NOTOCHORD
&
PHARYNGEAL ARCHES
Dr. Saleem Shaikh
Introduction
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The notochord is a midline structure, that develops
between the primitive streak and prochordal plate.
The cranial end of primitive streak becomes thickened
and is known as primitive knot
A depression is seen in the middle of this knot known
as blastopore.
The cells of the primitive knot multiply and pass
cranially between endoderm and ectoderm to reach the
prochordal plate – this is known as the notochord.
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Functions of notochord
Structure - acts as a rigid axis around which the
embryo develops
Skeletal - foundation upon which the vertebral column
(vertebral bodies) will form
Induction - will bring about formation of the neural
tube (future nervous system)
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Ectoderm overlying the notochord becomes thickened
to form the neural plate
The neural plate is converted to neural groove and then
to neural tube
The neural tube gives rise to brain and spinal cord
The cells near the neural plate have characteristics of
both ectoderm and mesoderm and are known as neural
crest cells.
The connective tissue of head and neck is derived from
these cells and is hence known as ectomesenchyme
PHARYNGEAL ARCHES
PHARYNGEAL ARCHES
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The pharyngeal or branchial arches are rod like
thickenings of mesoderm present on the wall of the
foregut
During development, after the rupture of buccopharyngeal membrane the neck region of the embryo
shows a series of mesodermal thickenings.
Initially there are six arches but the fifth arch
disappears and only five remain.
The first arch is known as the mandibular arch and the
second arch is known as the hyoid arch
The third, fourth and sixth arch do not have any special
names.
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Arch
Endodermal pouch
Ectodermal cleft
Each arch contains –
◦ A skeletal element (cartilage - bone)
◦ Muscle
◦ Artery (blood vessel)
◦ Nerve – pretrematic and post trematic nerve
Fate of the Ectodermal Clefts
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The first cleft develops to form the lining of the
external acoustic meatus (outer ear)
The second cleft grows rapidly then the rest and covers
the remaining clefts.
The space between the second cleft and the other clefts
is known as the cervical sinus.
Fate of Endodermal Pouches
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First pouch –
◦ Ventral part – obliterated
◦ Dorsal part – part of the ear (tubotympanic process)
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Second pouch –
◦ ventral part – tonsil
◦ Dorsal part - part of the ear (tubotympanic process)
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Third pouch –Inferior parathyroid and thymus
Fourth pouch – superior parathyroid and part of
thyroid?