1994;
Huffman et al., 1999b; Karlen et al., 2006; Olkowicz et al., 2008).
The boundaries of thalamic nuclei in short-tailed opossums
were also compared to previously published thalamic boundaries
reported in the closely related Virginia opossum (Oswaldo-Cruz
and Rocha-Miranda, 1968; Jones, 2007).
In the rostral-most sections where label was found, numerous
midline nuclei were identified. CeM was located at the midline,
was darkly stained, and had densely packed neurons. It was most
apparent in Nissl-stained tissue, but was also visible in CO and
vGluT2 preparations (Figures 3A–D). Dorsal and lateral to CeM
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squares projecting to the S1 face injection in (A). Dashed lines indicate
borders that were more difficult to determine from the sections available. The
gray box corresponds to the region of the light field and fluorescent images in
(C–F). Light field image of tissue stained for (C) vGluT2, (D) CO, (E) CTB, and
(F) unstained tissue visualized under fluorescence microscopy. The arrows in
each box indicate the presence of blood vessels use to align tissue. The scale
bar for (C–F) are all 500 µm. The boxes in (E) and (F) are magnified in (G) and
(H), respectively, showing (G) neurons labeled with CTB and (H) FR. The scale
bar for (G,H) is 100 µm. See Table 1 for abbreviations.
lies another darkly stained nucleus termed CL (which is part
of the intralaminar group; Figures 3A–D). As with CeM, CL is
most apparent in Nissl-stained tissue but can also be seen in
tissue stained for CO and vGluT2. Medial to CL and dorsal to
CeM is a large oval nucleus, MD, that has moderate neuronal
packing density in Nissl-stained sections (Figure 3B), and stains
lightly for both CO and vGluT2 (Figures 3A,C). Boundaries separating these nuclei can be readily determined by the sharp
contrast between the darkly stained CeM and CL and the lightly
stained MD.
Lateral to these midline nuclei in more rostral portions of
the thalamus we observed labeled neurons in the ventral anterior and the ventrolateral nuclei (VA and VL, respectively).
The boundaries between these two nuclei were not always distinct in CO and Nissl-stained tissue, and so throughout the
quantitative analysis performed in these studies they were combined into a single category (“VL/VA”). Both of these nuclei
stained moderately for CO and showed a medium neuronal
packing density in Nissl stains (Figures 3A,B). When tissue
was stained for vGluT2, VA stained more darkly than VL and
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FIGURE 3 | Boundaries of nuclei in rostral sections of the dorsal
thalamus in the short-tailed opossum. Three different stains were used
to delineate the boundaries of thalamic nuclei including CO (A) Nissl
(B) and vGluT2 (C). Reconstructions of thalamic nuclei derived from these
stains are drawn in (D). Midline nuclei, including CeM, CL and MD, can be
observed using each stain, but are most obvious in Nissl and vGluT2.
Ventral and lateral to midline nuclei, VL, VA, and VM can also be seen. VA
and VM boundaries are most apparent in vGluT2 (C). Lateral to these
structures, LGNd and LGNv can be identified. The boundaries of these
structures can be readily identified in CO (A) and vGluT2 (C) stained tissue.
In all images, dorsal is up and lateral is to the left. Scale bar in (A) is
500 µm, and is the same for all images.
was located medial and ventral to VL (Figure 3C, boundary
denoted by the dotted line in Figure 3D). Just medial to VA
was VM, a densely packed nucleus that stained darker than
the surrounding nuclei in Nissl-stained sections (Figure 3B).
However, VM stained moderately for CO and lightly for vGluT2
(Figures 3A,C). VM extended further caudal than VL and VA
(Figures 4A–D).
The reticular nucleus, Rt, could be identified throughout most
of the rostrocaudal extent of the thalamus (e.g., Figures 3, 4).
Rt was heterogeneous in appearance due to the number of fiber
bundles passing throughout this nucleus. Despite its heterogeneous appearance, Rt stained darker then the surrounding tissue
in all stains, and appeared particularly dark in tissue stained for
vGluT2 (Figures 3C, 4C). Throughout most of its rostral-caudal
extent, Rt was bounded dorsomedially by the external medullary
lamina (eml), which was clearly visible in both CO and vGluT2
stained tissue as a very lightly stained structure (see Figures 3A,C,
4A,C).
The ventral posterior nucleus (VP) was located caudal to VL
and VA. It is bordered ventrally by iml and Rt and laterally by
eml. VP stained darkly for CO, Nissl and vGluT2 (Figures 4A–C).
VP could be further subdivided into lateral (VPl) and medial
(VPm) divisions, with VPl corresponding to the representation
of the body and VPm corresponding to the representation of
the face. Further, VPm has been described as having a higher
Frontiers in Neuroanatomy
FIGURE 4 | Boundaries of nuclei in the dorsal thalamus at the level of
the ventral posterior nucleus in the short-tailed opossum. As in the
previous figure, three different stains were used to delineate boundaries;
CO (A), Nissl (B), and vGluT2 (C), with reconstructions derived from these
stains illustrated in (D). The ventral posterior complex, including VPm and
VPl can be seen as a darkly staining region in CO (A) and vGluT2 (C) stained
tissue, with VPm staining more darkly. VMb can be seen medial to VP as a
lightly stained line of tissue between two darkly stained sections. Lateral to
VP, eml stains lightly in all three series. At this level of the thalamus, Pol is
intermingled with eml. As in Figure 3, LGNd and LGNv are lateral to the
ventral posterior complex, and are most apparent in CO (A) and vGluT2
(C) stained tissue. Near the midline, PF can be seen in CO (A) and vGluT2
(C) stained tissue, being slightly darker than the surrounding nuclei. Finally,
dorsal to VMb and medial to VP, the caudal extent of VM could be identified
as a lightly stained nuclei in all three series. Conventions as in Figure 3.
density of cells compared to VPl, and thus appeared darker
than surrounding tissue in a Nissl stain, and stained darker for
CO. The boundary between VPl and VPm was most apparent in tissue stained for CO (see Figure 4A). Dorsal, caudal,
and in some sections lateral to VP was the posterior nucleus
composed of both a medial (Pom) and lateral (Pol) division.
Pom extended rostrally and is located just medial to VP, staining lightly for both CO and Nissl (Figures 4A,B). Medial to
VP we identified the ventral medial basal nucleus (VMb). This
nucleus stained moderately for CO and Nissl, and often appeared
lighter than regions immediately dorsal, ventral and medial to it
(Figures 4A,B). In tissue stained for vGluT2, VMb stained darkly
(Figure 3C).
As has been described previously (Huffman et al., 1999a;
Karlen et al., 2006), in Monodelphis both LGNv and LGNd stained
darkly for CO, and are separated from each other by a thin lightly
stained region (see Figures 3A, 4A). LGNv contained a lateral
portion that stained very darkly for both CO and in Nissl. LGNv
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January 2015 | Volume 8 | Article 163 | 7
Dooley et al.
Thalamocortical projections to parietal cortex
extended medially to the lateral border of Rt (Figures 3A–D,
4A–D). In tissue stained for vGluT2, LGNd stained much more
darkly than the surrounding tissue, and individual laminae could
be observed (Figures 3C, 4C). Dorsomedial to the LGNd is the
pulvinar (Pul, previously termed the lateral posterior thalamic
nuclei; Kahn et al., 2000b). Neurons in Pul showed a moderate
packing density in tissue stained for Nissl and CO (Figures 4A,B).
Pul was moderately stained by vGluT2, and the caudal subdivision stained more intensely near the dorsal-most extent
(Figure 4C; Baldwin et al., 2013). This subdivision is indicated
with a dotted line in Figure 4D.
In the caudal most portions of the thalamus, Pol could be
identified. In the rostral sections where it was identified, Pol was
mostly intermingled with eml (see Figures 4A–C, 5A–C, 6D–G).
While the boundaries of Pol were at times difficult to identify
with the stains used in the present studies, in favorable sections
it could be identified as a lightly staining nucleus lateral to VP but
medial to LGN. In the more rostral sections Pol is bordered ventrally by Rt. Finally, the MGN began to emerge just caudal and
lateral to VP. In CO and vGluT2 stained tissue, fiber bundles running perpendicular to the tissue could be seen separating MGN
from VP (Figures 5A–C, 6H). As the MGN begins to emerge,
the region surrounding and between these nuclei was identified
as the caudal extension of Pol described above (Figures 5A–C,
6H). In more caudal sections of the thalamus MGN became
larger and was located further laterally, and could be further
subdivided into different divisions, including the darkly stained
MGNv as well as the more lightly staining MGNd and MGNm
(Figure 6I).
