[Development of the internal ear during the 1st trimester of pregnancy. Differentiation of the sensory cells and formation of the 1st synapses].

The inner ear of seventeen embryos, aged from 7 to 14 weeks after the fecondation, was investigated by optic and electron (transmission and scanning) microscopy. The timing of the first stages of the auditory and vestibular human receptor development is similar to the timing reported in animal studies, the vestibular epithelium development preceding by 2 to 3 weeks the cochlear development. The nerve fibers enter the sensory epithelia before the hair cell differentiation was histologically observed. At 9 weeks in the vestibular organs and 11 weeks in the cochlea, the hair cells are well differentiated and they exhibit typical synapses with nerve endings. Very early in the embryonic life, the auditory and vestibular receptors start their maturation and establish their connections with the peripheral and central nervous system; this indicates that the third month of pregnancy is a particularly sensitive period as far as the inner ear development is concerned.