Human beings are ‘shrink-wrapped’ in a material of enormous complexity. The skin covers the body and is its largest tissue, covering 1.7 square metres and weighing about 3 kilogrammes. It is flexible, waterproof and self-renewing, and is the first line of defence against invasion by bacteria and other infections. It affords mechanical protection against injury to deeper and more delicate organs. It also plays a role in regulating body temperature, the production of vitamin D, excretion, and as a sensory organ that keeps us ‘in touch’ with the outside world.
Structure of the skin
There are two main layers of skin – the epidermis, which is closest to the surface, and the deeper lying dermis. The epidermis is covered with a layer of keratin (a tough protein substance) composed of deadsoftened by greasy secretions from sebaceous glands and moistened by sweat. Fat solvents or emul-sifiers such as detergents remove grease and leave the keratin stiff and harsh. Prolonged contact with water leads to the eventual breakdown of the skin’s waterproofing and the absorbion of water, making the skin thick and white – as seen in ‘washerwoman’s finger’. Below this keratin layer are several layers of living , and below these a layer of actively-dividing cells. As dead cells are rubbed from the surface the new cells quickly move upwards to replace them. The keratin on the epidermis of the fingers is determined genetically and forms a pattern of fingerprints unique to each individual.
Whereas the epidermis provides protection, it is the dermis that gives skin its elasticity and strength. The dermis is built around a network of protein fibres and contains nerve endings, sweat glands, lymphatic andvessels and hair follicles.
Sweat glands are widely distributed throughout the skin, mainly in the palms, soles of feet, axillae (armpits) and groin. The duct of the gland passes through both the dermis and the epidermis to open as a minute, or pore, on the surface of the skin. Sweat glands excrete water, salt and other products from the body. The broken-down products of such foods as garlic and curry can also be found (and can sometimes even be smelled) in sweat.
Hair follicles consist of a downward growth of epidermal cells into the dermis and often into the tissue below the skin. Hair growth occurs by multiplication of a cluster of cells called a bulb, which is situated at the base of the follicle. Each hair consists of the shaft, which protrudes from the surface, and the root below the surface of the skin.
Hair follicles are connected to little bundles of muscle fibres which, on contraction, erect the hair and ele- vate the skin around the hair causing ‘goose pimples’. Hair colour depends on the amount of the pigment melanin present. White hair is the result of the replacement of melanin by tiny air bubbles. The average adult has 100,000 head hairs, growing at the rate of 12cm each year.
The sebaceous glands associated with hair follicles consist of secretory cells. They pour their secretions, collectively known as sebum, into the follicle. They are present all over the body, except for the soles of the feet and palms, but are mainly found in the skin of the scalp, face, axillae and groin. Sebum keeps hair soft, pliable and shiny. It provides some waterproofing and prevents drying. Excess sebum clogs up the pores, encouraging the formation of pimples and acne .
Human nails are equivalent to claws, horns and hooves of animals and serve to protect the fingers and toe tips. They are composed of hard, horn-like, keratinized dead cells. A nail has its root embedded in skin and it grows from the nail bed to cover the dermis at the tops of the toes and fingers.
Pigmentation and skin colour
This is produced by mobile cells containing pigments, of which the two most important are melanin – a brown pigment found in cells in the epidermis called melanocytes – and carotene – a yellow pigment found in the keratin layer of the epidermis. All races have the same number of melanocytes; it is genetic differences that determine the amount of melanin manufactured in the skin. If pigmentation is completely lacking, the skin and hair are white – a condition known as albinism. Freckles occur in places where small groups of melanocytes are active.
Melanin production is stimulated by ultraviolet radiation from the sun. As melanin absorbs sunlight it acts as a protection from these harmful rays. At the same time vitamin D, which is essential for normal bone formation and maintenance, is formed in the skin.
Regulation of body temperature
Body temperature is maintained by controlling the volume offlowing close to the surface of the skin. In hot weather this is increased, with a subsequent increase in heat loss. In cold weather the reverse occurs.
With age, the elastic fibres of the dermis become less efficient, leading to inelastic skin which settles into age creases. Facial wrinkles lie at right-angles to the line of pull of the underlying muscles, so there are horizontal wrinkles on the forehead, crow’s feet wrinkles at the outer angles of the eyes and vertical wrinkles above and below the lips.