THALAMIC CONNECTIONS OF THE PRIMARY SOMATOSENSORY AREA
(S1)
Of the six cases in which injections were made in S1, two of
the injections were restricted to the body representation of S1
(Figures 6, 7), two were restricted to the face representation of S1
(Figure 8; case 09-18, not shown), and two were placed across the
S1face/S1body representations (Figure 9; case 13-73, not shown).
In all 6 cases, the majority of projections originated from VP
(mean = 57.5%, see Table 3 for the percentages from individual cases), with the percentage of labeled cells that originated
from VPm and VPl varying dramatically depending on the location of the cortical injection within S1. In one case, the injection
was placed in the medial portion of S1 (in the expected representation of the forepaw, case 13-114, Figure 6A), resulting in
the vast majority of labeled cells residing in VPl (Figures 6D–I).
Conversely, a cortical injection placed in the lateral portion of S1
(Figure 8A, confirmed with electrophysiological recording to be
restricted to the vibrissae representation of the face) resulted in
the vast majority of labeled cells located in VPm (Figures 8D–H).
In one case, two injections were placed such that one was
restricted to S1 body representation and another was restricted
to S1 face representation (extending rostrally into SR, Figure 2A).
In this case, VP neurons projecting to S1 face representation (dark
green squares) are restricted to VPm while neurons projecting to
S1 body representation (light green circles) are almost entirely
restricted to VPl (Figure 2B). In nearly all cases in which S1 was
injected, labeled cells were not spatially restricted to a particular
portion of VPl or VPm, but were found throughout each of these
subdivisions suggesting a rather loose topographic organization
of these nuclear subdivisions (e.g., Figures 2B, 7G).
In addition to projections from VP, all injections in S1 resulted
in strong projections from Pom (mean = 18.6%) and moderate
projections from VM and VMb (mean = 4.9 and 6.0% respectively). For both of these nuclei we observed a difference in the
percentage of labeled neurons depending on the placement of the
injection within S1. Injections restricted to the body representation in S1 did not have projections from VM, while all injections
into the face representation of S1 had moderate to strong projections from VM (range = 2.8–12.2%). Projections from VMb to S1
were not present in every case, but did not appear to differ systematically for body versus face representations in S1 (see Table 3).
However, the two injections with strong projections from VMb
are located rostrally within S1, closer to the expected location of
the representation of oral structures (Figure 6A; see Figure 1 for
location of oral representation). Moderate to weak projections
were also observed from PF and VA/VL, while all other nuclei
in which labeled cells were found had weak and inconsistent
projections to S1 (e.g., CL, CeM, see Table 3).
THALAMIC CONNECTIONS OF THE CAUDAL SOMATOSENSORY AREA
(SC)
FIGURE 5 | Boundaries of nuclei in the dorsal thalamus at the
caudal-most extent of the ventral posterior nucleus in the short-tailed
opossum. All three stains used are the same in Figures 3, 4, along with an
illustration of nuclear boundaries. In these sections VP, Pol and MG can be
identified. Pol is lateral to VP and extends outward, surrounding the
emerging MGN. Conventions as in previous two figures.
Frontiers in Neuroanatomy
The three cases in which injections were restricted to SC had
very consistent patterns of thalamic connectivity (Figures 10, 11,
case 08-29, not shown). As with S1, the strongest projections
originated from VP (mean = 39.3%), however SC also received
strong projections from Pol (M = 19.4%), VA/VL (mean =
11.6%), and Pom (mean = 9.7%). These strong projections from
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January 2015 | Volume 8 | Article 163 | 8
Dooley et al.
Thalamocortical projections to parietal cortex
FIGURE 6 | Projections to the S1 body representation from in case
13-114. (A) Reconstruction of the flattened cortical hemisphere, showing
the core of the injection site of FR (black dot) surrounded by the halo
(green circle) in the body representation of S1. (B) Fluorescent image of
the FR injection site and surrounding halo. (C) Percent of labeled
VL/VA were not spatially restricted, and were located throughout these nuclei (Figures 10C,D). Additionally, labeled neurons
in Pol were observed throughout the nucleus (Figures 10E–G,
11H–L). Moderate projections to SC also originated from VM
(mean = 6.0%). Notably, weak projections from LGNd and Pul
(two visual nuclei) were also present in all three cases (mean =
1.4 and 2.8% respectively). When comparing the thalamocortical projection profiles of S1 and SC, the most notable differences
are the density of projections of VP, Pol and VA/VL (Table 3). S1
had more dense projections from VP while SC had more dense
projections from Pol and VA/VL. Further, SC had weak but consistent projections from visual nuclei of the thalamus which were
not observed for S1 projections.
THALAMIC CONNECTIONS OF THE MULTIMODAL REGION (MM) AND
THE ROSTRAL SOMATOSENSORY AREAS (SR)
One injection was entirely restricted to MM (Figure 12A). In this
case, MM received strong projections from VP and Pol (26.8 and
27.7% respectively, see Figures 10F,G). Additionally, MM also
Frontiers in Neuroanatomy
neurons originating from various thalamic nuclei (D–I) Rostral to caudal
progression of CO stained tissue; corresponding thalamic borders and
labeled neurons are shown below (D′ –I′ ). In this case, the majority of
label was found within VPl, VMb, and Pom. All conventions as in
previous figures.
received significant projections from VL/VA (12.7%) and moderate projections from both LGNd and the pulvinar (6.6% for
both nuclei). Thus, MM is distinguished from SC by its relatively dense projections from LGNd and the pulvinar, giving
MM a more multimodal connectional profile compared to the
primarily somatosensory SC, Additionally, we had one injection
entirely restricted to SR (Figure 13). Unlike other cortical parietal
areas described thus far, SR received substantially more projections from both VM (20.6%) and Pom (20.1%) than from VP
(10.3%, see Figures 13D–H). SR also received very dense projections from several midline structures that do not consistently
project to S1, SC, or MM. For example, SR received strong projections from MD (16.1%), while VA/VL (8.9%), VMb (8.4%),
and PF (5.5%) all have moderate projections to SR. Weak projections were also observed from CL and CeM (1.1 and 1.2%,
respectively).
Taken together, these data demonstrate that S1, SC, and MM
received the strongest projections from VP; however the strength
of those projections decreased from S1 to SC and MM (see
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January 2015 | Volume 8 | Article 163 | 9
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Thalamocortical projections to parietal cortex
FIGURE 7 | Projections to the S1 body representation in case
12-13. (A) Reconstruction of the FR injection into the body
representation of S1. (B) Fluorescent image of the FR injection site
and surrounding halo. (C, D) Distribution of labeled neurons were
observed in similar nuclei to those shown in Figure 4, but were
Figure 14, Table 3). Conversely, MM had stronger projections
from visual structures (LGNd and Pul) while SC has only sparse
connections from these nuclei and S1 had no projections from
these nuclei. Mimicking the pattern of VP connections, the
strength of projections from Pom was weaker as injections moved
farther caudal in cortex (e.g., from S1 to SC to MM). Strong
projections from Pol were only found in areas caudal to S1
(Figure 14F). Located immediately rostral to S1, SR showed a pattern of projections from thalamic nuclei that was notably different
from the other three areas described. Strong projections to SR
were seen from midline structures including VM and MD, which
only weakly or inconsistently projected to the other cortical areas
examined (Figures 14B,F).
DISCUSSION
The present investigation is part of a broader objective of our laboratory to appreciate the cortical organization and connectivity
of early mammals, and how this basic plan was modified in different lineages to produce the remarkable variability in behavior
Frontiers in Neuroanatomy
also located in more rostral portions of the thalamus in VL/VA. (E–H)
Labeled neurons throughout VPl and Pom have a similar distribution
that was illustrated in the previous figure. (I) Percent of labeled
neurons originating from various thalamic nuclei. All conventions as in
previous figures.
observed in extant species. Comparative studies in general allow
us to infer aspects of organization of our early ancestors; however, some species can provide a more accurate representation
of early mammals based on their phylogenetic status, lifestyle,
as well as brain and body morphology (Kemp, 2005; Kaas, 2011;
Rowe et al., 2011). One such species is the marsupial Monodelphis
domestica, or the short-tailed opossum. Previously, we examined the corticocortical connections of several somatosensory
areas in Monodelphis and found evidence for three somatosensory fields (SR, S1, and SC) as well as a multimodal region
(MM; Dooley et al., 2013). In the present experiments we investigated the thalamocortical projections of these cortical fields
and demonstrate that each cortical field has a unique pattern of
thalamocortical connections, supporting our parcellation of cortex. Additionally, we demonstrate different thalamic projections
to the S1 body representation and the S1 face representation,
which we interpret in the context of sensorimotor integration
for specializations of the face versus the body in Monodelphis
domestica. Finally, thalamocortical connections to all cortical
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January 2015 | Volume 8 | Article 163 | 10
Dooley et al.
