ACTIN BUNDLES IN HUMAN HAIR-FOLLICLES AS REVEALED BY CONFOCAL LASER MICROSCOPY

A confocal laser microscope was used to examine the distribution patte rn of actin bundles in whole-mounts of human hair follicles stained wi th fluorescently labeled phalloidin. Actin bundles were found exclusiv ely in the epithelial outer root sheath of the lower and middle portio ns of the follicle. In the growth stage, the lower follicle was charac terized by well-developed actin bundles arranged circumferentially in the innermost and outermost cell layers of the outer root sheath. Acti n bundles in the innermost cells were aligned end-to-end so that they formed complete circular bands surrounding the inner root sheath. In t he outermost cells, actin bundles ran underneath the basal plasma memb rane to which they attached at both ends. In contrast, in the quiescen t stage, actin bundles in the lower follicle were disposed radially to ward the follicle surface where they terminated perpendicular to the b asal plasma membrane. In the middle follicle, circumferential actin bu ndles were found only in the intermediate layer of the outer root shea th throughout the hair cycle. Immunofluorescent anti-myosin and anti-a -actinin staining showed a striated pattern along actin bundles. Vincu lin was localized at both ends of actin bundles, corresponding to the cell-to-cell or cell-to-substrate adherens junctions. Glycerinated fol licles changed in shape on the addition of MgATP, suggesting a contrac tion of actin bundles. From these observations, we conclude that actin bundles in the hair follicle are comparable to stress fibers and that they serve as a tensile scaffold for the growth and integrity of the follicle.