True regarding the development of ear: (PGI Dec 2007)
Ans: A (Eustachian tube..); B (Eustachian tube.,); C (Pinna develops..) & D (Growth of the inner,.) Eustachian Tubeit connects nasopharynx with the tympanic cavity. In adults it is about 36 cm long & runs downwards, forwards media] ly from its tympanic end, forming an angle of 45 with the horizontal.The tympanic end of the tube is body & is situated in the anterior wall of the middle ear. A little above the level of the floor. The pharyngeal end of the tube is slit-like & situated in the lateral wall of the nasopharynx, 1-1.25 cm behind the posterior end of the inferior turbinate.it develops from the first arch & partly from the second aarch -Dhingra 5th/14Development of EarInterna Ear It is the first part of the ear to develop Early in the 4th Meek (22 days), a thickening of surface ectoderm the otic placodes appear, which invaginate into underlying mesenchyme to form otic pit (27 days) and then otic or auditory vesicles (otocyst)- the primordium of membranous labyrinth (4.5 weeks Ventralsaccular component of otic vesicle gives rise to saccule & cochlear duct, whereas dorsal utricular part forms small endolymphatic duct, utricle & semicircular canals. Development of membranous internal ear is completed by the 16th week and reaches its adult size by 20-22 weeksQ, when the cochlea is developed sufficiently. Middle Ear Develops in 4 weeks. & competes at 30 week Tympanic cavity, eustachian tube, mastoid antrum & air cells develop from endoderm of first pharyngeal pouch3 The lateral end of pouch expands & forms a tubotympanic recess, which given rising to primitive tympanic cavity whereas the proximal part forms auditory (eustachian) tube. Malleus & incus a re-derived from mesoderm (cartilage) of 1 from 2 arch, except its footplate and annular ligament which are derived from the otic capsule.External Ear Tympanic membrane develops from all three germinal layers, i.e.* ectoderm of 1st pharyngeal cleft* mesenchyme of 1 & 2 pharyngeal arches* endoderm of 1 pharyngeal pouch (tubotympanic recess). External auditory meatus develops from dorsal portion of 1 pharyngeal groove (cleft) Meatal plug is formed at the beginning of 3rd month & dissolves in 7 months. Auricle (pinna) develops from 6mesenchymai proliferations at thedorsal ends of 1 & 2 pharyngeal arches - utricular hillocks - surrounding the first pharyngeal cleft-* Auricle begins to develop at the base of the neck in 6 weeks and assume the normal position at 32 weeks, as the mandible develops.