The organization of actin filaments in the stereocilia of cochlear hair cells.

Abstract

Within each tapering stereocilium of the cochlea of the alligator lizard [Gerrhonotus multicarinatus] is a bundle of actin filaments within > 3000 filaments near the tip and only 18-29 filaments at the base where the bundle enters into the cuticular plate. At the base the filaments splay out as if on the surface of a cone, forming the rootlet. Decoration of the hair cells with subfragment 1 of myosin reveals that all the filaments in the stereocilia, including those that extend into the cuticular plate forming the rootlet, have unidirectional polarity, with the arrowheads pointing towards the cell center. The rest of the cuticular plate is composed of actin filaments showing random polarity and numerous fine, 30 .ANG. filaments connecting rootlet filaments to each other, to the cuticular plate and to the membrane. Examination of the packing of the actin filaments in the stereocilia by thin section and by optical diffraction reveals that the filaments are packed in a paracrystalline array with the crossover points of all the actin helices in near-perfect register. In transverse sections, the actin filaments are not hexagonally packed but arranged in scalloped rows presenting a festooned profile. This profile is demonstrated as a product of the crossbridges by examining serial sections, sections of different thickness and the same stereocilium at 2 different cuttng angles. The filament packing is not altered by fixation in different media, removal of the limiting membrane by detergent extraction or incubation of extracted hair cells in EGTA, EDTA [ethyleneglycolbis(.beta.-aminoethyl ether) N,N''-tetraacetic acid], and Ca2+ and ATP. The stereocilia of the ear, unlike the brush border of intestinal epithelial cells, apparently are not designed to shorten and the filaments do not slide past one another. The stereocilium is like a large, rigid structure designed to move as a lever.