FIGURE 8 | Projections to the face
(A) The location of an FR injection
the representation of the face. (B)
injection site and surrounding halo.
Thalamocortical projections to parietal cortex
representation in
site in the lateral
Fluorescent image
(C–H) Progression
case 13-126.
portion of S1 in
of the FR
of the thalamic
areas examined in the present study are compared to projections
in other marsupials and in other mammals to better appreciate common patterns of this thalamocortical network as well as
derivations.
THALAMIC PROJECTIONS TO S1 IN MARSUPIALS
Early studies in marsupials such as the Virginia opossum demonstrated that unlike placental mammals, marsupials do not appear
to have a separate cortical motor area. Rather, they possess what
has been called a “somatosensory-motor amalgam” (Lende, 1963;
Killackey and Ebner, 1973). The somatosensory-motor amalgam
refers to the complete overlap of the Virginia opossum’s topographic representation of cutaneous receptors of the body and
face in the primary somatosensory area (S1) with motor maps
of the body in the primary motor area (M1). Given that marsupials represent a very early mammalian radiation whose ancestors
diverged over 150 million years ago (Nilsson et al., 2010), one current hypothesis is that their sensory motor amalgam represents
Frontiers in Neuroanatomy
sections with labeled neurons, showing labeled neurons largely
restricted to VPm, Pom and VM. (H) Percent of labeled thalamic
neurons in this experiment. (I) Percent of labeled neurons originating
from various thalamic nuclei. Conventions as in previous figures.
a primitive form of mammalian neocortex, which ultimately
segregated into separate sensory and motor areas in placental
mammals (Kaas, 2004). While our understanding of the relationship between sensory and motor cortex has undergone enormous
transformations in the last decade (Hatsopoulos and Suminski,
2011; Kaas et al., 2012), the initial observation of a complete lack
of a separate motor cortex in marsupials has been validated in
other marsupials including short-tailed opossums and the bigeared opossums (Magalhaes-Castro and Saraiva, 1971; Frost et al.,
2000). Because it is likely that this type of organization does
indeed represent a more primitive form of cortical organization,
determining the connectivity, particularly with somatosensory
and motor subcortical structures, is a critical step in understanding how the cortical motor system in placental mammals ultimately came to generate sophisticated motor control necessary
for complex, goal-directed movements.
There are only a few studies of thalamocortical connections
in marsupials. The thalamic projections to S1 in the Virginia
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Thalamocortical projections to parietal cortex
FIGURE 9 | Thalamic projections to the border of the face-body
representation in S1 in case 12-18. (A) Location of the FR injection site.
(B) Fluorescent image of the FR injection site and surrounding halo. (C–F)
Labeled neurons can be seen throughout the medial and lateral divisions of
VP, along with labeled cells in Pom and VMb. (G) Percent of labeled thalamic
neurons projecting to S1. Conventions as in previous figures.
Table 3 | Percentage of labeled neurons observed in different thalamic nuclei out of total labeled neurons in the thalamus.
Case
Area injected
VA/VL
VM
CL
CeM
MD
VPm
VPl
VP
12-13
13-114
VMb
Pom
Pol
PF
LGNd
Pul
Other
S1 body
6.0
0.0
0.0
0.0
0.0
0.0
72.0
72.0
S1 body
0.0
0.0
0.0
0.0
0.0
2.8
38.9
41.7
1.0
18.0
0.0
0.0
0.0
0.0
3.0
10.2
30.6
0.0
4.6
0.0
0.9
12.0
13-126
S1 face
2.3
12.2
3.8
0.8
3.8
52.7
3.1
55.7
0.8
12.2
0.0
5.3
0.0
0.0
3.1
09-18
S1 face
0.0
2.8
0.0
0.0
0.0
43.4
26.4
69.8
10.4
11.3
1.9
2.8
0.0
0.0
0.9
12-18
S1
0.0
3.0
0.0
0.0
0.0
13.4
36.6
50.0
9.7
22.4
1.5
7.5
0.0
0.0
6.0
13-73
S1
4.0
11.4
4.0
1.5
0.0
23.8
32.2
55.9
4.0
16.8
0.0
0.0
0.0
0.0
2.5
Average
2.0
4.9
1.3
0.4
0.6
22.7
34.9
57.5
6.0
18.6
0.6
3.4
0.0
0.2
4.6
SEM
1.0
2.2
0.8
0.3
0.6
8.8
9.1
4.7
1.9
2.9
0.4
1.2
0.0
0.2
1.6
13-73
SC
10.4
7.4
2.8
0.0
1.2
9.0
24.2
33.3
0.0
16.6
17.6
0.0
0.5
4.2
6.2
09-18
SC
3.5
1.2
0.0
0.0
0.0
11.0
44.1
55.1
1.2
2.8
26.4
0.0
2.4
2.4
5.1
08-29
SC
20.7
9.5
3.8
1.1
0.2
14.0
15.5
29.5
0.7
9.6
14.4
0.5
1.3
1.8
7.0
Average
11.6
6.0
2.2
0.4
0.4
11.4
27.9
39.3
0.6
9.7
19.4
0.2
1.4
2.8
6.1
SEM
5.0
2.5
1.1
0.4
0.4
1.5
8.5
8.0
0.3
4.0
3.6
0.2
0.5
0.7
0.6
08-80
MM
12.7
0.9
0.5
0.0
1.4
9.9
16.9
26.8
0.5
8.5
27.7
0.0
6.6
6.6
8.0
08-80
SR
8.9
20.6
1.1
1.2
16.1
7.4
2.9
10.3
8.4
20.1
0.0
5.5
0.0
0.1
7.7
opossum are from the VP as well as from VL/VA, nuclei associated
with the motor system (Killackey and Ebner, 1973; Donoghue and
Ebner, 1981b; Beck et al., 1996). Projections from VPl and VPm
project to the body and face representations of S1, respectively,
and a previous study in the big-eared opossum has described
even finer somatotopy within VP (Sousa et al., 1971). While
the current investigation did not examine somatotopy at this
Frontiers in Neuroanatomy
level of specificity, our results are consistent with this previous
report. Additionally, strong projections to S1 have also been
reported from the medial division of the posterior nucleus, Pom
(Killackey and Ebner, 1973; Donoghue and Ebner, 1981b). In
two Australian marsupials, brush-tailed possums and Northern
quolls, cortical projections of VL and VP were overlapping, such
that a notably large portion of parietal cortex (presumed to be
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Thalamocortical projections to parietal cortex
FIGURE 10 | Thalamic projections to area SC in case 08-29. (A) An
injection of FE into SC which extends slightly into the caudal-most portion of
S1. (B–D) Densely packed labeled cells can be seen in VL/VA and VM, (D–G)
as well as throughout VPm and VPl. Large numbers of labeled cells can also
S1) received projections from both nuclei (Haight and Neylon,
1978a,b, 1981a,b). Notably, for projections from both VL and VP,
thalamic projections corresponding to distal body parts (e.g., digits of the paws) projected to more rostral portions of cortex, and
thalamic projections corresponding to proximate body parts (e.g.,
the trunk) projected to more caudal portions of cortex. In contrast, in placental mammals the topography of projections to cortex from VL are mirrored relative to those of VP, with distal body
parts (e.g., digits of the paws) projecting to relatively caudal portions of cortex compared to proximate body parts. Unfortunately,
neither electrophysiological nor histological techniques were used
to confirm the placement of injections into parietal cortex in these
Australian marsupials, making thalamocortical projections from
VL and VP to S1 difficult to compare across species.
Frontiers in Neuroanatomy
be seen throughout Pol, sometimes extending laterally into LGNd and Pul.
