Structure and function of the brush-border cytoskeleton. Cold Spring Harbor:. Syrup. 46

Our understanding of cell structure and movement has been advanced by the application of modern mi-croscopic and biochemical methods to the study of sev-eral biologic systems. The brush border of absorptive epithelial cells is one such system that historically has provided important information about the interaction between actin and its associated proteins in the cyto-plasm. Recent biochemical studies have identified the actin-associated proteins found in the brush border, but an understanding of the functional roles of these proteins first requires knowledge of the morphological and biochemical characteristics of the cytoskeleton. Early electron microscopy studies by Millington and Finean (1962) revealed that the brush border is com-posed of a densely packed, hexagonal array of micro-villi and the terminal web. The terminal web is com-posed of a meshwork of filaments which excludes large cell organelles uch as mitochondria nd ribosomes (Fig. 1). An axial core of microfilaments extends from the tip of the microvillus basally into the terminal web. These microfilaments contain actin (lshikawa et al. 1969) as identified by heavy meromyosin (HMM) or S I decoration and are unidirectionally polarized with respect o the membrane, i.e., the arrowheads in SI-decorated microvilli all point away from the distal tip of the microvillus and toward the terminal web (Moo-seker and Tilney 1975; Begg et al. 1978). The core microfilaments are packed into a bundle, which, in turn, is associated with the overlying membrane through two types of attachments. In the first type, the distal end of the core bundle terminates at an electron-dense region in the microvillus tip, whereas in the sec-ond type, the core bundle is connected laterally to the membrane by cross filaments (Mukerjhee and Stae-helin 1971). In longitudinal sections through Mg + '-extracted microvilli, these cross filaments appear spaced every 33 nm along the l ngth of the core bundle (Mooseker and Tilney 1975). The core bundle extends into the terminal web as rootlets of the microvilli. Sur-rounding the rootlets is a meshwork of filaments that includes microfilaments (5-7-nm dia) (Ishikawa et ai