• Derivatives of the skeletal elements
• Nerves and muscle of the arches
• Fate of ectodermal cleft
• Fate of endodermal pouches
• Development of the thymus
• Development of the parathyroid gland
• Development of thyroid gland
• Timetable of some mentioned events
• Pharyngeal arches?
• Are rod-like thickening of mesoderm present in
the wall of the foregut.
• How many arches are there?
• At first they are six aches. The fifth arch
disappears and only five remain.
• Where the aches meet?
• The ventral ends of the arches of the right and
left sides meet in the middle line in the floor of
• Endodermal /pharyngeal pouches
• In the interval between any two arches, the
endoderm is pushed outwards to form a series
• Ectodermal cleft
• The surface ectoderm dips inwards opposite
• Each pharyngeal arch contains :
1. a skeletal element (cartilage).
2. Striated muscle (supplied by a nerve of the arch
and an arterial arch.
• The cartilage of the 1st arch (Meckle’s cartilage)
gives rise to the incus and malleus.
• The cartilage of the 2nd arch forms
a. The stapes.
b. The styloid process.
c. Part of the hyoid bone.
• The cartilage of the 3rd arch forms the greater
part of the hyoid bone.
• The cartilage of 4th and 6th arches give rise to the
cartilage of the larynx.
• Nerves of the arches:
• 1st arch -----------------------mandibular.
• 2nd arch ----------------------facial.
• 3rd arch ---------------------- glossopharyngeal.
• 4th arch ---------------------- superior laryngeal.
• 5th arch ---------------------- recurrent laryngeal.
The muscles are supplied by these nerves
are derived from the mesoderm of the
• The external acoustic meatus develops from the 1st ectodermal
• Tubotympanic recess develops from the 1st and 2nd endodermal
• The middle ear and the auditory tube develops from the tubotympanic recess.
• The palatine tonsil arises from the 2nd pouch.
• The inferior parathyroid gland and the thymus are derived from
the 3rd pouch.
• The superior thyroid gland is derived from the 4th pouch.
• The thyroid gland develops mainly from the thyroglossalduct.
• Thyroglossal duct is formed as a median diverticulum arising from the floor of
the pharynx (at the foramen caecum0.
• After establishment of the head fold, the foregut is bounded ventrally by
pericardium and dorsally by developing brain.
• Cranially, it is separated at first separated from the stomatodaeum by
the buccopharyngeal membrane.
• When this membrane breaks down, the foregut opens to the exterior
through the stomatodaeum.
• At this stage, the head is represented by the bulging caused by the
developing brain, and
• While the pericardium occupying the future thorax,
• The two are separated by the stomatodaeum which is the future mouth.
• The neck is not yet present//////////
• The neck is formed by the elongation of the region between the
stomatodaeum and the pericardium
• The elongation is due to the appearance of the pharyngeal/ or branchial arches
(the mesodermal thickening).
• At this stage, the wall of the foregut is separate from the surface ectoderm by a
layer of mesoderm.
• The mesoderm comes to arrange in the form of six bars.
• These bars run dorsoventrally in the side wall of the foregut.
• Each of these “bars” grows ventrally in the floor of the developing pharynx and
fuses with the corresponding “bar” of the opposite side to form a PHARYNGEAL or
• In the interval between any two adjoining arches, the endoderm extends
outwards in the form of a pouch (endodermal or pharyngeal pouch) to meet the
ectoderm which dips into this interval as an ECTODERMAL CLEFT.
• The 1st arch is also called
he mandibular arch;
• The 2dn arch is called,
the hyoid arch.
• The 3rd , 4th ,and 6th arches do not have special names.
• The 5th arch disappears soon after its formation, so that only five
• The following structures are formed
in the mesoderm of each arch:
• A skeletal element:
• This is cartilaginous to begin with.
• It may remain cartilaginous,
• It may develop to bone,
• It may disappear.
• Striated muscle:
• Supplied by nerve to the arch.
• May , or may not, retain its attachment to the skeletal
elements derived from the arch.
• May subdivide to form a number of distinct muscles, which
may migrate away from the pharyngeal region.
• When they do so, they carry their nerve with them,
• And their embryological origin can determined from their
• An arterial arch:
• Ventral aorta develops ventral to the foregut.
• Dorsal aorta develops dorsal to the foregut.
• Aortic arches (a series of arches) connect ventral and dorsal
• One arterial arch lies in each pharyngeal arch.
• The arrangement of these arteries will be greatly modified ( will be discussed later).
• Each pharyngeal arch is supplied by a nerve.
• In addition to supplying the skeletal muscle of the arch, it
supplies sensory branch to the overlying ectoderm and
(continue)!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! You can skip it!!!
• In the human embryo, a double innervation is seen only in the
1st pharyngeal arch.
• In some lower animals, each arch is supplied by two nerves:
A. Post-trematic nerve.
B. Pre-trematic nerve.
(Trema = trench).
