Skin appendages

Hair, skin glands and nails arise from epithelial buds, which grow into the dermis. Hairs form hair follicles that each enlarge at their deep end to form a hair bulb. It comes to surround a dermal papilla of vascular mesoderm. The innermost cells of the hair follicle become keratinized. This forms the hair shaft, which is moved towards the surface by the division of matrix cells in the hair root. The outer cells of the hair follicle form the epithelial root sheath. A fibrous root sheath wraps around the epithelial root sheath, with a thick basement membrane (glassy membrane) lying in between. The arrector pili muscle is attached at the fibrous root sheath. Above this site of muscle attachment, the primordium of the sebaceous gland sprouts from the wall of the epithelial root sheath.
During the transition from the embryonic to the fetal period, primary hairs appear on the eyebrows, the upper lip and the chin. These hairs are initially fine and woolly (lanugo). From month 8 until birth, they are replaced by a second generation of hairs: the thicker terminal hairs on the eyebrows, eyelids and the head, and elsewhere the vellus-hairs, which look like lanugo hairs. Further terminal hairs appear during puberty (pubis, armpits and face).