The ultrastructure of the basilar papilla of the chick.

The basilar papilla of the chicken was studied by transmission and scanning electron microscopy. It is composed of two distinct types of hair cells, a tall hair cell (THC) and a short hair cell (SHC). Gradual transitions exist between these two forms. The THC occurs mainly in the anterior part of the neuroepithelium and is situated on the superior fibro-cartilagenous plate, while the SHC dominates in the posterior part and is found on the free basilar membrane. The THC is a tall columnar cell with an elongated hexagonal apical surface. One kinocilium and 150 to 220 stereocilia protrude from the center of its surface. The THC makes synaptic contact with several large afferent nerve terminals and with small bouton-shaped efferent terminals. The SHC is short and thick, having a much wider surface area than the THC. From the inferior part of the surface of the SHC 90 to 120 stereocilia protrude, whereas the kinocilium is lacking. The nucleus is located near the base of the cell. Small afferent nerve terminals and extremely large efferent nerve endings synapse with the SHC.