Development of Head and Neck: Pharyngeal Arches
Development of Head and Neck: Pharyngeal Arches
Development of Head and Neck: Pharyngeal Arches
Pharyngeal arches
Pharyngeal pouches
Pharyngeal grooves
Pharyngeal membranes
PHARYNGEAL ARCHES
These arches begin to develop early in the fourth week as neural crest
cells migrate into the future head and neck regions. Initially, each pharyngeal
arch consists of a core of mesenchyme (embryonic connective tissue) and is
covered externally by ectoderm and internally by endoderm. The first pair of
arches, the primordium of the jaws, appears as surface elevations lateral to the
developing pharynx. Other arches soon appear as obliquely disposed, rounded
ridges on each side of the future head and neck regions. By the end of the fourth
week, four pairs of arches are visible externally. The fifth and sixth arches are
rudimentary and are not visible on the surface of the embryo. The arches are
separated from each other by the pharyngeal grooves (clefts). Like the arches,
the grooves are numbered in a craniocaudal sequence. The arches support the
lateral walls of the primordial pharynx, which is derived from the cranial part
of the foregut. The stomodeum (primordial mouth) initially appears as a slight
depression of the surface ectoderm. It is separated from the cavity of the
primordial pharynx by a bilaminar membranethe oropharyngeal membrane
composed of fused ectoderm and endoderm. The oropharyngeal membrane
ruptures at approximately 26 days, bringing the primordial pharynx and foregut
into communication with the amniotic cavity. The arches contribute extensively
to the formation of the face, nasal cavities, mouth, larynx, pharynx, and neck.
The first arch develops two prominences: the smaller maxillary
prominence and the larger mandibular prominence.
The second arch (hyoid) makes a major contribution to the formation of
the hyoid bone.
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A cartilaginous rod that forms the skeleton of the arch.
A muscular component that is the primordium of the muscles in the head and
neck.
A nerve that supplies the mucosa and muscles derived from each arch.
The transformation of the arch arteries into the adult arterial pattern of
the head and neck is described in the section on the pharyngeal arch artery.
The dorsal end of the first arch cartilage becomes ossified to form two
middle ear bones, the malleus and incus. The middle section of the cartilage
regresses, but its perichondrium forms the anterior ligament of the malleus and
sphenomandibular ligament. Ventral parts of the first arch cartilage form the
horseshoe-shaped primordium of the mandible. Each half of the mandible forms
lateral to and in close association with its cartilage. The cartilage disappears
as the mandible develops around it by intramembranous ossification.
The dorsal end of the second arch cartilage contributes to the stapes of
the middle ear and the styloid process of the temporal bone. The part of the
cartilage between the styloid process and the hyoid bone regresses; its
perichondrium forms the stylohyoid ligament. The ventral end of the second
arch cartilage ossifies to form the lesser cornu of the hyoid bone.
The third arch cartilage ossifies to form the greater cornu of the hyoid
bone.
The fourth and sixth arch cartilages fuse to form the laryngeal cartilages,
except for the epiglottis. The epiglottic and thyroid cartilages appear to develop
from neural crest cells. The cricoid cartilage develops from mesoderm.
The muscular components of the arches form various muscles in the head
and neck; for example, the musculature of the first arch forms the muscles of
mastication and others.
Each arch is supplied by its own cranial nerve (CN). The special visceral
efferent (branchial) components of the cranial nerves supply muscles derived
from the pharyngeal arches. Because the mesenchyme from the pharyngeal
arches contributes to the dermis and mucous membranes of the head and neck,
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these areas are supplied with the special visceral afferent nerves. The facial
skin is supplied by the fifth cranial nerve (CN V, or trigeminal nerve); however,
only the caudal two branches (maxillary and mandibular) supply derivatives of
the first pharyngeal arch. CN V is the principal sensory nerve of the head and
neck and is the motor nerve for the muscles of mastication. Its sensory branches
innervate the face, teeth, and mucous membranes of the nasal cavities, palate,
mouth, and tongue (see Fig. 10-6C). The seventh cranial nerve (CN VII, or
facial nerve), the ninth cranial nerve (CN IX, or glossopharyngeal nerve), and
the 10th cranial nerve (CN X, or vagus nerve) supply the second, third, and
caudal (fourth to sixth) arches, respectively. The superior laryngeal branch of
the vagus nerve supplies the fourth arch, whereas its recurrent laryngeal branch
supplies the sixth arch. The nerves of the second to sixth pharyngeal arches
innervate the mucous membranes of the tongue, pharynx, and larynx.
