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Embryology
Lec: 14 Prof Dr. Al-Hubaity
Brachial Apparatus
(Pharyngeal arches, pouches & clefts)
The mesoderm at the sides of head region shows a series of 6 cylindrical
thickenings which grow ventrally on each side of the cranial part of the
foregut (adult pharynx) and are termed as the Brachial (pharyngeal)
arches .These are 6 on each side and are separated from each other by
furrows. These furrows are termed as Cleft (externally) covered by
ectoderm, and Pouches lined by endoderm (internally).
Each arch has 4 elements:
1- Skeletal element i.e. cartilaginous bar.
2- Muscular element i.e. striated muscle.
3- Vascular element i.e. aortic arch.
4- Neural element i.e. one of the cranial nerves for each arch.
The Derivatives of Arches:
The First Arch (Mandibular arch):
1- Skeletal element gives:
a) Malleus & incus.
b) Sphenomandibular ligament & anterior ligament of
malleus.
c) part of the mandible ( most of the mandible is formed
by membrane ossification)
2- Muscular element gives:
a) The 4 muscles of mastication.
b) Myelohyoid and anterior belly of digastric.
c) Tensor palatini and tensor tympani supplied by
mandibular division of trigeminal.
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Second Arch (Hyoid arch):
1- Skeletal elements gives:
a) Stapes.
b) Styloid process.
c) Stylohyoid ligament.
d) Lesser horn and upper part of hyoid bone.
2- Muscular element gives:
a) Muscles of facial expression.
b) Platysma.
c) Stapedius muscle.
d) Stylohyoid muscle and posterior belly of digastric
supplied by facial nerve.
Third Arch:
1- Skeletal elements gives:
a) Greater horn of hyoid bone.
b) Lower part of hyoid bone.
2- Muscular element gives the stylopharyngeus muscle
supplied by glossopharyngeal nerve.
Fourth Arch:
1- Skeletal element gives: thyroid cartilage.
2- Muscular element gives cricothyroid muscle which is
supplied by external laryngeal nerve.
Fifth Arch disappears completely.
Sixth Arch:
1- Skeletal element gives the cricoid & arytenoid cartilages.
2- Muscular element gives all the intrinsic muscles of the
larynx except cricothyroid & they are supplied by recurrent
laryngeal nerves of the vagus.
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Derivatives of the pharyngeal clefts
They are 4 in number and covered by the ectoderm.
The first cleft gives the external auditory meatus.
2
nd
-4
th
cleft will form the floor of the depression known as cervical
sinus, the sinus gradually reduce in size and finally obliterated by
approximation of its wall.
Congenital Anomalies
1) Brachial cyst is developed from the remnant of the cervical sinus.
2) Brachial fistula which may open on the external surface of the neck
(just anterior to the lower 1/3 of the sternomastoid muscle) as
external fistula or may open in to the pharynx near the tonsil as an
internal fistula.
Derivatives of the pharyngeal pouches
First pouch gives the tubo-tympanic recess which forms:
1) Tympanic cavity (middle ear).
2) Pharyngo-tympanic tube.
Second pouch gives the palatine tonsil where endodermal
proliferation will form a group of buds, then canalization appears
in these buds converting them into crypts, then lymphocytes
(mesodermal in origin) invades the tonsillar tissue.
Third pouch gives rise to:
1) Thymus gland which migrates caudally to enter the chest.
2) Inferior parathyroid gland.
Fourth pouch gives rise to:
1) Superior parathyroid glands.
2) A small contribution to the lobes of the thyroid gland (most
of the thyroid glands comes from the thyroglossal duct).
Fifth pouch gives ultimobranchial body with C cells to the thyroid.