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A three-ply thatch

The skin is made up of hundreds of millions of cells arranged in three layers (left). The top layer is called the epidermis; the middle is the dermis and the deepest layer is known as the subcutis, where fat is stored.

The epidermis itself is made up of four - layers. The outer layer - the stratum corneum - is composed of 25 to 30 layers of dead cells. The basal layer of the epidermis divides to produce "daughter" cells which rise to the surface to become part of dead layers of the stratum corneum. As they rise, they flatten and become filled with the protein keratin, which makes them tougher. The basal layer also contains cells that produce the pigment melanin (the body's natural sun-block), sensory cells and other cells which strengthen the body's immune responses.

The dermis is made up of horizontal bundles of collagen fibres bound by elastin fibres, which give skin its strength and elasticity. This layer also contains capillaries, hair and hair shafts, attached to which are muscles and sebaceous glands, sweat glands and receptors that can measure pain, temperature and pressure.

The inner layer of the skin - the subcutis - is made up of fat cells which provide some nutrition storage, insulation and cushioning.

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