(H) A small number of cells were seen projecting from the medial geniculate
complex. (I) Percent of labeled thalamic neurons projecting to SC.
Conventions as in previous figures.
These results described in other marsupials are consistent
with those described in the present investigation in the shorttailed opossum with respect to VP and Pom. However, shorttailed opossums differ from other marsupials studied in that
they do not have significant projections from VL/VA throughout
S1, with an average of only 2% of thalamic projections originating from VL/VA, and half of S1 injections resulting in no
projections from this nucleus (Table 3). The lack/inconsistency
of projections from VL/VA to S1 does not appear to differ
based on the somatotopic origin of the injection site, a finding
that is surprising considering previous results which demonstrate a difference in the motor movements elicited following
intracortical microstimulation (Frost et al., 2000; see below for
further discussion).
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Thalamocortical projections to parietal cortex
FIGURE 11 | Thalamic projections to area SC in case 13-73.
(A) Location of cortical injection sight of FR into SC. (B–E) Labeled cells
can be seen throughout VL/VA and VM. (F–J) Additional numerous
labeled cells can be seen throughout VPl and VPm, as well as Pom.
(K,L) As with Figure 8, labeled cells can be seen throughout Pol,
THALAMIC PROJECTIONS TO S1 IN OTHER MAMMALS
Studies in monotremes, an order of mammals whose ancestors
radiated approximately 220 million years ago (Van Rheede et al.,
2006), reveal the diversity of sensorimotor organization of early
mammals. There are only three extant species of monotremes
and studies of cortical connections in these species are extremely
Frontiers in Neuroanatomy
sometimes extending laterally into LGNd and Pul, along with labeled cells
at the caudal-most extent Pol as the Medial Geniculate complex begins
to emerge. (M) A single cell was observed in the medial geniculate
complex. (N) Percent of labeled thalamic neurons projecting to SC.
Conventions as in previous figures.
limited. Within the neocortex, the platypus and echidna have
been found to have three somatosensory areas; S1, a rostral area
termed R, and a second somatosensory area, which may correspond to S2 or PV. Studies also suggest the existence of a motor
area rostral to R (Krubitzer et al., 1995). Thalamic projections
to S1 have only been described in two studies in the echidna;
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January 2015 | Volume 8 | Article 163 | 14
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Thalamocortical projections to parietal cortex
FIGURE 12 | Thalamic projections to multimodal region (MM) in case
08-80. (A) Injection site of FE in MM. (B) Fluorescent image of the FE
injection site and surrounding halo. (C–G) Labeled neurons can be seen
projecting to MM from large portions of VL/VA as well as from VPl and VPm.
one using degeneration methods (Welker and Lende, 1980) and
one using retrograde tracers (Ulinski, 1984). Welker and Lende
(1980) demonstrated that lesions to S1 resulted in degenerated
neurons throughout VP and in a nucleus dorsal and caudal to
VP (possibly Pom). Ulinski (1984) described projections exclusively from VP to S1. Neither study describes projections from
VL to S1; however the delineation of thalamic nuclei in the
echidna has undergone significant revisions since these studies
were performed (Mikula et al., 2008), and thus these early studies
Frontiers in Neuroanatomy
Additionally, densely packed label neurons are observed in Pol; moderate
numbers of labeled neurons in LGNd and Pul. (H) Percent of labeled thalamic
neurons projecting from various thalamic nuclei to MM. Conventions as in
previous figures.
may have failed to properly differentiate the VP/VL boundary.
Regardless, these studies support the basic mammalian pattern of
projections from VP to S1, and suggest projections from VP and
VL do not overlap in cortex in the echidna.
There are numerous studies of thalamocortical projections to
S1 in placental mammals, which report slight modifications on
the same basic plan of thalamocortical connectivity. Because a
complete review of this literature is beyond the scope of this discussion, we will focus on the pattern of thalamic projections in
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Thalamocortical projections to parietal cortex
FIGURE 13 | Thalamic projections to SR in case 08-80.
(A) Reconstruction of the injection of FR in SR. (B) Fluorescent image of
the FR injection site and surrounding halo. (C) Percent of labeled thalamic
small-brained placental mammals, such as rodents. Studies in
rats, mice, squirrels, and naked-mole rats describe thalamic projections to S1 from VP and Pom (Wise and Jones, 1977; Krubitzer
and Kaas, 1987; Henry and Catania, 2006; Liao et al., 2010). More
recently, it has been demonstrated in rats that projections to S1
from the thalamus are not homogeneous; instead there are parallel pathways to different cortical layers as well as to the granular
and dysgranular zones within S1. VP projects to the granular
zones and Pom projects to the dysgranular zones (Liao et al., 2010;
Viaene et al., 2011; Ohno et al., 2012). It has been suggested that
these two thalamic pathways to cortex are functionally distinct,
with VP driving activity in S1 and Pom having more of a modulatory role (Hoogland et al., 1987; Liao et al., 2010; Viaene et al.,
2011; Ohno et al., 2012). The present investigation confirms that
as in rodents, VP and Pom project to S1, but injections in the
present study were not restricted to a particular layer of cortex and
there are no architectonically distinct granular and dysgranular
zones in Monodelphis.
Weak and/or topographically restricted projections from VL
and VM to S1 have also been described in some rodent species.
VL projections to the forelimb and hindlimb representations in
S1 have been demonstrated in rats; these representations display properties of both S1 and M1 in this species (Donoghue
et al., 1979; Cicirata et al., 1986; Aldes, 1988) and other rodents
(Henry and Catania, 2006; Liao et al., 2010). Sparse projections
from VM to S1 are reported in diverse rodent species ranging
from rats to squirrels to naked mole rats (e.g., Donoghue et al.,
Frontiers in Neuroanatomy
neurons projecting to SR. (D–H) Unlike previous injections, the majority of
label cells were in the midline nuclei, including PV, MD, and PF.
Conventions as in previous figures.
1979; Krubitzer and Kaas, 1987; Giannetti and Molinari, 2002;
Henry and Catania, 2006), although these projections are often
weak and not found in every case. In rats, VM has been shown
to project primarily to rostral cortical areas, and more weakly to
parietal cortex (Herkenham, 1979). Interestingly, in naked mole
rats, Henry and Catania (2006) describe weak projections from
VM to the S1 forepaw representation in every case, and inconsistent projections from VM to the S1 incisor representation,
although a similar pattern is not noted in other studies in rodents.
Thus, despite the presence of an architectonically distinct M1 in
rodents, there still appears to be some sensorimotor integration
of the thalamic projections to S1 within most species of rodents
that have been examined.
SOMATOSENSORY/MOTOR INTEGRATION IN SHORT-TAILED OPOSSUM
As mentioned in the introduction, the short-tailed opossum is
unique among mammals investigated in that it does not have
a motor representation of its entire body; only the head has a
cortical motor representation (Frost et al., 2000). These results
inspired our hypothesis that the body and face representations in
S1 would have different patterns of thalamic projections, specifically in the case of projections from nuclei associated with the
motor system such as VL/VA. As noted above, however, VL/VA
only showed weak and inconsistent projections to S1, and these
weak projections did not differ between S1 face and S1 body
(Figures 14C,F). This finding differs from the evidence of a complete somatosensory motor amalgam previously described for the
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Dooley et al.
Thalamocortical projections to parietal cortex
FIGURE 14 | Continued
projecting to (B) SR, (C) S1, (D) SC, and (E) MM. Data are mean ± SEM
when applicable. Axes are the same throughout A-D to aid in comparison.
Projections to VPm and VPl are summed above VP, with the projections
from VPm under the horizontal line and the projections from VPl above the
line. (F) Summary of strong projections from the 5 cortical areas discussed
throughout the paper. All projections shown make up > 10% of the total
thalamic projections to that area.
FIGURE 14 | Summary of projections across cortical areas investigated
in these studies. (A) Illustration of cortical areas, using the same colors as
the graphs and diagrams below. Percent of labeled thalamic neurons
(Continued)
Frontiers in Neuroanatomy
Virginia opossum, with extensive projections form VL/VA and
VP throughout S1 (Killackey and Ebner, 1973). One explanation for these differences between marsupials is that connections
between VL/VA and S1 have been lost in short-tailed opossums;
another possibility is that the thalamocortical projection patterns in Monodelphis actually represent the ancestral mammalian
plan. However, because both thalamic projections and functional
organization have been assessed in only two opossum species we
cannot distinguish between these two alternative hypotheses.
