In the early embryo, the fore-gut develops between the brain above and the primitive heart below.
The mouth is separated from the pharynx by the buccopharyngeal membrane which disappears around the end of 3rd week when a series of arches appears in the wall of pharynx. These arches are formed by mesodermal condensations and are known as the branchial arches.The arches fuse ventrally, thus forming u-shaped structures which support the pharynx. Initially there are 6 of these arches, but the 5th is vestigial and rapidly disappear.
There are 4 branchial pouches internally, 4 branchial grooves externally. These separate the remaining five branchial arches.
Each branchial pouch is lined by endoderm and branchial grooves by endoderm. These pouches and grooves are separated by a thin layer of mesoderm. If this endoderm, mesoderm & ectoderm layers break down a branchial fistula is formed.
A central core of cartilage develop in each arch and muscles differentiate from the surrounding mesoderm.
Each arch is supplied by Cranial Nerve and by an artery.
1st Branchial arch –
2nd Branchial arch-
3rd Branchial arch
4th Branchial arch
5th Brachial arch
1ST Branchial Pouch:
Growth laterally to form: The Eustachian tube and the middle ear.
2nd Branchial Groove:
Deepens to form (only Dorsal part) External auditory meatus and remaining disappear but if it persist collaural fistula is formed.
Number of tubercles appear at the dorsal each of the first and 2nd branchial arches. These tubercles surrounds the dorsal end of the 1st groove which is to be form the External Auditory Canal.
2nd /3rd /4th Branchial pouches and grooves
2nd pouch –Dorsal part contribute to middle ear
ventral part – supra tonsillar fossa.
Externally 2nd Branchial arch grow caudally and covers over 3rd 4th 6th arch
3rd pouch –Dorsal part Inferior parathyroid gland
Ventral part- Thymic part
Deep pit/sinus lined by ectoderm. Its lip fuse and ectoderm lining cystic space is produced which disappear later.