2008 - Huppertz - The Anatomy of The Normal Placenta
2008 - Huppertz - The Anatomy of The Normal Placenta
2008 - Huppertz - The Anatomy of The Normal Placenta
com
Review
ABSTRACT
The placenta is the fetal organ providing the interchange
between mother and fetus. This organ needs to provide
its function such as transport and secretion even during
its development and thus all developmental changes need
to be in accordance with its function. This review
describes development of the placenta during the first
few weeks of pregnancy until the villous trees with their
vasculature are established. The macroscopic anatomy of
the delivered placenta as well as the microscopic
anatomy and histology of this organ are also described.
This includes the different types of villi and the most
important cellular components of the villi such as villous
trophoblast, Hofbauer cells, mesenchymal cells and
endothelium. Fibrinoid and its localisation is also
described.
A brief introduction to the development of the
human placenta is given, followed by a description
of the structural characteristics of a delivered term
placenta.
Prelacunar stage
The localisation of the inner cell mass defines the
part of the trophoblast cover that makes the final
attachment of the blastocyst to the uterine
epithelium (fig 1). Only those trophoblasts overlying the inner cell mass (referred to as polar
trophoblast) seem to be able to finally lead to
implantation.2 Rotation of the blastocyst at that
stage may even lead to failure of pregnancy due to
reduced contact of the polar trophoblast to the
uterine epithelium (fig 1). It has been described
that varying the orientation of the blastocyst at
the time of attachment and implantation results in
abnormalities of umbilical cord insertion into the
1296
Lacunar stage
Eight days after conception, fluid-filled spaces
occur within the syncytiotrophoblast and soon
coalesce to form larger lacunae. The remaining
syncytiotrophoblastic masses between the lacunae
are termed trabeculae and are of great importance
for the further development of the villous trees of
the placenta. As soon as the lacunae have developed, the three fundamental zones of the placenta
can be defined: the early chorionic plate facing the
embryo; the lacunar system together with the
trabeculae developing into the intervillous space
and the villous trees; and the primitive basal plate
in contact with the maternal endometrium.
During implantation the invading syncytiotrophoblast penetrates into the interstitium of the
endometrium and comes into contact with maternal capillaries and the superficial venous plexus of
the endometrium. Erosion of these small vessels
leads to the presence of first maternal blood cells
within the lacunae of the syncytiotrophoblast.6 7
The appearance of first maternal blood cells in
these lacunae has been equated with the onset of
the maternal circulation in the placenta. However,
as was pointed out more than 50 years ago, at that
J Clin Pathol 2008;61:12961302. doi:10.1136/jcp.2008.055277
Review
From the primitive basal plate they (now termed interstitial
(extravillous) trophoblast) further invade the endometrial
stroma between glands and capillaries. A subset of these cells
(endovascular trophoblast) reaches and invades the walls of
spiral arteries from the interstitium, finally entering the lumen
of these vessels.11 12 This physiological transformation of spiral
arteries involves the destruction of the arterial muscular wall
and the replacement of the endothelium by trophoblast.12
Villous stage
Review
chorionic veins are direct continuations of the veins of the
villous trees and usually cross the chorionic arteries underneath.
The chorionic veins give rise to the single umbilical vein.
Mesenchymal villi
Mesenchymal villi (100250 mm in diameter) are the forerunners of the intermediate villi and can be found predominantly in
the earliest stages of pregnancy.22 23 Up to six weeks postmenstruation, mesenchymal villi are the only villous type
present in the developing placenta. Their stromal core is only
weakly organised and contains a large number of mesenchymal
cells and developing vessels, the latter sometimes still lacking a
vessel lumen. Mesenchymal villi persist until delivery, but due
to their ongoing differentiation into intermediate villi, their
number becomes extremely low after the first trimester of
pregnancy.
Stem villi
Stem villi derive from differentiation of immature intermediate
villi and are the villi with the largest diameter (1003000 mm in
diameter). They serve to give mechanical support to the villous
tree.2 20 Their villous core is characterised by centrally located
arteries and veins in a dense fibrous stroma.24 25 Capillaries are
rare, and thus it is speculated that this villous type plays only a
small part in materno-fetal exchange. Rather, their physiological
significancebesides their role to mechanically stabilise the
villous treeslies in the fact that their vascular system is
surrounded by a perivascular contractile sheath.26 27 Vessel
media and sheath act together as a functional myofibroelastic
unit, which contributes to support tensile and/or contracting
forces within the stem villus blood vessels.