The largest difference we found was that the sensory-motor
face representations received moderate projections from VM
while the S1 body representation did not receive any projections
from VM (Table 3, Figure 14C). In other species, VM receives
input from the cerebellum and projects to numerous (particularly
rostral) cortical areas as well as other subcortical structures that
are part of the motor system (Donoghue et al., 1979; Herkenham,
1979; Kuramoto et al., 2013). In addition to the differential thalamic projections to the face versus the body representation in S1,
we also demonstrate that the face representation receives input
from both VPm and VPl while the body representation receives
input only from VPl (Table 3). In our previous investigation of
corticocortical connections, we observed that intrinsic connections of S1 are generally heterotropic, and thus not restricted to
similar somatic representations (Dooley et al., 2013). Thus, the
S1 face representation receives projections from thalamic nuclei
associated with motor processing (VM) as well thalamic input
from both the face and body somatosensory representations. This
suggests that motor control of portions of the snout and face
as well as inputs processed within the face representation of S1
together form specialized differentiated, highly integrated system
that differs from the rest of the body.
When considering why the S1 face representation is so highly
integrated with different cortical areas and thalamic nuclei, it is
important to consider the peripheral prominence of the vibrissae
and the cortical magnification of the face in light of its ethological
significance. Studies of whisking behavior in short-tailed opossums have found that they do display active vibrissal exploratory
behavior (Mitchinson et al., 2011; Grant et al., 2013). This suggests that prominent sensory face hairs may have been a shared
feature of the common ancestor of placental and marsupial mammals, and thus may be one of the earliest and most pronounced
forms of sensory reception. Ultimately, the representation of vibrissae within the cortex may be an example of a significant,
and perhaps ubiquitous, site of sensorimotor integration. As
mammalian species radiated, different morphological structures
and associated sensory arrays, such as the hands and eyes in
primates, have undergone specialization and an accompanying
cortical expansion of motor and posterior parietal cortex (see
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January 2015 | Volume 8 | Article 163 | 17
Dooley et al.
Thalamocortical projections to parietal cortex
Cooke et al., 2014). However, the organization and connectivity of
the opossum vibrissae system suggests that strong sensorimotor
integration of ethologically significant body parts has long been a
general feature of the mammalian neocortex.
THALAMIC PROJECTIONS TO SR, SC, AND MM IN OTHER SPECIES
Among marsupials, SR and SC have been identified based on
receptive field characteristics and stimulus preference in the
Virginia opossum (Beck et al., 1996), the brush-tailed opossum (Elston and Manger, 1999), the northern quoll, the striped
possum (Huffman et al., 1999b), and the big-eared opossum
(Anomal et al., 2011). Across the species investigated, neurons
in SR and SC respond predominantly to stimulation of deep
receptors in muscles and joints, show a coarse topographical organization, and have larger receptive fields compared to neurons in
S1. In most of these studies, these functionally defined fields have
been directly related to cortical architecture (see above) and in
some species to cortical connections. Corticocortical connections
of both SR and SC have been reported in the Virginia opossum
(Beck et al., 1996), the brush-tailed possum (Elston and Manger,
1999), the short-tailed opossum (Dooley et al., 2013), and for SC,
the big-eared opossum (Anomal et al., 2011). In all of these studies, the pattern of projections provided further evidence that SR
and SC are distinct cortical fields and part of a somatosensory
network. It has been proposed that SR is homologous to 3a or
dysgranular cortex in rodents and SC is homologous to posterior parietal cortex in rats (for review, see Karlen and Krubitzer,
2007; Krubitzer et al., 2011). Our studies of corticocortical connections as well as previous electrophysiological recording studies
demonstrate an additional region of cortex just caudal to SC in
which neurons respond to more than one modality of stimulation
(Huffman et al., 1999a; Kahn and Krubitzer, 2002), and which
receives input from visual and somatosensory cortex (Dooley
et al., 2013). We term this field MM or the multimodal region.
Despite consistent evidence for both SR and SC in all marsupials studied, only one study in marsupials examined the thalamocortical connections of these fields (Virginia opossum; Donoghue
and Ebner, 1981b). In this early study, several injections were
placed immediately rostral and caudal to S1. The rostral injections
sites were in an area termed post-orbital cortex (likely equivalent to SR in the present investigation) and projections to this
area were from several midline and intralaminar nuclei including
CL, PF, MD, VL, and Pom (identified as CIN), with sparse projections from VP (see Figure 9 in Donoghue and Ebner, 1981b).
These projections are consistent with the patterns of projections
we observed, only with a smaller contribution from CL and
more projections from Pom and VMb. Additionally, while in the
present investigation only one injection was found to be entirely
restricted to SR, numerous injections spanned SR and S1 (e.g.,
the light green injection in Figure 2A), showing a similar pattern
of thalamic projections, just with a greater percentage of neurons
originating from VP (Figure 14B).
As mentioned above, we have proposed that SR may be
homologous to dysgranular cortex (3a) in rodents. In both
rats and squirrels, dysgranular cortex receives strong projections from Pom, along with moderate projections from VP,
VL/VA, and CL, as well as projections from VM (Koralek et al.,
Frontiers in Neuroanatomy
1988; Gould et al., 1989). Notably, dysgranular cortex does not
receive projections from midline nuclei MD and PF. However,
apart from this exception projections to dysgranular cortex/3a
are similar to those to SR in short-tailed and the Virginia
opossums.
Donoghue and Ebner (1981b) did not delineate cortical areas
caudal to S1, instead referring to the region caudal to S1 and rostral to peristriate cortex (V2) as posterior parietal cortex, or PP.
Their injection, however, appears to be in a location similar to
MM in the present study rather than to SC, since it borders the
rostral edge of peristriate cortex (see Figure 8 in Donoghue and
Ebner, 1981b). They report projections from VL/VA, Pol (termed
Po), and weak projections from intralaminar nuclei. Notably, they
do not see any labeled neurons in VP (termed VB) following any
injections into PP. In the present investigation, both SC and MM
receive strong projections from VL/VA and Pol, as was found in
the Virginia opossum, however projections differ in two important ways (Figures 14D–F). First, there are still strong projections
from VP to both SC and MM, although not as strong as was
found for S1. Second, both of these areas (although especially
MM) display consistent connectivity with visual thalamic nuclei
LGNd and the pulvinar. Thus, the general trend of projections
from the thalamus to SC and MM in the short-tailed opossum
is consistent with projections described in PP of Virginia opossum. However, the short-tailed opossum shows projections from
primary sensory relay nuclei from both the visual and somatosensory systems not found in Virginia opossums, further suggesting
that these thalamic primary sensory nuclei display more exuberance in their projections in short-tailed opossums compared to
other species.
This exuberance of the projections of these primary sensory
nuclei in short-tailed opossums compared to other marsupial and
mammalian species builds upon previous work in cortex that
suggests that brain circuits become more segregated as the brain
increases in size (Ringo, 1991). Increased long range connections
have been documented in small-brained rodents (Campi et al.,
2010; Henschke et al., 2014) as well as primates (Palmer and Rosa,
2006). Thus, it is possible that the observed exuberance of thalamocortical connectivity in the short-tailed opossum is due to
the small size of their brain. Whether this thalamocortical exuberance in the small-brained short-tailed opossum is unique to
the species, to small-brained marsupials, or a shared property
across other groups of mammals requires a larger comparative
analysis.
The location and thalamocortical connectivity of Monodelphis
MM and SC (PP in Virginia opossums) suggest that they
are homologous to some portion of posterior parietal cortex
described in eutherian mammals. Posterior parietal cortex is a
region of the mammalian neocortex which has undergone massive expansion in primates, and thus has been of interest to
numerous researchers interested in the organization of the human
brain (see Cooke et al., 2014 for review). Despite this, comparing SC and MM in the present investigation to posterior
parietal areas in other species is challenging, as very little is known
about the functional role of these areas in opossums and other
small-brained mammals such as rodents (see Krubitzer et al.,
2011). There is, however, an area with similar cortical architecture
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January 2015 | Volume 8 | Article 163 | 18
Dooley et al.
Thalamocortical projections to parietal cortex
and connections that has been identified in rats (termed posterior parietal cortex, or PPC; Reep et al., 1994) and squirrels
(termed parietal medial area, or Pm; Slutsky et al., 2000). Among
small-brained animals, PPC in rats has been the most studied, where, numerous behavioral studies have implicated PPC
as part of a larger thalamo-cortical-basal ganglia network that
plays an important role in multimodal spatial attention (Reep and
Corwin, 2009). Thalamic projections to PPC include the lateral
dorsal nucleus and the lateral posterior nucleus (pulvinar), as well
as projections from VL and Po (Giannetti and Molinari, 2002).
