Download 02-pharyngeal arches ,pouchs

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
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
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
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.
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
An aortic arch that passes around the
primordial pharynx to enter the dorsa
aorta .
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 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.
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
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..
Myoblasts from the occipital
myotomes to form the tongue
musculature
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
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.
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 ).
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 overgrows 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.
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 ) .
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 .
.
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.
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.
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.
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
).
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.
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.