Morphogenesis of the Hair Follicle during the Ontogeny of Human Skin

Abstract

Hair follicle development in the human foetus begins subsequent to epidermal stratification at 60 days estimated gestational age when clusters of basal cells form a hair germ. Hair germs bud into the dermis, elongate (hair pegs) and become surrounded by an accumulation of fibroblastic cells. Hair immunolabeling with anti-keratin antibodies suggests that the outer layer of follicle cells is derived from basal keratinocytes, whereas cells in the follicle core are continuous with intermediate cells. During the fourth month the follicle develops into a bulbous hair peg; bulges representing primordia of the apocrine gland, the sebaceous gland (intermediate) and the bulge (distal) form on its posterior aspect. , The tetminus infolds to form a concavity that encompasses cells of the dermal papilla. Matrix cells lining the roof of the bulb give rise to concentric layers of the inner root sheath and hair. These layers can be recognized morphologically and by labeling with several monoclonal antibodies. An intra-epidermal hair canal forms within the hair tract and the infundibulum of the follicle by keratinization identical to that of the interfollicular epidermis. Hairs grow into the hair canal and are exposed around 19–21 weeks by erosion of the roof of the hair canal at the skin surface.