046 THE MORPHOLOGY OF THE HUMAN HAIR FOLLICLE IN VERTICAL AND HORIZONTAL SECTIONS

David A. Whiting, Baylor Hair Research and Treatment Center

The human hair follicle comprises a permanent upper follicle with infundibulum and isthmus, and an impermanent lower follicle extending down from bulge to bulb. In anagen, the dermal papilla penetrates the bulb and forms a continuous unit with the dermal sheath, which surrounds the bulb. It nourishes the hair matrix containing transient amplifying cells, which form the hair shaft. The developing hair shaft is molded by the inner root sheath, which keratinizes in layers, first Henle, then cuticle, then Huxley. It is surrounded by external root sheath (trichilemma), hyaline membrane, and dermal fibrous sheath. The downward pointing cuticular cells of the inner root sheath interlock with the upwardly projecting cells of the hair shaft cuticle. The inner root sheath is shed in the isthmus, where it is replaced by trichilemmal keratin, which is in turn replaced by the keratinizing epithelium from the infundibulum. The dermal papilla is dragged upward in anagen when the hair bulb ascends the follicle to the level of the bulge, its path marked by the trailing fibrous tract or stela. The trichilemma is shrunk by apoptosis and the hyaline membrane thickens and convolutes. In telogen, the clubbed hair root is cushioned by a sac of trichilemmal keratin with the remains of the convoluted hair bulb below, forming the telogen germinal unit. The next anagen phase is indicated by a downward budding from the resting telogen root. The anatomy of the cycling and resting follicle will be displayed by matching vertical and horizontal sections.