Skin is the outermost part of the human body which covers and protects the internal organs from injurious effects of the environment and the harmful organisms. It is made up of three layers of the tissue, i.e. epidermis, dermis and subcutaneous tissue. In addition, it also has two appendages, i.e. hair and nail, and two glands, i.e. sweat gland (eccrine and apocrine glands) and sebaceous gland.
Epidermis is the outermost layer of the skin and consists of stratum corneum or horny cell layer, stratum granulosum, stratum malpighii or prickle cell layer and basal cell layer.
The basal cell layer, also known as stratum germinatum, is made up of one layer of columnar epithelial cells which keep on dividing constantly. These cells are gradually displaced outwards by the newly formed cells and transform into polygonal shape. These polygonal cells are known as stratum malpighii which are attached to each other and to the cells of basal cell layer by a specialised structure called desmosome. These cells are also covered with a proteinous substance called intercellular cement. Towards the surface, these cells flatten, lose their prickles and acquire numerous granules in their cytoplasm and are known as granular cells. The stratum corneum is the outermost layer of the epidermis and is made up of stratified horn cell layers and form a tough, elastic and supple surface of the skin. The horn cells are dead tissue without any nuclei and are made up of a fibrous protein called keratin. These cells are constantly shed from the surface and are continuously replaced by the newly keratinised cells from the deeper tissues. This process is known as keratinisation. It takes about 28 days for the basal cell to reach the stage of mature horn cell.
In palms and soles there is an additional layer of cells called stratum lucidum between the stratum granulosum and corneum. These are nucleated cells with dense cytoplasm and opaque membranes.
In the basal cell layer and hair matrix, there are cells which do not stain with routine haemotoxyline and eosin. These cells are known as melanocytes and are responsible for synthesis of melanin pigment. With special stain they show long branching processes called dendrites. There are other dendritic cells found in the epidermis called Langerhans cells which are responsible for phagocytosis.
The dermis represents the main portion of the skin and is about 3 mm thick. It is divided into two layers, the superficial portion is called papillary dermis and the deeper part is called reticular dermis. It consists of a network of fibrous tissue which supports the glands, hair follicles, blood vessels, lymphatics and nerves. The fibres are collagen fibres, elastic fibres and ground substance. Collagen fibres form the bulk of the dermis. Immature collagen fibres are known as reticulum. The elastic fibres are present in between the collagen bundles. A gelatinous material called ground substance is found in between these fibres.
Three types of cells are found in the dermis, namely fibroblast, histiocyte and mast cells. Fibroblasts are spindle-shaped cells responsible for the production of reticulum, collagen and possibly elastic fibres too. Histiocyte is difficult to differentiate from fibroblast except that it has power of phagocytosis. Mast cells are seen by special staining and are responsible for the production and release of heparin and histamine.
The subcutaneous tissue, the innermost layer of the skin, consists of fat. It varies in thickness from place to place being maximum in palms and soles and almost absent in the eyelids.
Blood supply of the skin is through a large number of arteries which anastomose in the subdermal region, from which arterioles ascend into the dermis. These are again interconnected to each other. From this the arterioles run upward and form a second network of subpapillary capillary plexus in the upper dermis. Terminal arterioles ascend from there to the papillae forming capillary loops which drain into connecting venules.
Blood returns to the large veins in subcutaneous tissue through four venous networks of increasing size lying parallel to the surface.
The arterial portion of the vascular system is contractile due to the presence of muscles in its wall whereas capillaries and small veins are not contractile and are like an inert tube. The temperature of the skin depends on the rate of blood flow—greater the rate, warmer is the skin.
The skin contains a good network of lymphatics which drain into a few large vessels in the subcutaneous tissue. The main function of the lymphatics is to drain away protein particles which are too large to be drained by the venous part of capillary circulation.
The nerve supply of the skin consists of a motor sympathetic part arising from the sympathetic ganglions and sensory part arising from the dorsal root ganglions. Adrenergic fibres of the sympathetic part innervate the blood vessels, errector pilorum muscles and apocrine ducts and are responsible for contractions and constriction of the structures supplied. The cholinergic fibres supply sweat glands.
Sensory part of the nerve breaks up into numerous fine terminals in between the epidermal cells which present as free nerve endings and also as specialised nerve endings in the dermis and around cutaneous appendages. These nerve ending carry the sensations of touch, pain, temperature, wetness and itch.
The most important function of the skin is to protect the body by various means and mechanisms. Due to constant shedding of stratum corneum it acts as a mechanical barrier and does not allow organisms to stay or penetrate into the skin. Triglycerides, free fatty acids, etc. produced from the breakdown of sebum act as antibacterial agents. Sebum secreted from sebaceous glands makes the skin supple. Melanin produced by the melanocytes present in the basal layer of the epidermis protects the body from ultra-violet radiation. Langerhans cells present in the epidermis phagocytose agents which invade the skin. Body temperature is controlled by sweat glands through secretion and evaporation of sweat. Vitamin D is synthesised in the skin in the presence of sunlight.