Structure of the Human Fetal Hair Canal and Initial Hair Eruption

Late stages of hair follicle development and the eruption of the first fetal hairs were studied from several body regions of 15-, 17-, and 21-week-old human fetuses by scanning electron microscopy. At 15-17 weeks of age epidermal and subepidermal portions of the follicles were seen from the epidermal basal layer surface as cellular cords that formed the hair tracts (intraepidermal) and hair pegs (dermal). At this stage keratinization was restricted to follicular structures of the scalp; the epidermal surface of the entire body was still covered with periderm. By 21 weeks fetal age, various stages in development of hair could be recognized in all areas of the body by observation of the epidermal surface. On the lower extremities and buttocks, regularly spaced, elongate ridges marked the position of intraepidermal hair canals which had been excavated within the hair tract. Over certain of these ridges the periderm was interrupted, and keratinized cells that lined the canal were exposed. Hair canals were even longer on the trunk and upper extremities and the epidermal surface at these sites had thinned to the extent that hairs contained within the canals were visible within cellular sheaths or were partially released. Long hairs projected from the surface of the face and scalp, and in these regions exclusively, the interfollicular epidermis also was keratinized. These studies by SEM showed that hairs developed within elongated hair canals which opened at the epidermal surface to release fully elongated hairs. Hair release occurred concomitant with epidermal keratinization and both events proceeded in a cephalocaudal direction. Studies by light and transmission electron microscopy were carried out to confirm the state of the follicle and hair development and to supply cytological detail of the developing hair canal