Putting on a good face – the chemistry of cosmetics
Box 2 | Only skin deep?
Our skin is our boundary with the outside world. With protective, regulatory and sensory roles, the skin is far more than just a covering to the body. It is more complex than a quick look at its surface suggests. Like all other body organs it is make up of numerous cells in various tissues that are organised together to perform its functions. It is the body’s largest organ, comprising about 12 per cent of the total body weight.
The skin helps the body regulate the correct fluid balance by reducing water loss through its surface and preventing the absorption of too much water during baths or swimming. It helps the regulation of temperature – fatty tissues conserve heat, and the sweat glands cool the body surface when necessary. Variation in blood supply to its surface also regulates the amount of heat lost. Skin also helps with the disposal of waste products, especially through the sweat glands. It is involved in the synthesis of vitamin D and the metabolism of some hormones. The skin has several important sensory functions, particularly the detection of contact and heat.
The layers of the skin
Human skin consists of two distinct layers – the epidermis (on the outside) and the dermis.
The epidermis is made up of several layers of cells
The epidermis consists of layers of cells. Old cells are continuously shed or worn away from the surface and replaced by new cells generated by the cube-shaped cells of the basal layer. Newly formed epidermal cells move upwards through the middle layers until they reach the outermost layer, from which they are eventually shed. As the cells migrate, they mature and become filled with a tough protein, keratin. Finally they compact to form the flat keratin plates that make up the skin’s surface.
Melanocytes are specialised pigment-producing cells in the basal layer of the epidermis. Their long ‘arms’ extend between the cells of the epidermis. They produce melanin, which is the protein responsible for skin colour. Melanocytes inject melanin into the surrounding epidermal cells, which then carry the pigment upwards to the surface as they mature. All humans, fair and dark, have about the same number of melanocytes but the rate of pigment production varies enormously. The amount of melanin produced is in part determined genetically, but it also increases in response to exposure to sunlight. In darker skin, pigment is produced much faster and in greater amounts than in fair skin. If melanin is produced unevenly freckles form.
The dermis layer contains various important structures
The dermis consists largely of supportive tissue, but it also contains important structures like hair follicles, sweat glands and sense organs.
The supportive tissue, which is a network of protein fibres embedded in a jelly-like substance, gives the skin its strength, elasticity and resilience. The protein fibres and the jelly-like substance are produced by special cells, the fibroblasts, that are scattered through the dermis.
The hair grows in follicles in the dermis. Hair itself is a roughly cylindrical strand of dead cells. It grows from the bottom of the follicle tube. Each hair has a small muscle that pulls it from its normal slanting position to a more erect one and gives us the feeling that ‘our hair is standing on end’ when we are frightened or cold.
Sweat glands also originate in the dermis and open to the surface of the epidermis. Each gland consists of a coiled narrow tube with a long duct to the surface. In releasing sweat to the surface, where it cools the skin by evaporation, sweat glands perform an important heat regulatory function.
The nerve fibres of the skin also form finely branching networks just below the epidermis. Some of the nerve fibres are specially adapted to carry sensation such as touch, heat, cold and pain. Areas of sensitive skin such as the fingertips contain large numbers of these specialised nerve endings.
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Box 1. Ingredient labelling
Related site
The
skin (St Vincent’s Hospital, Melbourne)
Posted October 2004.