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Development of the hair follicles in fetal human skin. (A) Basal epidermal cells become columnar and bulge slightly into the dermis. (B) Epidermal cells continue to proliferate, and dermal mesenchyme cells collect at the base of the primary hair germ to form a dermal papilla. (C) Hair shaft differentiation begins in the elongated hair germ. (D) The keratinized hair shaft extends from the hair follicle, the secondary bud forms the sebaceous gland, and beneath it is a region that may contain the hair stem cells for the next cycle of hair production. (E) Photograph of elongated hair germ. (After Hardy 1992 and Miller et al. 1993; photograph courtesy of W. Montagna.)