Neurofilament proteins in avian auditory hair cells.

The distribution of middle-weight neurofilament protein (NF-M), an intermediate filament of neurons, was examined in the developing and mature avian inner ear by using immunocytochemical techniques. NF-M was detected in auditory hair cells and VIIIth cranial nerve neurons. NF-M-positive hair cells are first detected at embryonic day 11 (E11) in superior hair cells in the mid-proximal (mid-frequency) region of the chicken basilar papilla. With time, increasing numbers of hair cells express NF-M. Two developmental gradients occur: 1) a radial gradient, in which superior hair cells are labeled first, and progressively more inferiorly located hair cells are labeled during ontogeny, and 2) a longitudinal gradient, in which hair cells in the mid-proximal region are labeled first, and then progressively more distal (low-frequency) hair cells are labeled. There is also a small proximally directed progression of NF-M expression. By E19, NF-M-positive hair cells are found throughout the distal and mid-proximal regions, and this expression is maintained through 3 weeks posthatching. By 22 weeks posthatching, NF-M staining in hair cells is markedly diminished; staining is seen in only a few tall hair cells in the distal one-fourth of the papilla and in short hair cells in the distal one-half of the papilla. NF-M is never expressed by hair cells at the proximal (high-frequency) end of the papilla at any time examined. These findings suggest that some cell types that have traditionally been classified as nonneural may express neurofilament and that the basilar papilla of the neonatal chicken is not morphologically mature.