The skin of an adult weighs aii average of 4 kg and covers an area of 2 nr. It has Ihree layers (Fig. 2.15). The outer avascular epithelium, the epidermis, is firmly attached to, and supported by, a tough tibroelastic dermis. The dermis contains blood vessels, nerves, sweat and sebaceous glands and hair follicles. The third layer, the hypodermis, is of loose connective tissue, often containing abundant fat. which underlies the dermis.
Kcratinocytes make up 90% of the epidermal cells. Then-primary function is to synthesise insoluble proteins, keratins, which arc the main component of the impervious surface of the epidermis, the horny layer. Kcratinocytes are generated by division of cells in the basal layers of the epidermis and move outwards, dying in the granular cell layer before being shed at the surface as anucleated horny squames (a significant proportion of common dust). The journey from the basal layer to the surface (epidermal turnover or transit lime) takes about 60 days but is greatly accelerated in some skin conditions, for example psoriasis.
Two types of dendritic cell make up most of the remaining epidermal cells. Melanocytes are found mainly in the basal ccll layer and are the only epidermal cells to contain tyrosinase, the enzyme essential for the synthesis of melanin from phenylalanine. Melanin is normally formed in the deepest layer of the epidermis and colours the skin brown or black. The amount present is largely determined by hereditary influences. The pigment increases or diminishes in amount with exposure to. or withdrawal from, ultraviolet light. Synthcsiscd melanin is transferred to surrounding kcratinocytes in the form of melanosomes. Langerhans cells form a network within the epidermis and are specialised macrophages which circulate between local lymph nodes and the skin. They are capable of presenting antigen to T lymphocytes (e.g. in allergic contact dermatitis), and play a part in immunosurveillance of viral and tumour antigens.
The dermis supports the epidermis both structurally and nutritionally. It is separated from the epidermis by a base ment membrane and has three principal components: cells (mainly fibroblasts and a few mononuclear phagocytes, lymphocytes, mast celts and Langerhans cells), fibres (collagen, reticulin and elastin) and an amorphous ground substance (mostly the glycosaminoglycans hyaluronic acid and dermatan sulphate). The functions of the skin arc listed in Table 2.7.
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