PHARYNGEAL POUCHES
The first pouch gives rise to the tubotympanic recess. The first
pharyngeal membrane contributes to the formation of the tympanic membrane
(eardrum). The cavity of the tubotympanic recess gives rise to the tympanic
cavity and mastoid antrum. The connection of the tubotympanic recess with the
pharynx forms the pharyngotympanic tube (auditory tube).
The second pouch is largely obliterated as the palatine tonsil develops. A
part of this pouch remains as the tonsillar sinus (fossa). The endoderm of the
second pouch proliferates and grows into the underlying mesenchyme. The
central parts of these buds break down, forming tonsillar crypts (pit-like
depressions). The pouch endoderm forms the surface epithelium and the lining
of the crypts. Lymphoid infiltration occurs approximately in the seventh month,
while germinal centers are not apparent until the neonatal period.
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The third pouch expands and develops a solid, bulbar,\ dorsal part and a
hollow, elongate ventral part. The connection between the pouch and pharynx is
reduced to a narrow duct that soon degenerates. By the sixth week of
development, the epithelium of each bulbar dorsal part begins to differentiate
into an inferior parathyroid gland. The epithelium of the elongated ventral
parts of the third pair of pouches proliferates, obliterating their cavities. These
parts come together in the median plane to form the thymus. The primordia of
the thymus and parathyroid glands lose their connections with the pharynx.
Later, the inferior parathyroid glands separate from the thymus and lie on the
dorsal surface of the thyroid gland, whereas the thymus descends into the
superior mediastinum. The mesenchyme surrounding the thymic primordium is
derived from neural crest cells.
The dorsal part of each fourth pouch develops into a superior
parathyroid gland, which lies on the dorsal surface of the thyroid gland. The
parathyroid glands derived from the third pouches descend with the thymus and
are carried to a more inferior position than the parathyroid glands that are
derived from the fourth pouches. The elongated ventral part of each fourth
pouch develops into the ultimopharyngeal body, which fuses with the thyroid
gland, giving rise to the parafollicular cells (C cells) of the thyroid gland. These
cells produce calcitonin, a hormone involved in the regulation of calcium. C
cells differentiate from neural crest cells that migrate from the pharyngeal
arches into the fourth pair of pharyngeal pouches.
If the fifth pharyngeal pouch develops, it is rudimentary and becomes
part of the fourth pharyngeal pouch.
PHARYNGEAL GROOVES
The head and neck regions of the embryo exhibit four grooves (clefts) on
each side during the fourth and fifth weeks. These grooves separate the
pharyngeal arches externally. Only one pair of grooves contributes to
structures; the first pair persists as the external acoustic meatus (ear canal).
The other grooves lie in a slit-like depressionthe cervical sinus and are
usually obliterated with it as the neck develops. Birth defects of the second
pharyngeal groove are the most common of such defects.
PHARYNGEAL MEMBRANES
These membranes form where the epithelia of the grooves and pouches
approach each other. The membranes appear in the floors of the grooves during
the fourth week. Only one pair of membranes contributes to the formation of
adult structures; the first membrane becomes the tympanic membrane.
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ARCH Arch Artery Skeletal Elements LIGAMENTS Muscles Cranial Nerve
First Terminal branch of Malleus Anterior ligament of Muscles of mastication Trigeminal (CN V)
(mandibular) maxillary artery Incus malleus Mylohyoid and
Sphenomandibular anterior belly of
ligament digastric
Tensor tympani
Tensor veli palatini
Second Stapedial artery Stapes (portion) Stylohyoid ligament Muscles of facial Facial (CN VII)
(hyoid) (embryonic), Styloid process expression
corticotympanic artery Lesser cornu of hyoid bone Stapedius
(adult) Stylohyoid
Posterior belly of
digastric
Third Common carotid artery, Greater cornu of hyoid bone Stylopharyngeus Glossopharyngeal
root of internal carotid (CN IX)
Fourth Arch of aorta (left side), Laryngeal cartilages (derived Constrictors of Superior laryngeal
right subclavian artery from the fourth-arch pharynx, cricothyroid, branch of vagus
(right side); original cartilage; originate from levator nerve (X)
sprouts of pulmonary neural veli palatini (originate
Arteries crest cells) from occipital somites
2 to 4)
Sixth Ductus arteriosus; roots Laryngeal cartilages (derived Intrinsic muscles of Recurrent laryngeal
of definitive pulmonary from the sixth-arch cartilage; larynx (originate from branch of vagus
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arteries originate from neural occipital somites 1 and nerve (X)
crest cells) 2)
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DEVELOPMENT OF FACE
The facial primordia appear around the stomodeum early in the fourth
week. Facial development depends on the inductive influence of three
organizing areas:
Forebrain (which establishes a gradient of SHH factor)
Frontonasal ectoderm
Developing eye
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separated from the maxillary prominence by a cleft called the nasolacrimal
groove.