Notably, despite close proximity to both S1 and peristriate cortex
in rat, PPC does not receive projections from VP or the LGNd.
Like PPC in rats, MM in Monodelphis has strong projections from
Pol and VL/VA, along with moderate projections with pulvinar
(lateral posterior nucleus in rat).
The finding that VL projects to posterior parietal areas has also
been described in rats, cats, and monkeys, although it has been
suggested that different populations of neurons within VL are
projecting to motor cortex and parietal areas (Divac et al., 1977;
Kasdon and Jacobson, 1978; Hendry et al., 1979; Giannetti and
Molinari, 2002). Considering the finding that posterior parietal
areas receive projections from VL/VA in the present investigation, we provide additional support for the idea that MM in
the short-tailed opossum may be homologous to portions of
posterior parietal cortex in other mammals, in part due to its
connectivity to somatosensory, visual and motor nuclei of the
thalamus. Further, this finding in Monodelphis provides further
evidence that the mammalian ancestor of both marsupials and
placentals possessed a cortical area similar to posterior parietal
cortex, with a role integrating visual, somatosensory and motor
inputs.
AUTHOR CONTRIBUTIONS
CONCLUSION
REFERENCES
In summary, thalamic projections to parietal cortical areas in
the short-tailed opossum are similar not only to other marsupials, but other small-brained eutherian mammals. Thus, these
results provide further evidence for homologies between 3a and
SR as well as PP and MM in the short-tailed opossum, suggesting that these regions were present in early mammals. The
projections to these regions, however, differ in short-tailed opossums in two notable ways. First, unlike other marsupials studied,
there are not extensive projections from VL/VA to S1. This result
highlights the need for additional comparative studies to more
accurately infer the cortical organization of the common mammalian ancestor. Second, in areas such as SR and SC, as well as
MM, there are strong projections from VP, along with direct projections from LGNd to MM. This highlights what appears to be
a more general feature of the brain of the short-tailed opossum:
Increased exuberance of the thalamic projections from primary
sensory relay nuclei to non-primary cortical areas. This finding suggests that our small-brained mammalian ancestors may
have also possessed increased thalamocortical connectivity, similar to the increased cortical connectivity found in many extant
small-brained mammals. However, we underscore the need for
additional comparative studies of mammals in order to determine
whether this feature has independently evolved in small brained
mammals, or is a retained ancestral trait.
Aldes, L. D. (1988). Thalamic connectivity of rat somatic motor cortex. Brain Res.
Bull. 20, 333–348. doi: 10.1016/0361-9230(88)90063-9
Anomal, R. F., Rocha-Rego, V., and Franca, J. G. (2011). Topographic organization
and corticocortical connections of the forepaw representation in areas s1 and sc
of the opossum: evidence for a possible role of area sc in multimodal processing.
Front. Neuroanat. 5:56. doi: 10.3389/fnana.2011.00056
Baldwin, M. K. L., Nguyen, H., Sekizaki, D., and Krubitzer, L. (2013). “Subcortical
connections of the superior colliculus and VGLUT2 staining in short-tailed
opossums (Monodelphis Domestica),” in Poster Presentation - Society for
Neuroscience (SanDiego, CA).
Beck, P. D., Pospichal, M. W., and Kaas, J. H. (1996). Topography, architecture, and connections of somatosensory cortex in opossums: evidence for five
somatosensory areas. J. Comp. Neurol. 366, 109–133.
Campi, K. L., Bales, K. L., Grunewald, R., and Krubitzer, L. (2010). Connections
of auditory and visual cortex in the prairie vole (Microtus ochrogaster): evidence
for multisensory processing in primary sensory areas. Cereb. Cortex 20, 89–108.
doi: 10.1093/cercor/bhp082
Catania, K. C., Jain, N., Franca, J. G., Volchan, E., and Kaas, J. H. (2000). The
organization of somatosensory cortex in the short-tailed opossum (Monodelphis
domestica). Somatosens. Mot. Res. 17, 39–51. doi: 10.1080/08990220070283
Cicirata, F., Angaut, P., Serapide, M. F., Papale, A., and Panto, M. R. (1986). Two
thalamic projection patterns to the motor cortex in the rat. Boll. Soc. Ital. Biol.
Sper. 62, 1381–1387.
Cooke, D. F., Goldring, A., Recanzone, G. H., and Krubitzer, L. (2014). “The evolution of parietal areas associated with visuomanual behavior: from grasping
to tool use,” in The New Visual Neurosciences, eds L. Chalupa and J. Werner
(Cambridge, MA: MIT Press), 1049–1063.
Divac, I., Lavail, J. H., Rakic, P., and Winston, K. R. (1977). Heterogeneous afferents
to the inferior parietal lobule of the rhesus monkey revealed by the retrograde
Frontiers in Neuroanatomy
All authors had full access to the data in the study and take
responsibility for the integrity of the data and the accuracy of the
data analysis. Study concept and design: James C. Dooley, João
G. Franca, Dylan F. Cooke, and Leah A. Krubitzer. Acquisition of
data: James C. Dooley, João G. Franca, Adele M. H. Seelke, Dylan
F. Cooke, and Leah A. Krubitzer. Histological processing of tissue:
James C. Dooley, João G. Franca, Dylan F. Cooke, and Leah A.
Krubitzer. Analysis and interpretation of data: James C. Dooley,
João G. Franca, Adele M. H. Seelke, Dylan F. Cooke, and Leah A.
Krubitzer. Drafting of the article: James C. Dooley and Leah A.
Krubitzer. Critical revision of the article for important intellectual content: James C. Dooley, João G. Franca, Adele M. H. Seelke,
Dylan F. Cooke, and Leah A. Krubitzer. Obtained funding: Leah
A. Krubitzer. Study supervision: Leah A. Krubitzer.
FUNDING
This project was supported by funds to Leah Krubitzer from
NINDS (R21 NS071225) and NEI (R01 EY022987); funds to João
Franca from CNPq—Brazil and FAPERJ—Brazil, and funds to
James Dooley from NEI (T32-EY015387-05).
ACKNOWLEDGMENTS
The authors thank Cindy Clayton, UC Davis School for
Veterinary Medicine, and the rest of the animal care staff at the
UC Davis Psychology Department vivarium. We also thank Becky
Grunewald and for technical assistance; Deepa Ramamurthy and
Michaela Donaldson for surgical assistance; Conor Weatherford
for assistance in XY stage encoding, and Anika K. Colopy for
additional data collection and analysis. Finally, the authors thank
Mary Baldwin for histological assistance and for helpful comments on the manuscript.
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January 2015 | Volume 8 | Article 163 | 19
Dooley et al.
Thalamocortical projections to parietal cortex
transport method. Brain Res. 123, 197–207. doi: 10.1016/0006-8993(77)
90474-7
Donoghue, J. P., and Ebner, F. F. (1981a). The laminar distribution and ultrastructure of fibers projecting from three thalamic nuclei to the somatic
sensory-motor cortex of the opossum. J. Comp. Neurol. 198, 389–420. doi:
10.1002/cne.901980303
Donoghue, J. P., and Ebner, F. F. (1981b). The organization of thalamic projections
to the parietal cortex of the Virginia opossum. J. Comp. Neurol. 198, 365–388.
doi: 10.1002/cne.901980302
Donoghue, J. P., Kerman, K. L., and Ebner, F. F. (1979). Evidence for two organizational plans within the somatic sensory-motor cortex of the rat. J. Comp. Neurol.
183, 647–663. doi: 10.1002/cne.901830312
Donoghue, J. P., and Wise, S. P. (1982). The motor cortex of the rat: cytoarchitecture and microstimulation mapping. J. Comp. Neurol. 212, 76–88. doi:
10.1002/cne.902120106
Dooley, J. C., Franca, J. G., Seelke, A. M., Cooke, D. F., and Krubitzer, L. A. (2013).
