Derivatives of the skin
Hair
There are three types of hair:
1. Lanugo hairs are fine and long, and are formed in the fetus at 20 weeks’ gestation. They are normally shed before birth, but may be seen in premature babies.
2. Vellus hairs are the short, fine, light-coloured hairs that cover most body surfaces.
3. Terminal hairs are longer, thicker and darker, and are found on the scalp, eyebrows, eyelashes and also on the pubic, axillary and beard areas. They originate as vellus hair; differentiation is stimulated at puberty by androgens.
Structure
The hair follicle is an invagination of the epidermis containing a hair. The portion above the site of entry of the sebaceous duct is the infundibulum. The hair shaft consists of an outer cuticle that encloses a cortex of packed keratinocytes with (in terminal hairs) an inner medulla (Fig. 1). The germinative cells are in the hair bulb; associated with these cells are melanocytes, which synthesize pigment. The arrector pili muscle is vestigial in humans; it contracts with cold, fear and emotion to erect the hair, producing ‘goose pimples’.
Nails
Structure
The nail matrix contains dividing cells which mature, keratinize and move forward to form the nail plate (Fig. 2). The nail plate has a thickness of 0.3–0.5 mm and grows at a rate of 0.1 mm/24 h for the fingernail. Toenails grow more slowly. The nail bed, which produces small amounts of keratin, is adherent to the nail plate. The adjacent dermal capillaries produce the pink colour of the nail; the white lunula is the visible distal part of the matrix. The hyponychium is the thickened epidermis that underlies the free margin of the nail.