The nerve that runs along the cranial border of the arch
is called the post-termatic nerve.
The nerve that runs along the caudal border of the arch
is called the pre-termatic nerve.
1. First Arch Derivatives
1. The cartilage of the 1st arch is called Meckel’s cartilage:
the following structures are formed from the dorsal end
cartilage of the 1st arch:
c) Anterior ligament of the malleus
d) Sphenomandibular ligament
e) Bone (the maxilla, the mandible, the zygomatic bone, the
palatine bone, and part of the temporal bone).
///// The ventral part of the cartilage is absorbed.
(it is surrounded by the developing mandible and is absorbed)
2. Second Arch derivatives
the following structures 5S are formed from
the cartilage of the 2nd arch:
B. Styloid process
C. Stylohyoid ligament (from sheath)
D. Smaller (lesser) cornu of hyoid bone
E. Superior part of body of hyoid bone
3. 3rd Arch derivatives
• the following structures are formed from the
cartilage of the 3rd arch:
a) Greater cornu of hyoid bone
b) Lower part of the body oh hyoid bone
4. The cartilages of the larynx
• are derived from the 4th and 6th arches
• with possible contribution from the 5th arch
• but, their exact derivation is controversial.
Nerves and Muscles of the Arches
• All muscles derived from a pharyngeal arch are supplied by the nerve
of the arch.
• Muscles can be identified by their nerve supply.
• The nerves supply :
• parts of skin
• and mucous membrane derived from the arch.
• The 1st arch has a double nerve supply; (anterior 2/3rd tongue)
• Mandibular nerve is the post-terematic nerve of the 1st arch.
• Chorda tympani is the pre-trematic nerve of the 1st arch
• Anterior 2/3rd of the tongue is derived from the ventral part of the
Arch Nerve of the Arch Muscles of the Arch
First Mandibular Medial + lateral pterygoid, Masseter,
Temporalis, Mylohyoid, Anterior belly
of digastric, Tensor tympani, Tensor
Second Facial Muscles of the face, Occipitofrontalis,
Platysma, stylohyoid, Posterior belly of
digasrtic, Stapedius, Auricular muscles
Third Glossopharyngeal Stylopharyngeus
Superior laryngeal Muscles of the larynx and pharynx
Fate of Ectodermal Clefts
• 1st cleft:
• The ventral part is obliterated.
• The dorsal part:
1. Epithelium lining of the external auditory meatus.
• 2nd arch,
• Grows much faster than the succeeding arch and comes
• The space between the overhanging 2nd arch and the 3rd, 4th,
and 6th arches is called cervical sinus.
Fate of Ectodermal Clefts
• Cervical sinus;
• The lower overhanging border of the 2ns arch fuses with
tissues caudal to the arches.
• The side of the neck becomes smooth.
• The cavity of cervical sinus become normally obliterated.
• Part of it may persist and give rise to branchial cyst.
• The cyst may open onto the surface ---- branchial fistula.
• Rarely it may open into the lumen of the pharynx in the
region of the tonsil.
Fate of Endodermal Pouches
• 1st pouch;
• The dorsal part is obliterated by formation of the tongue.
• The ventral part receives contribution from the 2nd pouch,
• These two form a diverticulum, called;
• Tubotympanic recess. (proximal and distal part).
• Proximal part --- auditory tube (pharyngotympanic).
• Distal part ----- middle ear cavity (including the tympanic
The derivatives of the branchial pouches. Note that the inferior parathyroid migrates
downwards from the 3rd pouch whereas the superior parathyroid (4th pouch) remains stationary.
Fate of Endodermal Pouches (continue)
• The epithelium of the ventral part contributes to the
formation of the tonsil.
• The dorsal part --- tubotympanic recess.
gives rise to the inferior parathyroid glands.
o Gives origin to the superior parathyroid glands.
o May contribute to the thyroid gland.
Fate of Endodermal Pouches (continue)
• 5th = ultimobranchial pouch
• Seen for a brief period during development.
• In some species it gives rise to the ultimobranchial body.
• Its fate is controversial in human.
• Generally it is believed to be incorporated into the 4th pouch.
• 4th + 5th pouches form the
• Caudal pharyngeal complex. This gives rise to the:
1. Superior parathyroid glands.
2. Parafollicular cells of the thyroid gland.
Development of the Thymus
• Arise from the epithelium of the 3rd pharyngeal pouch.
• Relatively large at birth.
• Continues to increase in weight till puberty.
• Then it gradually undergoes atrophy.
• Early in development, the 3rd pouch is cut off, both from the pharyngeal wall
and from the surface ectoderm.
• After separating from the inferior parathyroid rudiment, each thymic rudiment
• Thinner cranial part and broader caudal part.
• Thinner portion forms the cervical part of the thymus;
• Boarder parts of both sides enter the thorax and become united to each other
by connective tissue.
