The Organization of Actin Filaments

Within each tapering stereocilium of the cochlea of the alligator lizard is a bundle of actin filaments with >3,000 filaments near the tip and only 18-29 filaments at the base where the bundle enters into the cuticular plate; there 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 that show random polarity, and numerous fine, 30 A filaments that connect the rootlet filaments to each other, to the cuticular plate, and to the membrane . A careful 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, rather, are arranged in scalloped rows that present a festooned profile . We demonstrated that this profile is a product of the crossbridges by examining serial sections, sections of different thicknesses, and the same stereocilium at two different cutting 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, and Ca" and ATP. From our results, we conclude that the stereocilia of the ear, unlike the brush border of intestinal epithelial cells, are not designed to shorten, nor do the filaments appear to slide past one another. In fact, the stereocilium is like a large, rigid structure designed to move as a lever. There are 3-10 rows of stereocilia on the apical surface ofeach hair cell in the cochlea (organ of Corti) and a single cilium, the "kinocilium." The latter is present in submammalian vertebrates and in developing mammals but not in the mature mammalian cochlea . Within each stereocilium is a population of filaments (7) . The stereocilium, then, resembles a large microvillus rather than a cilium, but because the word stereocilium is firmly entrenched in the literature, we will continue to use it, although it is misleading. The filaments in the stereocilia of the equilibrium organ have recently been identified as actin filaments by their ability to decorate with myosin subfragment 1 (S1) (9) . Where it could be determined, the arrowheads produced from the interaction of S 1 with the actin filaments appeared to point towards the cell center indicating the polarity of the actin filaments . From these observations, Flock and Cheung (9) proposed that there is a similarity 244 LEWIS G . TILNEY, DAVID J . DEROSIER, and MICHAEL J . MULROY Department of Biology, University of Pennsylvania, Philadelphia, Pennsylvania 19104; The Rosenstiel Center, Brandeis University, Waltham, Massachusetts 02154, Department of Anatomy, Medical Center, University of Massachusetts, Worcester, Massachusetts 01605 between the stereocilia ofthe equilibrium organ and microvilli present in the apical surface of intestinal epithelial cells (20) . This latter preparation is capable of movement in vitro (18) . The hair cells in the equilibrium organ, in the lateral-line system of fishes and aquatic amphibians (see reference 8), and in the cochlea (see reference 8) all have a similar morphology . Accordingly, Flock and Cheung (9) suggested a functional homology among the stereocilia in the lateral line, in the equilibrium organ, in the cochlea, and in microvilli on intestinal epithelial cells, a contention with which we disagree . Our initial reason for investigating the stereocilia was that they represent another example of a highly ordered bundle of actin filaments . As we began a characterization of this unique bundle ofactin filaments, it became clear that an understanding of the structure of the stereocilium is essential for a determination of the mechanism of mechanical input into the cell . In THE JOURNAL Of CELL BIOLOGY " VOLUME 86 JULY 1980 244-259 © The Rockefeller University Press 0021-9525/80/07/0244/16 $1 .00 on Jne 7, 2017 D ow nladed fom Published July 1, 1980