Skin is the biggest and most visible human organ. It is up to 3 mm thick and covers a total area of about two square metres. And it has many functions - keeping out the rain and harmful germs, protecting us from sun, and helping to keep us cool. It is equipped with pain and touch receptors and it produces vitamin D and hormones, decorative hair and useful finger nails. Skin also produces our characteristic human odour.
Skin is a two-layered structure. The outer layer is called the epidermis and is composed of keratinocytes, cells that produce the fibrous proteins known as keratin. Keratin assembles into tough filaments and is particularly plentiful in finger/toe nails and hair.
The outer part of the epidermis is called the stratum corneum, composed of dead keratinocytes packed with keratin. This layer slows down the loss of body water to the outside and protects against the absorption of damaging chemicals.
Everyday wear and tear continually rub off dead keratinocytes but they are replaced by keratinocytes from the deeper epidermis. The entire skin surface is replaced in this manner about once a month. Another type of cell in the epidermis is the melanocyte which produces the brown pigment called melanin. The colour of skin is mainly due to melanin - darker-skinned people secrete more melanin than paler-skinned people.
The inner layer of the skin is called the dermis. It is thicker than the epidermis and confers strength and elasticity on the skin. It is mainly a mesh of strong (collagen) and elastic (elastin) protein fibres, embedded in a gel of sugary molecules.
Also present are blood vessels, nerve endings, and fibroblasts (cells that secrete collagen and elastin). The blood supplies nutrients to the cells and the nerves allow us to experience pain, cold, heat and the surface texture of substances that we touch.
Epidermal cells grow and divide at just the right rate to keep the skin fitting tightly over the body. In psoriasis too many cells are produced and excess cells flake off leaving red patches. The underlying cause of psoriasis is unclear.
Eczema, or dermatitis, is an inflammation of the epidermis and upper dermis resulting in severe itching. It can be caused by irritating chemicals. Dandruff is a mild dermatitis in which the shedding of dead cells from the stratum corneum is stimulated.
Skin contains two types of sweat-producing glands, eccrine and apocrine glands. Eccrine glands occur all over the body. Apocrine glands are concentrated in the groin and arm pit and develop at puberty. Perspiration produced by the eccrine glands is 99 per cent water that cools the body as it evaporates. Most of the time we are unconscious of this water loss - an adult loses about half a litre of water per day. In severe heat the rate of water loss from eccrine glands may increase ten-fold.
Apocrine glands produce a milky secretion that increases markedly when we are nervous. The secretion is odourless, but is broken down by skin bacteria to produce a volatile mixture of chemicals with a musky smell. Normal hygiene prevents the build up of this odour, but unhygienic habits quickly allow a powerful "hum" to develop. Deodorants temporarily turn down the apocrine glands and inhibit the growth of bacteria in the arm pit. The dermis also contains the sebaceous or grease-producing glands. They are attached to hair shafts and they produce a greasy liquid known as sebum. The sebaceous glands are quiescent until the onset of puberty. The sebaceous gland is connected to the skin surface by a duct. Acne results when a high production of sebum blocks the duct. Blackheads reflect a partial blocking of a duct. The dark centre is a concentration of melanin. A whitehead results from a complete blocking of a duct. Sex hormones are produced by the ovaries and the testicles at puberty, beginning the production of sebum, which leads to acne. Exposure of skin to sunlight is essential for good health. For example, exposure to sunlight produces vitamin D in the skin which is essential for healthy bones. But over-exposure to sunlight is dangerous.
The potentially dangerous component of sunlight is ultraviolet (UV) radiation, which can induce chemical changes in DNA that might eventually produce a cancer. The skin has a mechanism to protect against UV radiation - the UV-absorbing melanin pigment produced by the melanocytes. Some skin types are naturally better at this than others. At one extreme you have the black skin of the African which secretes massive amounts of melanin on stimulation by UV. At the other extreme is the milky white skin of the albino who suffers from a disorder in which no melanin is produced. Fair-skinned Celts have skins that are poorly protected against UV. The skin-darkening of the sun tan reflects diffusion of melanin pigment throughout the skin as it attempts to protect against UV radiation. Fair-skinned Celts do not tan easily on exposure to sun, tending rather to burn, which is dangerous. The only sensible response is for such people to wear light protective clothing in strong sunlight.
Skin cancer is the most common cancer, more common than all other forms of cancer combined. Skin cancer comes in several forms including basal cell carcinoma and malignant melanoma. Basal cell carcinoma develops from a keratinocyte, is the commonest form of cancer and usually develops in people aged 50 and over.
It can usually be treated successfully, once caught at an early stage. Malignant melanoma is a tumour of a melanocyte and is quite dangerous. It spreads quickly and is seriously life-threatening if not dealt with at a very early stage. The number of people dying of melanoma in this part of the world doubles about every 10 years, probably due to the great popularity of sun holidays.
William Reville is a senior lecturer on biochemistry at UCC.