NEUROFILAMENT PROTEINS IN AVIAN AUDITORY HAIR-CELLS

The distribution of middle-weight neurofilament protein (NF-M), an int ermediate filament of neurons, was examined in the developing and matu re avian inner ear by using immunocytochemical techniques. NF-M was de tected in auditory hair cells and VIIIth cranial nerve neurons. NF-M-p ositive hair cells are first detected at embryonic day 11 (E11) in sup erior hair cells in the midproximal (midfrequency) region of the chick en basilar papilla. With time, increasing numbers of hair cells expres s NF-M. Two developmental gradients occur: 1) a radial gradient, in wh ich superior hair cells are labeled first, and progressively more infe riorly located hair cells are labeled during ontogeny, and 2) a longit udinal gradient, in which hair cells in the midproximal region are lab eled first, and then progressively more distal (low-frequency) hair ce lls are labeled. There is also a small proximally directed progression of NF-M expression. By E19, NF-M-positive hair cells are found throug hout the distal and midproximal regions, and this expression is mainta ined through 3 weeks posthatching. By 22 weeks posthatching, NF-M stai ning 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 sho rt hair cells in the distal one-half of the papilla. NF-M is never exp ressed by hair cells at the proximal (high-frequency) end of the papil la at any time examined. These findings suggest that some cell types t hat have traditionally been classified as nonneural may express neurof ilament and that the basilar papilla of the neonatal chicken is not mo rphologically mature. (C) 1997 Wiley-Liss, Inc.