Occurrence of actinlike microfilaments in the outer root sheath cells of the hair follicle: a possible role of a cytoskeleton.

Broad bundles of actinlike microfilaments are found in the basal cells of the outer root sheath of the hair follicle. The filaments react specifically with heavy meromyosin to form a fuzzy structure, and disappear from the cells after the actin depolymerization treatment. An array of the filament bundles alters along with the cell shape changes of the basal cells. In the flattened cells of the hair follicle bulb, the bundles appear to run parallel to the long axis of the cells, and are present in the basal cytoplasm. In the cuboidal cells of the suprabulbar and the more superficial portion of the follicle, the bundles are perpendicular to the basal plasma membrane, and are disposed mainly in the basal cytoplasm. The microfilaments are associated with the basal plasma membrane directly or via insertion into the plaque of the membrane, where filamentous or amorphous materials link the membrane and the basal lamina. In vitro treatment of cytochalasin B for up to 8 hr causes no visible change in the distribution pattern of the bundles nor abnormal changes in the cell shape. Thus, the actinlike microfilament bundles appear to provide a cytoskeletal system responsible for the maintenance of the cell shape change, since they are disposed in the direction and area in which a maintenance force for the cell shape change may be required, and other cytoskeletal systems of the cells such as microtubules and tonofilament-desmosome complexes are poorly developed, and no hemidesmosomes are present in the cells.