Formation of the cupula cristae ampullaris: development in vivo and in vitro.

The development of the cupula cristae ampullaris in the mouse starts on the 14th gestational day and occurs mainly during the 15th and 16th gestational days. Subsequently the secretory activity of the epithelial cells diminishes. The cupula is formed from the underlying epithelium when there are only a small number of differentiated hair cells present. Secretion of cytoplasmic material occurs mainly from supporting cells. The surface of the epithelium at a location corresponding to the planum semilunatum in the adult organ has a thick layer of microvilli and demonstrates a particularly high secretory activity on the 15th gestational day. After the 16th gestational day there are few morphologic signs of secretion. The cupula also develops in inner ear anlages that have been explanted at the otocyst stage on the 12th or 13th gestational day and cultured for eight days in vitro.