A connection to the past: Monodelphis domestica provides insight into the organization and connectivity of the brains of early mammals. J. Comp. Neurol. 521,
3877–3897. doi: 10.1002/cne.23383
Elston, G. N., and Manger, P. R. (1999). The organization and connections
of somatosensory cortex in the brush-tailed possum (Trichosurus vulpecula):
evidence for multiple, topographically organized and interconnected representations in an Australian marsupial. Somatosens. Mot. Res. 16, 312–337. doi:
10.1080/08990229970384
Foster, R. E., Donoghue, J. P., and Ebner, F. F. (1981). Laminar organization of
efferent cells in the parietal cortex of the Virginia opossum. Exp. Brain Res. 43,
330–336.
Frost, S. B., Milliken, G. W., Plautz, E. J., Masterton, R. B., and Nudo, R. J.
(2000). Somatosensory and motor representations in cerebral cortex of a primitive mammal (Monodelphis domestica): a window into the early evolution of
sensorimotor cortex. J. Comp. Neurol. 421, 29–51. doi: 10.1002/(SICI)10969861(20000522)421:1<29::AID-CNE3>3.0.CO;2-9
Gallyas, F. (1979). Silver staining of myelin by means of physical development.
Neurol. Res. 1, 203–209.
Giannetti, S., and Molinari, M. (2002). Cerebellar input to the posterior parietal cortex in the rat. Brain Res. Bull. 58, 481–489. doi: 10.1016/S03619230(02)00815-8
Gould, H. J. III., Whitworth, R. H. Jr., and Ledoux, M. S. (1989). Thalamic
and extrathalamic connections of the dysgranular unresponsive zone in
the grey squirrel (Sciurus carolinensis). J Comp. Neurol. 287, 38–63. doi:
10.1002/cne.902870105
Grant, R. A., Haidarliu, S., Kennerley, N. J., and Prescott, T. J. (2013). The evolution
of active vibrissal sensing in mammals: evidence from vibrissal musculature and
function in the marsupial opossum Monodelphis domestica. J. Exp. Biol. 216,
3483–3494. doi: 10.1242/jeb.087452
Haight, J. R., and Neylon, L. (1978a). An atlas of the dorsal thalamus of the
marsupial brush-tailed possum, Trichosurus vulpecula. J. Anat. 126, 225–245.
Haight, J. R., and Neylon, L. (1978b). The organization of neocortical projections from the ventroposterior thalamic complex in the marsupial brush-tailed
possum, Trichosurus vulpecula: a horseradish peroxidase study. J. Anat. 126,
459–485.
Haight, J. R., and Neylon, L. (1981a). An analysis of some thalamic projections
to parietofrontal neocortex in the marsupial native cat, Dasyurus viverrinus
(Dasyuridae). Brain Behav. Evol. 19, 193–204. doi: 10.1159/000121642
Haight, J. R., and Neylon, L. (1981b). A description of the dorsal thalamus of the
marsupial native cat, Dasyurus viverrinus (Dasyuridae). Brain Behav. Evol. 19,
155–179. doi: 10.1159/000121640
Hatsopoulos, N. G., and Suminski, A. J. (2011). Sensing with the motor cortex.
Neuron 72, 477–487. doi: 10.1016/j.neuron.2011.10.020
Hendry, S. H., Jones, E. G., and Graham, J. (1979). Thalamic relay nuclei for cerebellar and certain related fiber systems in the cat. J. Comp. Neurol. 185, 679–713.
doi: 10.1002/cne.901850406
Henry, E. C., and Catania, K. C. (2006). Cortical, callosal, and thalamic connections from primary somatosensory cortex in the naked mole-rat (Heterocephalus
glaber), with special emphasis on the connectivity of the incisor representation. Anat. Rec. A Discov. Mol. Cell. Evol. Biol. 288, 626–645. doi: 10.1002/ar.
a.20328
Henschke, J. U., Noesselt, T., Scheich, H., and Budinger, E. (2014). Possible anatomical pathways for short-latency multisensory integration processes in primary
Frontiers in Neuroanatomy
sensory cortices. Brain Struct. Funct. doi: 10.1007/s00429-013-0694-4. [Epub
ahead of print].
Herkenham, M. (1979). The afferent and efferent connections of the ventromedial thalamic nucleus in the rat. J. Comp. Neurol. 183, 487–517. doi:
10.1002/cne.901830304
Hoogland, P. V., Welker, E., and Van Der Loos, H. (1987). Organization of the projections from barrel cortex to thalamus in mice studied with Phaseolus vulgarisleucoagglutinin and HRP. Exp. Brain Res. 68, 73–87. doi: 10.1007/BF00255235
Huffman, K. J., Molnar, Z., Van Dellen, A., Kahn, D. M., Blakemore, C., and
Krubitzer, L. (1999a). Formation of cortical fields on a reduced cortical sheet.
J. Neurosci. 19, 9939–9952.
Huffman, K. J., Nelson, J., Clarey, J., and Krubitzer, L. (1999b). Organization
of somatosensory cortex in three species of marsupials, Dasyurus hallucatus, Dactylopsila trivirgata, and Monodelphis domestica: neural correlates of
morphological specializations. J. Comp. Neurol. 403, 5–32.
Jones, E. G. (2007). The Thalamus, 2nd Edn. Cambridge, UK. Cambridge University
Press.
Kaas, J. H. (2004). Evolution of somatosensory and motor cortex in primates. Anat.
Rec. A Discov. Mol. Cell. Evol. Biol. 281, 1148–1156. doi: 10.1002/ar.a.20120
Kaas, J. H. (2011). Reconstructing the areal organization of the neocortex of the
first mammals. Brain Behav. Evol. 78, 7–21. doi: 10.1159/000327316
Kaas, J. H., Stepniewska, I., and Gharbawie, O. (2012). Cortical networks subserving upper limb movements in primates. Eur. J. Phys. Rehabil. Med. 48, 299–306.
Available online at: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3695617/
Kahn, D. M., Huffman, K. J., and Krubitzer, L. (2000a). Organization and connections of V1 in Monodelphis domestica. J. Comp. Neurol. 428, 337–354. doi:
10.1002/1096-9861(20001211)428:2<337::AID-CNE11>3.0.CO;2-2
Kahn, D. M., Huffman, K. J., and Krubitzer, L. (2000b). Organization and connections of V1 in Monodelphis domestica. J. Comp. Neurol. 428, 337–354. doi:
10.1002/1096-9861(20001211)428:2<337::AID-CNE11>3.0.CO;2-2
Kahn, D. M., and Krubitzer, L. (2002). Massive cross-modal cortical plasticity and
the emergence of a new cortical area in developmentally blind mammals. Proc.
Natl. Acad. Sci. U.S.A. 99, 11429–11434. doi: 10.1073/pnas.162342799
Karlen, S. J., Kahn, D. M., and Krubitzer, L. (2006). Early blindness results in
abnormal corticocortical and thalamocortical connections. Neuroscience 142,
843–858. doi: 10.1016/j.neuroscience.2006.06.055
Karlen, S. J., and Krubitzer, L. (2007). The functional and anatomical organization
of marsupial neocortex: evidence for parallel evolution across mammals. Prog.
Neurobiol. 82, 122–141. doi: 10.1016/j.pneurobio.2007.03.003
Karlen, S. J., and Krubitzer, L. (2009). Effects of bilateral enucleation on the size
of visual and nonvisual areas of the brain. Cereb. Cortex 19, 1360–1371. doi:
10.1093/cercor/bhn176
Kasdon, D. L., and Jacobson, S. (1978). The thalamic afferents to the inferior
parietal lobule of the rhesus monkey. J. Comp. Neurol. 177, 685–706. doi:
10.1002/cne.901770409
Kemp, T. S. (2005). The Origin and Evolution of Mammals. New York, NY: Oxford
University Press.
Killackey, H., and Ebner, F. (1973). Convergent projection of three separate thalamic nuclei on to a single cortical area. Science 179, 283–285. doi: 10.1126/science.179.4070.283
Koralek, K. A., Jensen, K. F., and Killackey, H. P. (1988). Evidence for two complementary patterns of thalamic input to the rat somatosensory cortex. Brain Res.
463, 346–351. doi: 10.1016/0006-8993(88)90408-8
Krubitzer, L. (2007). The magnificent compromise: cortical field evolution in
mammals. Neuron 56, 201–208. doi: 10.1016/j.neuron.2007.10.002
Krubitzer, L. A., and Kaas, J. H. (1987). Thalamic connections of three representations of the body surface in somatosensory cortex of gray squirrels. J. Comp.
