Prenatal development study of rat Corti organ at the morphological level

Although the structural organization of the mature Corti organ is generally well defined, little is known about its prenatal development. Here, we have examined by photonic and electron transmission microscopies the morphological changes occurring in the cochlear epithelium from the embryonic day 16 to 19 in rat.
<br />At the embryonic day 16 (E16), whatever the region of the cochlear studied (base, medium, apex), the organ of Corti is not present. The cochlear epithelium appears as a pseudostratified epithelial tissue formed by two distinct regions according to the staining of the cells : a pale region on the striolar side of the cochlear canal and a darker region on the modiolar side. At E17, it appears in the basal and medium cells of the cochlear epithelium, at the transition between the pale and the dark one, a cell visible on the whole height of the epithelium. By electron microscopy, we also show that this cell has a rich content in glycogen, a characteristic reminiscent to pillar cells. These results suggest that the first differentiating cell in the cochlear epithelium could be a pillar cell. At E18, in the majority of sections realized at the base and medium of the cochlear canal, the organ of Corti is relatively well recognizable. We distinguish principally the inner hair cells but also to a lesser extent the outer hair cells as well as the supporting cells (pillar, phalengeal and Deiters cells). At E19, the organ of Corti is clearly recognizable in all the basal and medium cells. By contrast, it is yet differenciated at the apex.
<br />These data seem to indicate that the Corti organ in rat develops from base to apex starting by the pillar cells.