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 containing a core of actin filaments;  stereocilia (stereovilli), microvilli of unusual length; and  cilia, cytoplasmic processes containing bundle of micro- tubules.
Microvilli are fingerlike 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, bleblike 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 the cell’s absorptive capacity. Thus, cells that principally transport fluid and absorb metabolites have many closely packed, tall
FIGURE 5.2 • Electron micrographs showing variation in microvilli of different cell types. a. Epithelial cell of uterine gland; small projections. b. Syncytiotrophoblast of placenta; irregular, branching microvilli. c. Intestinal absorptive cell; uniform, numerous, and regularly arranged microvilli. All figures 20,000.
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 tubules), a distinctive border of vertical striations at the apical surface of the cell, representing an astonishing number of 15,000 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, any microvilli present are usually short and not numerous, which explains why they may escape detection in the light microscope. The variations seen in microvilli of various types of epithelia are shown in Figure 5.2. 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.
The internal structure of microvilli contains a core of actin filaments that are cross-linked by several actinbundling proteins.
Microvilli contain a conspicuous core of about 20 to 30 actin flaments. Their barbed (plus) ends are anchored to villin, a 95-kilodalton actin-bundling protein located at the tip of the microvillus. The actin bundle extends down into the apical cytoplasm. Here it interacts with a horizontal network of actin filaments, the terminal web, that lies just below the base of the microvilli (Fig. 5.3a). The actin filaments inside the microvillus are cross-linked at 10-nm intervals by other actin-bundling proteins such as fascin (57 kilodaltons), espin (30 kilodaltons), and fmbrin (68 kilodaltons). 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 micro-villus. The addition of villin to epithelial cells growing in cultures induces formation of microvilli on the free apical surface.