Neurol. 265, 549–580. doi: 10.1002/cne.902650408
Krubitzer, L., Campi, K. L., and Cooke, D. F. (2011). All rodents are not the same:
a modern synthesis of cortical organization. Brain Behav. Evol. 78, 51–93. doi:
10.1159/000327320
Krubitzer, L., and Dooley, J. C. (2013). Cortical plasticity within and across lifetimes: how can development inform us about phenotypic transformations?
Front. Hum. Neurosci. 7:620. doi: 10.3389/fnhum.2013.00620
Krubitzer, L., Manger, P., Pettigrew, J., and Calford, M. (1995). Organization
of somatosensory cortex in monotremes: in search of the prototypical plan.
J. Comp. Neurol. 351, 261–306. doi: 10.1002/cne.903510206
Kuramoto, E., Ohno, S., Furuta, T., Unzai, T., Tanaka, Y. R., Hioki, H., et al.
(2013). Ventral medial nucleus neurons send thalamocortical afferents more
www.frontiersin.org
January 2015 | Volume 8 | Article 163 | 20
Dooley et al.
Thalamocortical projections to parietal cortex
widely and more preferentially to layer 1 than neurons of the ventral anteriorventral lateral nuclear complex in the rat. Cereb. Cortex. 25, 221–235. doi:
10.1093/cercor/bht216
Lende, R. A. (1963). Cerebral cortex: a sensorimotor amalgam in the marsupiala.
Science 141, 730–732. doi: 10.1126/science.141.3582.730
Liao, C. C., Chen, R. F., Lai, W. S., Lin, R. C., and Yen, C. T. (2010). Distribution of
large terminal inputs from the primary and secondary somatosensory cortices
to the dorsal thalamus in the rodent. J. Comp. Neurol. 518, 2592–2611.
Magalhaes-Castro, B., and Saraiva, P. E. (1971). Sensory and motor representation
in the cerebral cortex of the marsupial Didelphis azarae. Brain Res. 34, 291–299.
doi: 10.1016/0006-8993(71)90282-4
Mikula, S., Manger, P. R., and Jones, E. G. (2008). The thalamus of the monotremes:
cyto- and myeloarchitecture and chemical neuroanatomy. Philos. Trans. R. Soc.
Lond. B Biol. Sci. 363, 2415–2440. doi: 10.1098/rstb.2007.2133
Mitchinson, B., Grant, R. A., Arkley, K., Rankov, V., Perkon, I., and Prescott, T. J.
(2011). Active vibrissal sensing in rodents and marsupials. Philos. Trans. R. Soc.
Lond. B Biol. Sci. 366, 3037–3048. doi: 10.1098/rstb.2011.0156
Nilsson, M. A., Churakov, G., Sommer, M., Tran, N. V., Zemann, A., Brosius, J.,
et al. (2010). Tracking marsupial evolution using archaic genomic retroposon
insertions. PLoS Biol. 8:e1000436. doi: 10.1371/journal.pbio.1000436
Ohno, S., Kuramoto, E., Furuta, T., Hioki, H., Tanaka, Y. R., Fujiyama, F., et al.
(2012). A morphological analysis of thalamocortical axon fibers of rat posterior
thalamic nuclei: a single neuron tracing study with viral vectors. Cereb. Cortex
22, 2840–2857. doi: 10.1093/cercor/bhr356
Olkowicz, S., Turlejski, K., Bartkowska, K., Wielkopolska, E., and Djavadian, R.
L. (2008). Thalamic nuclei in the opossum Monodelphis domestica. J. Chem.
Neuroanat. 36, 85–97. doi: 10.1016/j.jchemneu.2008.05.003
Oswaldo-Cruz, E., and Rocha-Miranda, C. E. (1968). The Brain of the Opossum
(Didelphis Marsupialis). A Cytoarchitectonic Atlas in Stereotaxic Coordinates. Rio
de Janeiro, Brasil. Instituto de Biofísica, Universidade Federal do Rio de Janeiro.
Palmer, S. M., and Rosa, M. G. P. (2006). Quantitative analysis of the corticocortical projections to the middle temporal area in the marmoset monkey:
evolutionary and functional implications. Cereb. Cortex 16, 1361–1375. doi:
10.1093/cercor/bhj078
Reep, R. L., Chandler, H. C., King, V., and Corwin, J. V. (1994). Rat posterior parietal cortex: topography of corticocortical and thalamic connections. Exp. Brain
Res. 100, 67–84. doi: 10.1007/BF00227280
Reep, R. L., and Corwin, J. V. (2009). Posterior parietal cortex as part of a neural
network for directed attention in rats. Neurobiol. Learn. Mem. 91, 104–113. doi:
10.1016/j.nlm.2008.08.010
Ringo, J. L. (1991). Neuronal interconnection as a function of brain size. Brain
Behav. Evol. 38, 1–6. doi: 10.1159/000114375
Rowe, T. B., Macrini, T. E., and Luo, Z. X. (2011). Fossil evidence on origin of the
mammalian brain. Science 332, 955–957. doi: 10.1126/science.1203117
Seelke, A. M., Dooley, J. C., and Krubitzer, L. A. (2012). The emergence of
somatotopic maps of the body in S1 in rats: the correspondence between functional and anatomical organization. PLoS ONE 7:e32322. doi: 10.1371/journal.pone.0032322
Slutsky, D. A., Manger, P. R., and Krubitzer, L. (2000). Multiple somatosensory areas in the anterior parietal cortex of the California ground
Frontiers in Neuroanatomy
squirrel (Spermophilus beecheyii). J. Comp. Neurol. 416, 521–539. doi:
10.1002/(SICI)1096-9861(20000124)416:4<521::AID-CNE8>3.0.CO;2-#
Sousa, A. P., Oswaldo-Cruz, E., and Gattass, R. (1971). Somatotopic organization and response properties of neurons of the ventrobasal complex
in the opossum. J. Comp. Neurol. 142, 231–247. doi: 10.1002/cne.901
420208
Turlejski, K., Djavadian, R. L., and Saunders, N. R. (1994). Projection of visuotopically organized afferents to the dorsal thalamus in the opossum, Monodelphis
domestica. Acta Neurobiol. Exp. (Wars) 54, 307–319.
Ulinski, P. S. (1984). Thalamic projections to the somatosensory cortex of
the echidna, Tachyglossus aculeatus. J. Comp. Neurol. 229, 153–170. doi:
10.1002/cne.902290203
Van Rheede, T., Bastiaans, T., Boone, D. N., Hedges, S. B., De Jong, W. W., and
Madsen, O. (2006). The platypus is in its place: nuclear genes and indels confirm the sister group relation of monotremes and Therians. Mol. Biol. Evol. 23,
587–597. doi: 10.1093/molbev/msj064
Viaene, A. N., Petrof, I., and Sherman, S. M. (2011). Properties of the thalamic projection from the posterior medial nucleus to primary and secondary somatosensory cortices in the mouse. Proc. Natl. Acad. Sci. U.S.A. 108, 18156–18161. doi:
10.1073/pnas.1114828108
Welker, W., and Lende, R. A. (1980). “Thalamocortical relationships in echidna
(Tachyglossus aculeatus),” in Comparative Neurology of the Telencephalon ed S.
O. E. Ebbesson (New York, NY: Plenum Press), 449–481.
Wise, S. P., and Jones, E. G. (1977). Cells of origin and terminal distribution of
descending projections of the rat somatic sensory cortex. J. Comp. Neurol. 175,
129–157. doi: 10.1002/cne.901750202
Wong, P., and Kaas, J. H. (2009). An architectonic study of the neocortex of the
short-tailed opossum (Monodelphis domestica). Brain Behav. Evol. 73, 206–228.
doi: 10.1159/000225381
Wong-Riley, M. (1979). Changes in the visual system of monocularly sutured or
enucleated cats demonstrable with cytochrome oxidase histochemistry. Brain
Res. 171, 11–28. doi: 10.1016/0006-8993(79)90728-5
Conflict of Interest Statement: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be
construed as a potential conflict of interest.
Received: 03 November 2014; accepted: 15 December 2014; published online: 07
January 2015.
Citation: Dooley JC, Franca JG, Seelke AMH, Cooke DF and Krubitzer LA (2015)
Evolution of mammalian sensorimotor cortex: thalamic projections to parietal cortical areas in Monodelphis domestica. Front. Neuroanat. 8:163. doi: 10.3389/fnana.
2014.00163
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