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Skin Structure

The skin has to be elastic so that it can expand and contract, be strong so that it can protect the body and it must be able to repair itself.  It also needs to be sensitive to touch, heat and pressure, be able to absorb and lose heat by sweating and radiating.  It must also act as a defence against infection.

There are three basic skin layers: the epidermis, dermis and subcutaneous tissue.  The surface of the epidermis is made up of layers of dead cells which are composed of keratin and which form a protective outer layer.  The dead cells come from a lower layer that is constantly dividing and producing more cells as others migrate upwards.  This process of formation and movement upwards takes approximately four weeks, during which time the cells undergo changes in chemical composition so that the surface layer contains mostly keratin in the form of flattened cells bonded into layers.

Melanocytes also exist in the lower layer of the epidermis.  These cells are responsible for producing the dark pigment melanin.  When UV penetrates into the base layer it triggers the melanocytes and they react by producing the melanin granules which spread throughout the keratin cells as they move upwards.  This produces a tan.

The second layer of skin is the dermis which contains the organs of the skin such as the sebaceous and sweat glands, hair follicles, blood vessels, nerves, and all the collagen and elastin fibres that give skin its strength, flexibility and elasticity.  Collagen is like a spiderweb network throughout the dermis.  Elastin fibre gives skin its elastic properties and allows it to spring back after stretching.

Elastin is damaged by ultraviolet light penetrating the skin.  It thickens and coarsens and thus loses its elasticity.  In aged skin it forms great knots of fibres that replace collagen.  This process is called elastosis and occurs mainly in the upper third of the dermis.

The collagen and elastin fibres are set into a jelly-like tissue called ground substance which is supported and held together by these fibres, and the surface appearance, thickness and resilience of the skin depend on these three things working together.

The third layer is the subcutaneous fatty tissue.

It has been estimated that the life of the skin in sunshine is between 100,000 and 200,000 hours of exposure.  However there are unpredictable variations according to skin type and intensity of exposure.  The important thing is that there is a limit to the amount of UVR exposure the skin can have before the collagen and elastin fibres in the dermis become aged or fatigued.