Identification of a cell layer containing alpha-smooth muscle actin in the connective tissue sheath of human anagen hair.

Immunohistochemical and immunoelectron microscopy studies revealed the presence of alpha-smooth muscle (alpha-SM) actin in fibroblasts located in the connective tissue sheath (CTS) of human anagen hair follicles. Immunostaining was positive from the base of the bulb to the upper part of the lower portion of the mature anagen hair follicles. The late catagen hair follicles did not stain. Ultrastructurally, alpha-SM actin was detected only in the fibroblasts located in the innermost layer of the transverse collagenous fibres. Since alpha-SM actin is located in cells with contractile potential, this newly identified layer may play an important role in the morphological changes of the lower portion of the hair follicle during the hair growth cycle.