By the end of the fifth week, six auricular hillocks primordia of the
auricles (mesenchymal swellings) form around the first pharyngeal groove
(three on each side), the primordium of the external acoustic meatus (ear
canal). Initially, the external ears are positioned in the neck region; however, as
the mandible develops, they ascend to the side of the head at the level of the
eyes.
By the end of the sixth week, each maxillary prominence has begun to
merge with the lateral nasal prominence along the line of the nasolacrimal
groove. This establishes continuity between the side of the nose, formed by the
lateral nasal prominence, and the cheek region, formed by the maxillary
prominence. The nasolacrimal duct develops from a rod-like thickening of
ectoderm in the floor of the nasolacrimal groove. This thickening gives rise to a
solid epithelial cord that separates from the ectoderm and sinks into the
mesenchyme.
Later, as a result of apoptosis (programmed cell death), this cord
canalizes to form the nasolacrimal duct. The cranial end of this duct expands to
form the lacrimal sac. In the late fetal period, the nasolacrimal duct drains into
the inferior meatus in the lateral wall of the nasal cavity. The duct usually
becomes completely patent (open) after birth.
Between weeks 7 and 10, the medial nasal prominences merge with each
other and with the maxillary and lateral nasal prominences, resulting in
disintegration of their contacting surface epithelia. This causes intermingling of
the underlying mesenchyme.
Merging of the medial nasal and maxillary prominences results in
continuity of the upper jaw and lip and separation of the nasal pits from the
stomodeum. As the medial nasal prominences merge, they form an
intermaxillary segment. The segment gives rise to the:
Median part (philtrum) of the upper lip
Premaxillary part of the maxilla and its associated gingiva (gum)
Primary palate
The lateral parts of the upper lip, most of the maxilla, and the secondary
palate form from the maxillary prominences. These prominences merge laterally
with the mandibular prominences. Recent studies indicate that the lower part of
the medial nasal prominences appears to have become deeply positioned and
covered by medial extensions of the maxillary prominences to form the
philtrum. The primordial lips and cheeks are invaded by myoblasts from the
second pair of pharyngeal arches, which differentiate into the facial muscles.
The myoblasts from the first pair of arches differentiate into the muscles of
mastication. The smallness of the face prenatally results from the following:
Rudimentary upper and lower jaws
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No erupted deciduous teeth
Small size of nasal cavities and maxillary sinuses
Paranasal Sinuses
DEVELOPMENT OF PALATE
The palate develops from two primordia: the primary palate and
secondary palate. Palatogenesis (a regulated morphogenetic process) begins in
the sixth week but is not completed until the 12th week. Multiple molecular
pathways, including Wnt and PRICKLE1, are involved.
The critical period of pathogenesis is from the end of the sixth week until
the beginning of the ninth week.
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Primary Palate
Early in the sixth week, the primary palate (median palatine process)
begins to develop from the deep part of the intermaxillary segment of the
maxilla. Initially, this segment is a wedge-shaped mass of mesenchyme between
the internal surfaces of the maxillary prominences of the developing maxillae.
The primary palate forms the premaxillary part of the maxilla. It represents
only a small part of the adult hard palate (the part anterior to the incisive
fossa).
Secondary Palate
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the adult hard palate by the incisive fossa. An irregular suture runs from the
incisive fossa to the alveolar process of the maxilla, between the lateral incisor
and the canine teeth on each side, indicating where the embryonic primary and
secondary palates fused.
DEVELOPMENT OF TONGUE
Near the end of the fourth week, a median triangular elevation appears
in the floor of the primordial pharynx, just rostral to the foramen cecum. This
swellingthe median lingual swelling (tongue bud)is the first indication of
tongue development. Soon, two oval lateral lingual swellings (distal tongue
buds) develop on each side of the median tongue swelling. The three lingual
swellings result from the proliferation of mesenchyme in the ventromedial parts
of the first pair of pharyngeal arches. The lateral lingual swellings rapidly
increase in size, merge with each other, and overgrow the median tongue
swelling.
The merged lateral swellings form the anterior two thirds (oral part) of
the tongue. The plane of fusion of the lateral swellings is indicated superficially
by the midline groove of the tongue and internally by the fibrous lingual
septum. The median lingual swelling forms no recognizable part of the adult
tongue.
