Pharyngeal apparatus

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Pharyngeal apparatus It is formed during
4th week from 1- pharyngeal arches 2-
pharyngeal pouches 3- pharyngeal grooves 4-
pharyngeal membranes These apparatus is
contributed to the formation of the head neck (
tongue, face, lips, jaws, palate, pharynx and
neck . . Most congenital anomalies in these
regions originate during transformation of the
pharyngeal apparatus into its adult derivatives

Pharyngeal arches begin to develop from
neural crest cells that migrate into the future
head neck . By the end of the 4th week, 4 pairs
of arches are visible externally. The 5th the
6th arches are rudimentary and are not visible on
the surface of the embryo. The arches are
separated externally, from each other by fissures
( grooves or clefts ) covered by ectoderm. Also,
the arches are separated internally by pouches (
balloonlike diverticula that are lined by
endoderm ).
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The first pair of arches ( the primordium of the
jaws) appears as surface elevations lateral to
the developing pharynex. The other arches appear
as rounded ridges on each side of the future head
neck . . The 1st arch is called
mandibular arch , from it two prominences
are developed The maxillary prominence gives
rise to the maxilla( upper jaw), Zygomatic bone
Squamous part of the temporal bone . The
mandibular prominence forms the mandible ( lower
jaw The 1st pair plays a major role in
facial development. These arches support the
lateral walls of the primordial pharynx, which is
derived from the cranial part of the foregut
.
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A 20 week fetus illustrating the area of the
face derived from the first pair of the
pharyngeal arches.
Derivatives of maxillary prominence Maxilla

Palatine bone
Zygomatic bone
Squamous temporal bone
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The primordial mouth or Stomodeum appears as a
slight depression of the surface ectoderm . It is
separated from the cavity of the primordial
pharynx by a bilaminar membrane (oropharyngeal
membrane ) It is composed of ectoderm externally
endoderm internally. It ruptures at about 26
days , thus the primordial pharynx foregut
become communicating with the amniotic cavity.
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Pharyngeal arch Components Each arch
consists of a core of mesenchyme and is covered
externally by ectoderm internally by endoderm.
The original mesenchyme is derived from mesoderm
in the third week. During the 4th week most of
the mesenchyme is derived from neural crest cells
which are the major source of the arches
connective tissue ( bone, cartilage and
ligaments) .
A typical pharyngeal arch contains
An aortic arch ,
an artery that arises from the truncus arteriosus
of the primordial heart. ( vascular endothelial
from the original mesenchyme ) .
A cartilaginous rod
that forms the skeleton of the arch . ( from the
neural crest ) . A muscular component
that differentiates into muscles of head neck (
original mesen.) A nerve that supplies the mucosa
muscles. It is derived from neuroectoderm of
the . Brain


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An aortic arch that passes around the
primordial pharynx to enter the dorsa aorta .

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Ventral parts of the 1st arch cartilage(
mandibular process) form the horseshoe-
shaped primordium of the mandible. The
cartilage disappears as the mandible develops
around it by intramembranous ossification
of mesenchymal tissue surrounding it .
The middle part regresses, but its
perichondrium forms the ligaments. The
dorsal end is closely related to the
developing ear , it ossifies and form two middle
ear bones malleus incus ). The ventral
end of the 2nd arch cartilage ossifies to
form the lesser cornu and the superior
part of the body of the hyoid bone The
middle part regresses its perichondrium forms
styloid ligament. The
dorsal end, ossifies to form stapes styloid
process. If stylohyoid ligament ossifies, it may
cause pain in palatine tonsil .
The ventral part of the 3rd arch cartilage,
ossifies to form the greater cornu and the
inferior part of the body of the hyoid bone.
The 4th
the 6th arch cartilages fuse to form the
laryngeal cartilages ,except the epiglottis.
The 5th arch is rudimentary and has no
derivatives.
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The 1st arch forms muscles of mastication,
tensor veli palatini, tensor veli tympani,
mylohyoid and anterior belly of digastric.
The 2nd arch forms the
muscles of facial expression the auricular
muscles. Occipitofrontalis, platysma, posterior
belly of digastric, stylohyoid and stapedius.
The 3rd arch forms the stylopharyngeus..
The 4th arch forms cricothyroid,
levator veli palatini, constrictor muscles of the
pharynx and striated muscles of esophagus.
The 6th arch forms The other intrinsic
muscles of the larynx .

Myoblasts from the occipital myotomes to form the
tongue musculature
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The derivatives of the 1st arch are supplied by
the caudal two branches of trigeminal nerve ,
5th, ( mandibular maxillary ) . It is the motor
nerve for muscles of mastication. It is sensory
to the face, teeth, and mucous membranes of the
nasal cavities, palate, mouth, and tongue.
The derivatives of the 2nd arch are
supplied by the facial nerve
The derivatives of the 3rd arch
are supplied by the glossopharyngeal nerve.
The derivatives of the 4th arch are
supplied by the superior laryngeal nerve of the
vagus . The derivatives of the 6th arch
are supplied by the recurrent laryngeal nerve of
the vagus. The nerves of the 2nd to 6th
arches have little cutaneous distribution
, they innervate the mucous membranes of
the tongue, pharynx, and larynx.

