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 x20,000.

its contractile ability, which could have the effect of decreasing the diameter of the apex of the cell, causing the microvilli, whose stiff actin cores are anchored into the terminal web, to spread apart and increase the intermi-crovillous space.

Stereocilia are unusually long, immotile microvilli

Stereocilia are not widely distributed among epithelia. They are, in fact, limited to the epididymis, to the proximal part of the ductus deferens of the male reproductive system, and to the sensory (hair) cells of the ear. They are included in this section because this unusual surface modification is traditionally treated as a separate structural entity.

Stereocilia of the genital ducts are extremely long processes that extend from the apical surface of the cell and facilitate absorption. Unique features include an apical cell protrusion from which they arise and thick stem portions that are interconnected by cytoplasmic bridges. Because electron microscopy reveals their internal structure to be that of unusually long microvilli, some histologists now use the term stereovilli (Fig. 4.4a). Seen in the light microscope, these processes frequently resemble the hairs of a paint brush because of the way they aggregate into pointed bundles.

Like microvilli, stereocilia are supported by internal actin filament bundles that are cross-linked by fimbrin. Unlike microvilli, a plasma membrane-associated molecule, erzin, anchors the actin filaments to the plasma membrane of stereocilia. The stem portion of the stereocilium and the apical cell protrusion contain the cross-bridge-forming molecule a-actinin (Fig. 4.4b). A striking difference between microvilli and stereocilia, other than size and the presence of erzin, is the absence of villin from the tip of the stereocilium.

Stereocilia of the sensory epithelium of the ear are uniform in diameter and possess an internal structure similar to that of genital duct stereocilia. However, they lack both erzin and a-actinin. These stereocilia serve as sensory receptors rather than absorptive structures.

Cilia are motile cytoplasmic structures capable of moving fluid and particles along epithelial surfaces

Cilia possess an internal structure that provides for their movement. In most ciliated epithelia, such as the trachea, bronchi, or oviducts, cells may have as many as several hundred cilia, all arranged in orderly rows. In the tracheobronchial tree, the cilia sweep mucus and trapped particulate material toward the oropharynx where it is swallowed with saliva and thus eliminated from the body. In the oviducts, cilia help transport ova and fluid toward the uterus.

In some epithelia, only a single cilium per cell may be present, e.g., the epithelial cells of the rete testis in the male reproductive tract and the vestibular hair cells of the ear. In these instances, the single cilium is thought to have a sensory role.

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