Download 03-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.
Development of the thyroid gland
copula
Tuberculum
impar
It is the first endocrine gland to develop in the
embryo . It begins to form about 24 days after
fertilization from a median endodermal
thickening in the floor of the primordial pharynx.
From this thickening a small outpouching is
formed.
As the embryo & tongue grow , It descends in
the neck passing ventral to the developing
hyoid bone & laryngeal cartilages.
It is connected to the tongue by a thyroglossal
duct
At first its primordium is hollow then it become
solid & divides into right and left lobes which
are connected by the isthmus, which lie anterior
to the developing 2nd & 3rd tracheal rings.
By 7th weeks , it has its definitive shape &
reached its final site in the neck. Also, at this
time the thyroglossal duct degenerate &
disappear
The proximal opening of the duct persists as a
small pit in the tongue ( the foramen cecum )
A pyramidal lobe extends superiorly from
the isthmus in a about 50 % of people . It
may be attached to the hyoid bone by
fibrous tissue and or smooth muscle (
levator of thyroid gland ) . These structures
represent a persistent part of the distal end
of the thyroglossal duct.
Histogenesis of the thyroid gland
The thyroid primordium consists of a solid
mass of endodermal cells
Later, this cellular aggregation breaks up into a
network of epithelial cords . This is due to its
invasion by the surrounding vascular
mesenchyme.
By the 10th week , the cords have divided into
small cellular groups.
In each cell group ( cluster ) , the cells become
arranged in a single layer around a lumen.
During the 11th week colloid begins to appear
in it . So , the thyroid follicles is formed. Also,
the synthesis of thyroid hormones can be
demonstrated.
Note that the follicular cells arise from
endoderm & the parafollicular cells arise from
mesoderm.
Thyroglossal Duct
Cysts
It is formed anywhere along
the course followed by the
thyroglossal duct during
descend of the primordial
thyroid gland from the
tongue.
It is formed by a remnant of
the thyroglossal duct. Which
persist..
Usually, it is formed inferior
to the hyoid bone in the
midline or in the tongue.
Following infection of a cyst, a perforation of the
skin occurs, forming a
sinus that
thyroglossal duct
opens in the median plane of the
neck, anterior to the laryngeal cartilages.
Most are observed by the
age of 5 years. It is a
painless, progressively
enlarging movable mass. It
may contain some thyroid
tissue.
Development of the tongue
Distal tongue duds
Near the end of the 4th week , a
median triangular elevation appears in
the floor of the primordial pharynx just
rostral to the foramen cecum , it is
called the median tongue bud (
Tuberculum impar ). Then two oval
distal tongue buds ( lateral lingual
swellings ) develop on each side of the
median tongue bud.
These three lingual buds result from
the proliferation of mesenchyme in the
ventromedial parts of the first pair of
pharyngeal arches.
The distal tongue buds rapidly
increase in size , merge with each
other, & overgrow the median tongue
bud. So, the distal tongue buds form
the anterior two – thirds ( Oral part ) of
the tongue.
The median tongue bud forms no
recognizable part of the adult
Formation of the posterior third (
pharyngeal part )of the tongue is
indicated by two elevations that
develop caudal to the foramen
cecum.
1- The copula forms by fusion of
the ventromedial parts of the 2nd
pair of pharyngeal arches.
2- The hypopharyngeal eminence
develops caudal to the copula
from mesenchyme in the
ventromedial parts of the 3rd & 4th
pairs of
arches.
As the tongue develops copula
disappears and gradually the
hypopharyngeal eminens
overgrown it .
So, the pharyngeal part of the
tongue is developed from the
rostral part of the
hypopharyngeal eminence.
The distal part of the
hypopharyngeal
eminence gives rise to
the epiglottis .
The arch mesenchyme forms
the connective tissue & the
vasculature of the tongue.
Most of the tongue muscles are
derived from myoblasts that
migrate from the occipital
myotomes.
The hypoglossal nerve
accompanies the myoblasts
during migration & innervates
the tongue muscle, except
palatoglossus which is supplied
by pharyngeal plexus by fibers
arising from the vagus nerve.
The entier tongue is within the
mouth at birth & its posterior
third descends into the
oropharynx by the 4 years of
age.
Fusion of the distal tongue buds is
indicated externally by a middle
groove, ( the median sulcus of the
tongue). & internally by the fibrous
lingual septum.
The line of fusion of the oral &
pharyngeal parts of the tongue is
roughly indicated by a V- shaped
groove ( the terminal sulcus ) .
At first , the tongue is adherent to
floor of the mouth. Later , its oral
part separates from the floor of the
mouth and the lower gum by a
horseshoe groove called alveololingual groove.
So, its inferior surface is connected
to the floor of the mouth by a
frenulum ( mucous memberane
fold.
Papillae & Taste Buds
Lingual papillae appear near the
end of the 8th week.
The vallate & foliate appear first ,
close to terminal branches of the
glossopharyngeal nerve.
These gustatory nerve cells are from the chorda
tympani, glossopharyngeal, and vagus nerves
Most taste buds form on the dorsal surface of the
tongue. Also, they develop on the palatoglossal
arches, palate, posterior surface of the epiglottis,
and the posterior surface of the oropharynx.
At 26 to 28 weeks, there is a reflex pathways
between taste buds & facial muscles by bittertasting substances.
The most common papillae is the
filiform, they are threadlike shape .
They develop from the 10 th to 11th
weeks.( early fetal period ). They
contain afferent nerve endings that
are sensitive to touch.
The fungiform appear later near
termination of the chorda tympani (
from facial nerve ).
Taste buds develop during
weeks 11 to 13 by interaction
between the epithelial cells of the
tongue & the invading gustatory
Nerve Supply of the tongue
The sensory supply to the mucosa
of the oral part is from the lingual
branch of the mandibular division of
the trigeminal nerve.
The superior laryngeal branch of the vagus
nerve of the 4th arch supplies a small area of
the tongue anterior to the epiglottis ( root of
the tongue )
The 2nd arch does not share in the
formation of the tongue mucosa,
But , the corda tympani ( branch
from facial nerve ) supplies the
taste buds in the oral part. , except
for the vallate papillae.which are
innervated by the glossopharyngeal
nerve of the 3rd arch. The reason is
that the mucosa of the posterior
third is pulled anteriorly as the
tongue develops.
The pharyngeal part is innervated
by the glossopharyngeal nerve .