Ultrastructural Changes of the Basal Lamina during the Hair Growth Cycle

The "hyaline membrane" (glassy or vitreous membrane), the noncellular layer that invests the entire hair follicle, is visible with the light microscope. During hair growth cycles (5), this membrane undergoes characteristic morphological modifications. Under the light microscope, the hyaline membrane appears thin and inconspicuous around the follicle during anagen and telogen (growing and resting phases of the hair follicle). At the onset of catagen (the transitional phase between anagen and telogen), the PAS-positive membrane around the lower part of the follicle becomes hypertrophied. In the later stages of catagen, this lower section of the follicle atrophies and the surrounding hyaline membrane appears like a thick wrinkled sac (2, 5, 13). Under the electron microscope, the hyaline membrane of anagen follicles consists of an inner basal lamina and two outer layers of orthogonally arranged collagen fibers (7, 9). The ultrastructure of the thickened hyaline membrane of the catagen follicle, however, has not been described. In this study, I have observed that during catagen the basal lamina undergoes an extensive pleating process that causes the hyaline membrane to resemble the thick wrinkled sac seen under the light microscope. The details of these ultrastructural changes are reported below.