The development of hair cells in the embryonic chick's basilar papilla.

During the 7th to 21st (hatching) days, hair cells of the embeyonic chick transform from an undifferentiated epithelium into cylindrically-shaped tall hair cells (THCs), pitcher-shaped short hair cells (SHCs), or intermediate hair cells that share structural characteristics of the first two. By the 11th day hair cell types are unambiguous. "Hairs" (stereocilia and a kinocilium) were first identified on the 7th day and resembled the adult pattern the 13th. The nucleus occupies relatively less volume as hair cells increase in size, becoming positioned centrally in THCs and basally in SHCs. Nucleoli, which are prominent throughout development, remain conspicuous even in newly hatched chicks. The cuticular cone begins to form the 11th day. By the 10th day, sensory nerve endings synapse on the bases of the HCs, which by the 11th day develop synaptic bars. Although efferent neurites were in evidence as early as the 11th day, synaptic contacts and their cisterns were identified by the 19th day but may form earlier. Supporting cells transform from cylindrically to flask-shaped cells with constricted necks and may secrete at least a portion of the tectorial membrane.