a) The cartilage of 1st arch is called ” THE MECKELS CARTILAGE”. The INCUS and MALLEUS of the middle ear are derived from the dorsal end. The ventral part of the cartilage is surrounded by the developing mandible and is absorbed. The part of the cartilage extending from the region of the middle ear to the mandible disappears but its sheath forms the anterior ligament of malleus and the sphenomandibular ligament.
b) The cartilage of 2nd arch forms the following:-
1) STAPES
2) STYLOID PROCESS
3) STYLOHOID LIGAMENT from sheath
4) SMALLER (lesser) CORNUA OF HYOID
5) SUPERIOR PART OF THE BODY OF HYOID.
c) The following structures are derived from the cartilage of the 3rd arch:-
1) GREATER CORNUA OF HYOID BONE
2) LOWER PART OF THE BODY OF HYOID BONE
d) The cartilages of larynx are derived from 4th and 6th arches with a possible contribution from 5th arch but there exact derivation is controversial.
The endodermal pouches take part in the formation of several important organs:-
1) FIRST POUCH:-
Its ventral part is obliterated by the formation of the tongue.
Its dorsal part receives a contribution from the dorsal part of second pouch and these together form a diverticulum that grows towards the region of developing ear. This diverticulum is called tubotympanic recess.
The proximal part of this recess gives rise to the auditory tube, the distal part of middle ear cavity including the tympanic antrum.
2) SECOND POUCH:-
a) The epithelium of the ventral part of the pouch contrivbutes to the formation of tonsil.
b) The dorsal part takes part in the formation of TUBOTYMPANIC RECESS.
3) THIRD POUCH:-
This gives rise to inferior parathyroid glands and thymus gland.
4) FOURTH POUCH:-
This gives origin to superior parathyroid gland and may contribute for the development of thyroid gland.
5) FIFTH or ULTIMO BRANCHIAL POUCH:-
A fifth pouch is seen for brief period during development. In some species it gives rise to ultimobranchial body. Its fate in man is controversial. It is generally believed to be incorporated into fourth pouch the together forming the caudal pharyngeal complex.
The superior parathyroid gland arises from this complex, the complex probably also gives origin to the parafollicular cells of thyroid.
After the formation of pharyngheal arches the region of the neck is marked on outside by series of grooves or ectodermal clefts.
1) The dorsal part of 1st cleft develops into epithelial lining of the external auditory meatus.
2) The pinna is formed from a series of swellings, which arise on the 1st and 2nd arches, where they adjoin the 1st cleft. The ventral part of this cleft is obliterated. (more…)
These are mesodermal bars of arches which develop in the region of the pharynx. Since in lower animals they form the respiratory organs they are called as BRONCHIAL or GILL ARCHES. Initially there are six pair of arches, later the 5th pair disappears.
Each arch has an outer ectodermal covering, inner endodermal and in the middle mesoderm. Each arch has its own nerve called the post-trematic nerve and a branch from next arch called pretrematic nerve and its own artery.