• Thymic endoderm is invaded by vascular mesoderm.
• Vascular mesoderm contains numerous lymphoblasts.
• Mesenchymal invading breaks up the thymic tissue into isolated masses, and
gives the thymus its lobulated appearance.
• Accessory thymic tissue may develop from fragmentation oh the cervical part.
• Accessory thymic tissue, present in relation to the SPG, and is believed to rise
from the 4th pouch.
Development of Parathyroid Glands
• Inferior parathyroid glands; parathyroid III
• Develop from endoderm of the 3rd pharyngeal pouch.
• Superior parathyroid glands; parathyroid IV.
• Develop from endoderm of the 4th pharyngeal pouch.
• Superior parathyroid glands are constant in position.
• Inferior parathyroid glands are carried caudally due to
its closely related to thymus.
• As thymus descends caudally parathyroidIII becomes
caudal to parathyroid IV.
The normal sites of the parathyroid glands (posterior aspect).
Development of Thyroid Gland
• Thyroid gland develops mainly from the thyroglossal duct.
• Parafollicular cells are derived from the caudal pharyngeal
complex (from 4th and 5th pharyngeal pouch).
• After formation of the pharyngeal arches, the medial ends of
the two mandibular arches are separated by
• Tuberculum impar =
A midline structure in the floor of the pharynx.
• Immediately the tuberculum impar the epithelium of the
floor of the pharynx shows a thickening.
• This thickening is soon depressed to form a diverticulum
called the thyroglossal duct.
Development of Thyroid Gland (continue)
• The site of the diverticulum is now seen as a depression called
the foramen caecum.
• The diverticulum grows down in the midline into the neck.
• Its tip bifurcates.
• Proliferation of the cells of this bifid end gives rise to the
• Two lobes of the thyroid gland.
• The developing thyroid gland comes into intimate contact with
The caudal pharyngeal complex and fuses with it.
• Cells arising from this complex are believed to give origin to the
parafollicular cells of the thyroid.
Anomalies of the Thyroid gland
1. Anomalies of the shape.
2. Anomalies of the position.
3. Ectopic thyroid tissue.
4. Remnants of the thyroglossal duct.
Anomalies of the shape
1. Isthmus may be absent
2. One of the lobe may be very small, or absent
3. Pyramidal lobe:
a) Regarded as a normal structure.
b) May arise from the isthmus, or from one of the
c) May have no connection with rest of the thyroid.
d) May be divided into two or more parts.
e) Its extent vary from a short stump to a process
reaching the hyoid bone.
Anomalies of the position
1. Lingual thyroid.
2. Intra-lingual thyroid.
3. Suprahyoid thyroid.
4. Infrahyoid thyroid.
5. Intrathoracic thyroid.
when thyroid tissue is present in the
anomalous position, an additional thyroid
may or may not be present at the normal site.
Normal and abnormal sites of the parathyroid glands (lateral view).
Ectopic thyroid tissue
• Small masses of thyroid tissue may be present at abnormal sites.
• Thyroid tissue may be observed in the:
• Larynx, trachea.
Lateral aberrant thyroids =
Masses of ectopic thyroid tissue have been described in
relation to the deep cervical lymph node.
Remnants of the thyroglossal duct
• These remnant may persist and lead to the
formation of the following:
1. Thyroglossal cyst, that may be anywhere along
the course of the duct.
2. Thyroglossal fistula, internal papilla (opening)
at foramen caecum.
3. Carcinoma of the thyroglossal cyst.
Timetable of Some Events Mentioned
in this Chapter
Age Developmental events
4th week (22nd day) Appearance of the 1st and
5th week (29th day) Four arches are seen
Thyroid, parathyroid and
thymus are start forming
7th week Thyroid gland reaches its
pharyngeal arch derivatives
• The mesodermal core of each pharyngeal arch differentiates
into three main types of tissue, and each arch becomes
associated with a particular cranial nerve.
• The tissues are:
• Skeletal tissue
(eg: cartilage, bone, ligaments).
• Muscle tissue
(striated musculature, but not all under voluntary control).
• Arterial arch
(which may or may not be converted into a definitive major artery).
• on the inside of the embryonic pharyngeal region
are endodermally-lined grooves between
adjacent pharyngeal arches.
• They contribute to the development of a
surprising diversity of structures, ranging from
the middle ear cavity to endocrine glands and
components of the lymphatic system.
• The last two pouches are difficult to distinguish
clearly, and are often considered as a single unit.
• are ectodermally-lined grooves on the outside of
the embryonic pharynx.
• Only the first cleft is important - it develops into
the external auditory meatus of the ear and
provides the outer epithelium of the tympanic
• (It used to be believed that the remaining clefts
were ‘submerged' beneath overgrowing folds
and then disappeared, but more recent work
shows that the clefts simply ‘fill out' by
proliferation of the underlying mesoderm).!!!!!!!