THE functions of the skin allow man to adapt to a wide variation in environment. The tough fibrous collagen of the lower layer, the dermis, withstands mechanical trauma. The epidermis made up of many layers of horny plates is relatively but not absolutely impermeable to fluids from without and prevents evaporation of tissue fluids, though a small transepidermal waste is inevitable.
On exposure to cold, reduction of skin blood flow insulates and maintains body temperature. Increased blood flow in the superficial blood vessels and evaporation of sweat from eccrine sweat glands increases heat loss and enables man to remain cool in hot climates.
The presence of pigment in the lower layers of the epidermis filters out most of the harmful ultra-violet radiation. Finally the sensory nerves can only receive impulses through normal skin.
There is a wide regional variation in the structure of the skin. As an example the thick horny epidermis in the palms and soles can be contrasted with the delicate thin epidermis of the eyelids and the density of hair follicles varies from the profuse growth on the scalp to complete absence on palms and soles.
The structure of the skin. The epidermis, the outer of the two layers of the skin, is derived from the embryological ectoderm. It is a many layered pavement of squamous epithelial cells devoid of blood vessels and nourished by a plexus of capillaries situated in the upper papillary part of the dermis. Although the epidermis can be divided morphologically into four different layers—the horny, granular, prickle cell and the lowest, the basal layer, each level in the skin represents a stage in the life of the epidermal cell.
The columnar cells of the basal or germinal layer give rise to the prickle cells by mitotic division. Prickle cells move upwards and as new cells are formed beneath them gradually change their polyhedral shape to a flatter configuration. As they approach the surface they take on their primary function, production of the protein keratin. Keratin is first formed in separate protein helices but under enzymatic action the helices become cross linked by disulphide bonds which make the keratin resistent to proteolytic digestion. At the level of the granular layer the cells are filled with granules of keratohyalin, and then lytic enzymes are released which destroy the cell nucleus and the unbonded keratin to leave a keratinous husk which is the squame of the horny layer. The horny layer is thus formed of dead epidermal cells.
The average time from the formation of a prickle cell to its loss from the skin surface is about 28 days approximately 14 being spent in the horny layer. This transit time may be shortened by disease but a feed-back mechanism normally controls the equilibrium of mitotic activity in the basal layer to maintain constant epidermal thickness in a particular site. Removal of the horny layer by stripping calls forth a burst of mitotic activity and constant friction and the wearing away of squames leads to a permanently increased thickness such as occurs in a callosity.
The prickle cells are held together by intercellular bridges, the prickles, which can be seen by light microscopy. Under the electron microscope these bridges or desmosomes can be seen to contain grouped bundles of tonofibrils which terminate at the attachment plates by which one cell is fixed to the next and to the basement lamina. There is a distinct intercellular space between adjacent cells which is important for the transport of metabolites to and from the cells.
Scattered between the basal cells are the pigment producing melanocytes which are derived embryologically from the nervous system. Under light microscopy they appear as clear cells between the deeply staining basal cells. With special staining methods they can be shown to have dendritic processes with which they inject pigment granules into the surrounding epidermal cells.
There is another group of dendritic cells situated in the middle zone of the epidermis, the Langerhans’ cells. Their function remains a mystery but they are believed to be drived from mesenchymal histiocytes and to be entirely unconnected with the pigmentary system.
The dermis is divided into the papillary layer adjacent to to the epidermis in which the fibres are thrown into ridges at right angles to the surface of the skin, and the deeper reticular tissue which extends to the subcutaneous fat and in which the collagen bundles tends to be parallel to the surface. The configuration of the ridges particularly on the fingertips are responsible for the finger-prints and the study of these ridges is known as dermatoglyphics.
The major constituent of”the dermis is a fibrous protein collagen which is embedded in ground substance containing a high proportion of mucopolysaccharide. The dermis has a great capacity for retaining water and is an important reservoir of body fluid. Collagen is formed by fibroblasts which are present in large numbers in the dermis. There are other cells, mast cells, which have the power of releasing histamine when damaged and histiocytes derived from the reticulo-endothelial system.
Two other fibres, reticulum and elastic weave a supporting web around the collagen bundles. The combination of all three, collagen, reticulum and elastic fibres give the dermis its physical properties. They form a framework through which run the hair follicles, sweat ducts, blood vessels, lymphatics and cutaneous nerves which terminate either in specialised endings beneath the epidermis or at the epidermal appendages themselves.
The keratin of the epidermis is maintained supple by the secretion of the sweat and sebaceous glands and fat produced by the epidermal cells themselves. The sweat glands, which are situated deep in the dermis, are formed of a coiled tube of cubical epithelium which leads by the sweat duct to open on the surface of the skin through spiral clefts between the epidermal cells. The sweat glands are under central nervous control and can be stimulated to secrete by the need for the body to lose heat, or by fear. Sweat is a true secretion of slightly acid pH which in general reflects the changes of the electrolytes in the plasma. The sebaceous glands are situated around and open into hair follicles forming a functional whole, the pilosebaceous unit. The sebaceous glands are composed of specialised epidermal cells which form the greasy product, sebum, by the degeneration of their own cell substance in a comparable way to keratin formation by the normal epidermal cell. Sebum production is not under nervous control but is dependent on the size and mitotic activity of the sebaceous cells which are themselves influenced by the pituitary and sex hormones. The apocrine glands, which are situated in the axillae, breasts and near the genitals have features of both sweat and sebaceous glands. They are odoriferous and are a secondary sex characteristic.
Hair and nails are specially modified keratin structures, both being formed by invaginations of the epidermis.