Derivatives of the skin

Hair

Hairs are found over the entire surface of the skin, with the exception of the glabrous skin of the palms, soles, glans penis and vulval introitus. The density of follicles is greatest on the face. Embryologically, the hair follicle has an input from the epidermis, which is responsible for the matrix cells and the hair shaft, and the dermis, which contributes to the papilla, with its blood vessels and nerves.

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’.

Fig. 1 Structure of the hair follicle.

Nails

The nail is a phylogenetic remnant of the mammalian claw and consists of a plate of hardened and densely packed keratin. It protects the fingertip and facilitates grasping and tactile sensitivity in the finger pulp.

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.