O The Apical Domain And Its Modifications

In many epithelial cells, the apical domain exhibits special structural surface modifications to carry out specific functions. In addition, the apical domain may contain specific enzymes (e.g., hydrolases), ion channels, and carrier proteins (e.g., glucose transporters). The structural surface modifications include

• Microvilli, cytoplasmic processes that extend from the cell surface

• Stereocilia (stereovilli), microvilli of unusual length

• Cilia, motile cytoplasmic processes

Microvilli are finger-like cytoplasmic projections on the apical surface of most epithelial cells

As observed with the electron microscope (EM), microvilli vary widely in appearance. In some cell types, microvilli are short, irregular, bleb-like projections. In other cell types, they are tall, closely packed, uniform projections that greatly increase the free cell surface area. In general, the number and shape of the microvilli of a given cell type correlate with its absorptive capacity. Thus, cells that principally transport fluid and absorb metabolites have many closely packed, tall microvilli. Cells in which transepithelial transport is less active have smaller, more irregularly shaped microvilli.

Among the fluid-transporting epithelia, e.g., those of the intestine and kidney tubule, a distinctive border of vertical striations at the apical surface of the cell, representing the close packed microvilli, is easily seen in the light microscope. In intestinal absorptive cells, this surface structure was originally called the striated border; in the kidney tubule cells, it is called the brush border. Where there is no apparent surface modification based on light microscope observations, microvilli, if present, are usually short and not numerous; thus, they may escape detection in the light microscope.

The variations seen in microvilli of various types of epithelia are shown in Figure 4.2. The microvilli of the intestinal epithelium (striated border) are the most highly ordered and are even more uniform in appearance than those that constitute the brush border of kidney cells. They also contain a conspicuous core of actin filaments (microfilaments). Actin filaments are anchored to villin located in the tip of the microvillus and extend down into the apical cytoplasm. Here, they interact with a horizontal network of actin filaments, the terminal web, which lies just below the base of the microvilli (Fig. 4.3a). The actin filaments inside the microvillus are cross-linked at 10-nm intervals by the actin-bundling proteins fascin and fimbrin. This cross-linkage provides support and gives rigidity to the microvilli. In addition, the core of actin filaments is associated with myosin I, a molecule that binds the actin filaments to the plasma membrane of the microvillus. The addition of villin to epithelial cells growing in cultures induces formation of microvilli on the free apical surface.

The terminal web is composed of actin filaments stabilized by spectrin, which also anchors the terminal web to the apical cell membrane (Fig. 4.3b). The presence of myosin II and tropomyosin in the terminal web explains

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