In contrast to the actin cytoskeleton in the main body of the cell where the actin network forms a gel-like structure that is isotropic, many cells have slender microvilli or processes in which the actin network is highly organized into cross-linked axial arrays in which the long axes of the actin filaments are parallel to one another. Four prominent examples of such structures are the brush border microvilli of the proximal tubule and small intestine, the hair cells of the inner ear and the cell processes of osteocytes. Except for the stereocilia on the hair cells in the inner ear, very little attention has been paid to these actin bundles as possible mechanotransducers. This is due in large measure to the large difference in function of the cells and the organs in which the cells reside. The stereocilia on hair cells have been extensively studied because it is well recognized that they respond to accoustic waves and sense the directionality of linear and angular momentum. The stereocilia also have a highly specialized structure in which they are arranged in rows of ascending height in which their tips are cross-linked in a special manner. Deflections across the rows lead to depolarization or hyperpolarization of the cells whereas bending along a row causes no electrical response. In this paper we will explore new hypotheses for the other three cell types as mechanotransducers. In particular we will want to examine the relationship between structure and function and quantitatively compare the magnitude of the deformation of the actin bundles for physiological loading. The close similarity between the actin cytoskeleton of brush border microvilli and the osteocytic cell process has only recently been realized, Tanaka-Kamioka et al.. Both actin bundles have the same cross-linking molecules, fimbrin and α-Actinin, and a filament density that is comparable. Curiously, the actin bundle structure that has been most elucidated at the molecular level is the intestinal microvillus, Furukawa and Fechheimer. The least is known about the proximal tubule brush border microvilli which appear to have the simplest structure, namely 6-8 central actin filaments, Maunsbach and Christensen.
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