Early fetal development of the human cochlea

The cochlear scalas are differentiated from a single tube with a lining by the tall epithelium, that is, the cochlear duct. However, we have no information about the mechanism involved in the formation of the scalas. We evaluated histological sections taken from 20 fetuses: eight each at 8--9 weeks [early stage; 28--45 mm crown-rump length (CRL)] and 11--12 weeks (middle stage; 52--74 mm CRL), and four at 14--15 weeks (late stage; 90--110 mm CRL) of gestation. In four of eight early-stage and in all eight middle-stage specimens, we observed irregular perilymphatic spaces and their fusion; these spaces tended to be larger in the future scala tympani than in the future scala vestibuli. The cochlear duct epithelium was positive for cytokeratin 19 in contrast to the other parts of the cochlea. The tectorial membrane appeared in two of eight middle-stage and all four late-stage specimens. After 16 weeks, mesothelial lining of the scala may follow the development of aquaporin-positive thin blood vessels along the scala wall. Notably, gap formation of the cochlear duct epithelium at a site facing the scala tympani consistently occurred before the development of S100 protein-negative organ of Corti. This gap is likely to correspond to a site occupied finally by Hensen's cells. All these steps likely started in the basal coil and extended to the apical side of the cochlea. These findings suggest that leakage through the epithelial gap of endolymph, with a high concentration of potassium ions, causes mesenchymal cell death, leading to the coalescence of vacuoles containing low potassium perilymph.