Brush borders and the resulting enlargement of the luminal cell membranes are characteristic of resorptive epithelium. Brush borders are highly specialized structures. They are dense with microvilli, which are covered by a glycocalix (see Figs. 77-79). This cross-section shows the proximal tubules of a nephron. A high brush border, stained light blue with azan, covers the single-layered, medium height epithelium of the proximal tubules. Approximately 6000-7000 microvilli cover each of the epithelial cells from the proximal tubule. The microvilli can be up to 0.2 |im long. The left half of the figure shows a cut through the center part of the distal tubules [2 of the kidney (cf. Figs. 73, 497, 498).
1 Brush border
2 Distal tubule
Stain: azan; magnification: x 800
oo Microvilli are finger-like protrusions from the plasmalemma. They are about ID 50-120 nm thick and up to 3 |imm long. In resorptive epithelium (e.g., in en-^ terocytes from the small intestine and epithelium from the proximal renal tubules) the microvilli form a dense lawn, which can be recognized as a brush border in light microscopy (cf. Figs. 73, 77, 78).
Electron microscopy reveals that the rod-shaped processes (microvilli), which extend from the free surface of the epithelial cells, are encased by the plasmalemma (three-layered structure). On the microvilli surface is a finely granular, sometimes filament-like material (cf. Figs. 77, 78). This layer is the glycocalix (cell coat) (cf. Fig.79). The glycocalix (calyx, Greek: chalice, bud) consists of the polysaccharide chains of glycolipids and glycoproteins. Enlargement will bring out these chains as antennulae microvillares. The glycocalix provides the basis for cell specificity. It exposes antigenic determinants, which determine the serological attributes of cells. The glycocalix mediates cell-cell recognition.
Microfilaments (microfilament bundles) □ traverse the microvilli parallel to the long axis. They consist of actin and actin-binding proteins (see Fig. 56) and extend to the terminal complex (terminal web) [2 in the apical cytoplasm. The terminal web also contains myosin at its border. This apical terminal web area of the epithelial cell is mostly free of organelles. Duodenal enterocytes.
1 Bundled actin filaments providing mechanical support (skeleton)
2 Terminal web
3 Crista-type mitochondria
Electron microscopy; magnification: x 34 000
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