Formation of the posterior third (pharyngeal part) of the tongue is
indicated by two elevations that develop caudal to the foramen cecum:
The copula forms by fusion of the ventromedial parts of the second pair of
pharyngeal arches.
The hypopharyngeal eminence develops caudal to the copula from
mesenchyme in the ventromedial parts of the third and fourth pairs of
pharyngeal arches.
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Lingual Papillae and Taste Buds
Lingual papillae appear by the end of the eighth week. The vallate and
foliate papillae appear first, close to the terminal branches of the
glossopharyngeal nerve (CN IX). The fungiform papillae appear later, near the
terminations of the chorda tympani branch of the facial nerve. The long and
numerous papillae are called filiform papillae because of their thread-like
shape. They develop during the early fetal period (1011 weeks). They contain
afferent nerve endings that are sensitive to touch.
Taste buds develop during weeks 11 to 13 by inductive interaction
between the epithelial cells of the tongue and invading gustatory nerve cells
from the chorda tympani, glossopharyngeal, and vagus nerves. Facial
responses can be induced by bitter-tasting substances at 26 to 28 weeks,
indicating that reflex pathways between taste buds and facial muscles are
established by this stage.
The sensory supply to the mucosa of almost the entire anterior tongue
(oral part) is from the lingual branch of the mandibular division of the
trigeminal nerve (CN V), the nerve of the first pharyngeal arch. Although the
facial nerve is the nerve of the second pharyngeal arch, its chorda tympani
branch supplies the taste buds in the anterior two thirds of the tongue, except
for the vallate papillae. Because the second arch component, the copula
(narrow part connecting two structures), is overgrown by the third arch
component, the facial nerve does not supply any of the tongue mucosa, except
for the taste buds in the anterior part of the tongue. The vallate papillae in the
anterior tongue are innervated by the glossopharyngeal nerve (CN IX) of the
third pharyngeal arch. The posterior third of the tongue is innervated mainly by
the glossopharyngeal nerve (CN IX) of the third pharyngeal arch. The superior
laryngeal branch of the vagus nerve (CN X) of the fourth arch supplies a small
area of the tongue anterior to the epiglottis. All muscles of the tongue are
supplied by the hypoglossal nerve (CN XII), except for the palatoglossus, which
is supplied from the pharyngeal plexus by fibers arising from the vagus nerve.
During the sixth and seventh weeks, the salivary glands begin as solid
epithelial buds from the endoderm of the primordial oral cavity. The buds
undergo branching morphogenesis and grow into the underlying mesenchyme.
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The connective tissue in the glands is derived from neural crest cells. All
parenchymal (secretory) tissue arises by proliferation of the oral epithelium.
The parotid glands are the first to appear (early in the sixth week). They
develop from buds that arise from the oral ectodermal lining near the angles of
the stomodeum. The buds grow toward the ears, branching to form solid cords
with rounded ends. Later, the cords canalize and become ducts by
approximately 10 weeks. The rounded ends of the cords differentiate into acini.
Secretions begin at 18 weeks. The capsule of the connective tissue develops
from the surrounding mesenchyme.
The submandibular glands appear late in the sixth week. They develop
from endodermal buds in the floor of the stomodeum. Solid cellular processes
grow posteriorly, lateral to the developing tongue. Later they branch and
differentiate. Acini begin to form at 12 weeks and secretory activity begins at 16
weeks. Growth of the submandibular glands continues after birth, with the
formation of mucous acini. Lateral to the developing tongue, a linear groove
forms that soon closes over to form the submandibular duct.
The sublingual glands appear in the eighth week, approximately 2 weeks
later than the other salivary glands. They develop from multiple endodermal
epithelial buds in the paralingual sulcus. These buds branch and canalize to
form 10 to 12 ducts that open independently into the floor of the mouth.
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thyroid-stimulating hormone and thyroxine begin to increase, reaching adult
levels by 35 weeks.
There are two pairs of parathyroid glands, the superior and the inferior placed
along the posterior borders of the thyroid gland. Parathyroids are endodermal
in origin and develop from the dorsal parts of the 3rd and 4th pharyngeal
pouches. Inferior parathyroids develop from the 3rd pharyngeal pouch with the
thymus. Third pharyngeal pouch gives rise to the thymus and the inferior
parathyroids. Descent of, thymus into the thorax carries the parathyroid III
caudally. The parathyroid IV remains static due to their attachment to the
lateral lobes of thyroid. It can be said that the parathyroids IV are static and the
parathyroids III are mobile. Parathyroids III may go to the superior
mediastinum with the thymus.
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