Starting at about mid-gestation, long slender mature intermediate villi (80120 mm in diameter) differentiate from
mesenchymal villi that emerge from stem villi.21 The gently
curving mature intermediate villi give rise to terminal villi at
intervals. Their villous core consists of a loose stroma with a
few small peripheral vessels and capillaries. All vessels present in
a villous cross section occupy maximally half the villous cross
sectional area.
Terminal villi
Terminal villi are the final branches of the villous trees. They
have a length of up to 100 mm and a diameter of about 80 mm,
and originate from mature intermediate villi.2 20 One of their
J Clin Pathol 2008;61:12961302. doi:10.1136/jcp.2008.055277
Review
Figure 3 Types of placental villi.
(A) Mesenchymal villi are rich in
mesenchymal cells and show syncytial
sprouting. (B) Immature intermediate villi
are characterised by stromal channels
containing fetal macrophages, Hofbauer
cells. (C) Stem villi are the largest villi and
show a perivascular contractile system
around their central vessels. (D) Mature
intermediate villi only contain smaller
vessels and capillaries in a loose stroma.
(E) Terminal villi possess sinusoids and
capillaries with a thin vasculo-syncytial
membrane. Light grey structure encircling
the villi, syncytiotrophoblast; dark grey
cells next to the syncytiotrophoblast,
cytotrophoblast; white centre of villi,
villous stroma; grey and dark grey cells in
stroma stromal cells; light grey cells in
stromal channels (B), macrophages; dark
grey circles and ovals with cells,
placental blood vessels with endothelial
cells; grey circles in centre of stroma (C),
central vessels (artery and vein) of stem
villi.
Villous cytotrophoblast
The mononucleated villous cytotrophoblasts (Langhans cells)
always stay in direct contact with the basement membrane by
J Clin Pathol 2008;61:12961302. doi:10.1136/jcp.2008.055277
Review
proliferation of cytotrophoblast stem cells throughout pregnancy, the pool of cytotrophoblasts increases and is able to
maintain the second layer, the syncytiotrophoblast.33 The
reason for the seeming reduction in the number of cytotrophoblasts is the rapid expansion of the villous surface leading to a
separation of the single cytotrophoblasts.
Undifferentiated cytotrophoblasts display a cuboidal shape
with a cytoplasm that contains only few organelles.34 35
Differentiation after leaving the cell cycle results in the
formation of intermediate cells, which display a morphological
appearance between the undifferentiated state and the syncytiotrophoblast.34 35 The cytoplasm of these intermediate cells
contains large numbers of mitochondria and free ribosomes
together with high amounts of rough endoplasmic reticulum
and mRNA.36 37 These highly active cells display activities of
enzymes involved in aerobic and anaerobic glycolysis.38 39 The
activity of these cells has also been demonstrated by the
incorporation of high amounts of 3H-uridine in vitro.40 41
The highly differentiated cytotrophoblasts display a concentration of organelles, proteins and mRNA that is much higher
than that of the overlying syncytiotrophoblast.38 These cells will
soon fuse with the syncytiotrophoblast and will become an
integral part of this syncytial layer, incorporating all the
organelles, proteins and nucleic acids into this layer.
1300
Review
fibroblasts secrete typical matrix proteins such as collagen types
I and III, as well as proteoglycans such as hyaluronic acid.52
Fetal vessels
A non-fenestrated endothelium lines the placental vasculature
throughout gestation with junctional complexes to link
neighbouring cells and to reduce paracellular transport. Larger
molecules cross the endothelial cells by means of vesicular
transport.54
Capillaries and sinusoids within terminal villi are surrounded
by a basement membrane without any further supporting cells
such as pericytes.14 Arteries and arterioles within stem and
intermediate villi possess a media with smooth muscle cells but
missing elastic laminae. The luminal diameter of placental
vessels has to be controlled by paracrine and autocrine factors
because there is no neural innervation in the placenta.55 The
endothelium of venules and veins has recently been shown to be
composed of a more immature endothelial cell phenotype
compared to placental arteries.56
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Fibrinoid
Fibrinoid is a homogeneous material that preferably binds acid
stains and can be found in paraffin sections of placentas of all
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doi: 10.1136/jcp.2008.055277
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References
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Notes