|MadSci Network: Development|
The vertebrate ear evolved from a modification of the first gill slit (closest to the mouth) in primitive fish into a spiracle which brought the external environment close enough to the braincase to use the balance organs for hearing. In lower reptiles, this is the extent of the ear - a spiracle opening onto a tympanum (ear drum) connected directly to the stapes (stirrup). Because of differences in their skulls, the earliest pre-mammals needed a way to transmit sound from the tympanum which could not contact the stapes directly. To accomplish this, two small lower jaw bones, the articular and the quadrate which were already in the area, were brought in between with the stapes and the tympanum, becoming the incus (anvil) and malleus (hammer), respectively.
During development, the first structures to be recognizable around the head are the optic cups and discs (which become the eyes), the nasal placodes (which become the nose), the otic placodes (which become the cochlea and balance organs of the inner ear), and the mouth/gill complex composed of branchial arches with external gill slits outside and pharyngeal pouches inside. As the otic placodes internalize to form the auditory vesicles, it comes in contact with the first pharyngeal pouch. The formation of the auditory vesicle also induces the nearby cartilaginous primordia to form the malleus, incus, and stapes between the auditory vesicle and the external gill slit. As the auditory ossicles, as these bones are called, form, the pharyngeal pouch extends around the bones to form the middle ear and eustacian tube. As the pharyngeal pouch extends upward, the gill slit extends inward, forming the tympanum where the two meet.
All of this occurs during the first trimester of a pregnancy. However, vestigial embryonic connective tissue fills most of the middle ear until after birth, preventing the auditory ossicles from moving properly. This prevents the ears from functioning fully, such that fine noises are lost between the tympanum and stapes. Within a few months after birth, this connective tissue is completely resorbed, and the hearing apparatus is fully functional.
Patten, BM, and Carlson, BM, Foundations of Embryology, McGraw-Hill, Inc., New York.
Walker, WF Jr, Functional Anatomy of the Vertebrates, Saunders College Publishing (CBS), New York.
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