Actin filaments, stereocilia, and hair cells of the bird cochlea. I. Length, number, width, and distribution of stereocilia of each hair cell are related to the position of the hair cell on the cochlea.

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Abstract
Located on the sensory epithelium of the sickle-shaped cochlea of a 7- to 10-day old chick are .apprx. 5000 hair cells. When the apical surface of these cells is examined by scanning microscopy, the length, number, width and distribution of the stereocilia on each hair cell are predetermined. A hair cell located at the distal end of the cochlea has 50 stereocilia, the longest of which are 5.5 .mu.m in length and 0.12 .mu.m in width; those at the proximal end number 300 and are maximally 1.5 .mu.m in length and 0.2 .mu.m in width. Along the cochlea from its distal to proximal end, stereocilia are seen on successive hair cells which gradually increase in number and width, yet decrease in length. Transversely across the cochlea where adjacent hair cells have the same length and number of stereocilia (they are the same distance from the distal end of the cochlea), the stereocilia of successive hair cells become thinner and the apical surface area of the hair cell proper, not including the stereocilia, decreases from a maximum of 80 to 15 .mu.m2. From the length of the longest stereocilium on a hair cell and the width of that stereocilium, the position of that hair cell on the cochlea can be pinpointed in 2 axes. From the number of stereocilia and the apical surface of a hair cell, the location of the cell can be pinpointed in 2 axes. The distribution of the stereocilia on the apical surface of the cell is also precisely determined. More specifically, the stereocilia are hexagonally packed and this hexagonal lattice is precisely positioned relative to the kinocilium. Because of the precision with which individual hair cells regulate the length, width, number and distribution of their cell extensions, this is a magnificent object with which to ask questions about how actin filaments that are present within the cell are regulated. Equally interesting is that the gradient in stereociliary length, number, width and distribution may play an important role in frequency discrimination in the cochlea.