Appendages of the skin - part I

These are nails, hairs, sebaceous, sweat and mammary glands, and are all derived from the epidermis.

Nails

The nail consists of an approximately rectangular plate of horny tissue found on the dorsum of the terminal phalanx of the fingers, thumb and the toes. They are a special modification of the two most superficial layers of the epidermis, particularly the stratum lucidum. Its transparency allows the pinkness of the underlying highly vascular nail bed to show through. The nail is partly surrounded by a fold of skin, the nail wall, and is firmly adherent to the underlying highly vascular nail bed with some fibres ending in the periosteum of the distal phalanx. It is this firm attachment which enables the nails to be used for scratching and as instruments for prizing open various objects.

The distal end of the nail is free, while the proximal covered part constitutes the nail root. There is an abundant supply of sensory nerve endings and blood vessels to the nail bed. The nails grow at approximately 1mm per week, being faster in summer than in winter.

Hairs

Hairs are widely distributed over the body surface, notable exceptions being the palm of the hand and the sole of the foot. Hairs vary as to their thickness and length. Most of them are extremely fine so that the skin may appear hairless. There is a marked sexual difference in the distribution of coarse hair, particularly on the face and trunk, and in its loss from the scalp. This coarse hair tends to become more prominent after puberty, particularly in the axilla, over the pubes, and on the face in males.

Except for the eyelashes, all hairs emerge obliquely from the skin surface, with the hairs in any one region doing so in the same direction. The part of the hair which projects from the skin surface is the shaft, with that part under the skin being the root, which is ensheathed in a sleeve of epidermis known as the follicle extending into the subcutaneous tissue. The shaft appears circular in cross-section. Throughout most of its length the hair consists of the keratinized remains of cells. Hair colour is due to pigment in the hair cells(melanin and a subtle red pigment), and to air within the shaft of the hair. The hair of the head has a life of between 2 and 4 years, but that of the eyelashes is only 3 to 5 months. All hairs are intermittently shed and replaced.

In the growing hair, the deepest part of the hair follicle expands to form a cap, known as the bulb of the hair, which almost completely surrounds some loose, vascular connective tissue, known as the papilla. The cells of the follicle around the papilla proliferate to form the various layers of the hair. In the resting hair follicle the bulb and papilla shrink, with the deepest part of the follicle being irregular in shape.

Associated with each hair are one or more sebaceous glands, which lie in the angle between the slanting hair follicle and the skin surface with their ducts opening into the neck of the follicle. Bundles of smooth muscle fibres(the arrector pullorum) attach to the sheath of the hair follicle, deep to the sebaceous gland, and pass to the papillary layers of the dermis on the side towards which the hair slopes(picture b). Contraction of the muscle causes hair to stand away from the skin, elevating the skin around the opening of the hair follicles, thereby producing “goose flesh”. This action also compresses the sebaceous glands causing them to empty their secretions onto the skin surface. Elevation of the hairs traps a layer of air against the skin surface in an attempt to produce an insulating layer to reduce heat loss, while the sebaceous secretions are important in “water-proofing” the skin surface and in aiding the absorption of fat-soluble substances through the skin.