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Abstract :
[en] 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. Moreover, we have used the periodic acid-thiocarbohydrazide-silver proteinate method to detect the glycogen particles on semithin and ultrathin sections of the cochlear duct.
<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 epithelium formed formed by two distinct regions according to the presence or the absence of microvilli at the apical surface of cells. The region with the microvilli is on the modiolar side and the presence of microvilli is a characteristic of cells in the greater epithelial ridge (GER). In basal part of the cochlear duct, glycogen particles are visible on ultrathin sections in only one cell. On both sides of this cell, apical part of the epithelium shows microvilli. At E17, on semithin sections of basal and medium parts of cochlear duct, one cell visible on the whole height of the epithelium shows rich content in glycogen . 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).We show that only the pillar cells have a rich content in glycogen. At E19, the organ of Corti is clearly recognizable in all the basal and medium parts of the cochlear duct. By contrast, it is yet differenciated in the apex.
<br />These data seem to indicate that the Corti organ in rat develops from base to apex in the greater epithelial ridge starting by the pillar cells.