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The skin has three layers:
1. The epidermis on the outside. This is made from layers of cells with a basal layer, which is always forming new cells through cell division. The new cells gradually move towards the surface, which takes 1-2 months. As they move up, they gradually die, become flattened and develop keratin and the outermost layer of flat dead cells is being continually worn away by friction. The keratin and oil from the sebaceous glands help to make the skin waterproof.
2. The dermis is the inner layer. The following tissues and structures can all be found in the dermis:
3. There is a layer of fat underneath and in the lower regions of the dermis. The thickness of this layer varies depending on the place in the body and from person to person. A store of fat is useful to the body as insulation, and it can be used for energy when the intake of nutrients is insufficient.
Body cells can detect how crowded they are – the cell density. When cell density decreases, cell division occurs to make new cells and when cell density increases the rate of cell division slows down. This process is usually strictly controlled in the body, however, occasionally, the control mechanism fails and cell division continues at an abnormally high rate. This is how tumours occur in cancer. Cell division is important in skin repair after a cut or other injury to the skin. On the cut surface cell density is lowered, stimulating cell division. New cells gradually fill the gap and, once normal cell density is reached again, cell division slows down to the normal rate. Usually a cut will first fill up with blood which clots and, with other fibres, forms a scab. Underneath the scab the new tissue is being produced to heal the wound.
In recent years scientists have found a way to make artificial skin using some human skin cells and synthetic materials. It can be used to cover, for example, a burn or a chronic ulcer, and it gradually integrates with the patient′s own skin.