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Pharyngeal membranes Appear
in the floors of the pharyngeal grooves where
ectoderm becomes nearer to the endoderm and
few mesodermal cells lie inbetween .
The membranes disappear
except for the 1st pair, which becomes the
tympanic membranes ( eardrum ).
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Pharyngeal Grooves The head neck regions
exhibit 4 pharyngeal grooves on each side during
the 4th 5th weeks . Only, the 1st pair persists
as the external acoustic meatus. The other
grooves lie in a slitlike depression the
cervical sinus . Development of the
neck During the 5th week , the 2nd arch enlarges
or grows downwards or caudally. Also, the 6th
arch elongates upwards or cranially. Thus
the previous 2 arches over- grows the 3rd 4th
arches, forming an ectodermal depression (
cervical sinus ) . So , the 2nd , 3rd and the 4th
pouches become hidden beneath the elongated two
arches. By the end of the 7th week, the 2nd, 3rd
4th grooves the cervical sinus have
disappeared, giving the neck a smooth contour.

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Pharyngeal Pouches There are 4 well- defined
pairs of pouches. The 5th pair is absent or
rudiment The 1st pouch It
expands into an elongate tubotympanic recess The
expanded distal part of this recess share in the
formation of the tympanic membrane ( eardrum ).
Its cavity gives rise to the tympanic cavity
mastoid antrum . The connection of the
tubo-tympanic recess with the pharynx elongates
to form the pharyngotympanic tube ( auditory tube
) .
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The second pouch It is obliterated by
palatine tonsil Its proximal part remains as the
tonsillar sinus or fossa . Its
endoderm proliferates grows into the
underlying mesenchyme.
The central parts of these buds
break down, forming crypts. So, its endoderm
forms the surface epithelium the lining of the
tonsillar crypts.
About 20 weeks mesenchyme
around the crypts differentiates into lymphoid
tissue which give rise to the lymphatic
nodules of the palatine tonsil .
.
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The third Pharyngeal Pouch It
expands proliferate during the 5th week and
forms small nodules on the dorsal aspect . Then
it develops a solid Dorsal bulbar part
and a hollow , elongate ventral part
. Its
connection with the pharynx degenerates .By the
6th week the epithelium of each dorsal bulbar
part begins to differentiate into an
inferior parathyroid gland . The
epithelium of the elongate ventral parts
proliferates obliterating their
cavities to form the thymus gland. These
bilateral primordia come together in the
median plane then descends into the
superior mediastinum. The bilobed form
remains throughout life The primordia of both
glands lose their connections migrate into
neck. The parathyroid separate from the
thymus lie on the dorsal surface of
the thyroid.
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The Fourth Pharyngeal Pouch It
expands into dorsal bulbar and elongate ventral
parts. By the 6th week , each
dorsal part develops into a superior parathyroid
gland , which lie on the dorsal surface of the
thyroid gland . The glands derived from the 3rd
pouch descend with the thymus to a more inferior
position than the glands derived from the 4th
pouches. The elongated ventral part of each
pouch develops into an ultimopharyngeal body
which fuses with the thyroid gland and its cells
give rise to the parafollicular cells of the
thyroid gland. These cells also called C cells
which produce calcitonin, a hormone that regulate
the calcium level in the body. C cells
differentiate from neural crest cells that
migrate from the arches into the 4th pouches.

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The fifth Pharyngeal pouch When it develops it
helps to form the ultimopharyngeal body
Histogenesis of Parathyroid Glands
The chief or principal cells differentiate
during the embryonic period . They regulate fetal
calcium metabolism .
The oxyphil cells differentiate 5 to
7 years after birth.
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Histogenesis of Thymus Epithelial tubes (
endoderm ) grow within the mesenchyme. These
tubes become solid cords that proliferate and
give rise to side branches . Each side branch
becomes the core of a lobule. Some cells of the
epithelial cords become the THYMIC CORPUSCLES (
Hassall ) (endodermal ). Other of the epithelial
cords form the epithelial reticular cells (
endodermal ) . The lymphocytes are derived from
hematopoietic stem cells ( mesodermal ) . A thin
layer of mesenchyme surround the gland to form
the capsule . The mesenchyme the macrophages
and the muscle cells ( smooth muscles of vessel )
are derived from neural crest cells ( mesoderrmal
) .
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Branchial Sinuses 1- external It
opens along the anterior border of the
sternocleidomastoid muscle in the inferior third
of the neck. It is uncommon. It results from
failure of the 2nd groove the cervical sinus to
oblitrate. Anomalies of the other grooves ( 1st,
3rd 4th )occure in about 5. It is detected
during infancy by discharge of mucous material
from them. These bilateral cervical sinuses are
in about 10 of cases and commonly associated
with auricular sinuses. 2- Internal
open into the pharynx are very rare. They
usually open into the tonsillar sinus or near the
palatopharyngeal arch. They result from
persistence of the proximal part of the 2nd
pouch.

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Branchial Fistula It is an abnormal canal
that internally open into the tonsillar sinus
externally in the side of the neck. It results
from persistence of parts of the 2nd groove 2nd
pouch. It passes between the internal and
external carotid arteries.
Branchial Cysts Remnants of parts of
the cervical sinus and or the 2nd groove persist
and form cyst. They are produce as a slowly ,
enlarging painless swelling in the neck. They
enlarge due to accumulation of fluid cellular
debris derived from desquamation of their
epithelial linings. They often lie inferior to
the angle of the mandible or anywhere along the
anterior border of the sternocleidomastoid
muscle. They observed also in the parathyroid
glands.
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