Structure and Functions of SkinChapter One

Skin is the largest organ of the body with an average surface area of 1.5 to 2 sq metre. It is derived from both ectoderm (epidermis) and mesoderm (dermis). Epidermis is an avascular multilayered structure, regularly regenerates itself through cell division at the basal layer. The basal cells are elongated vertical cells with basophilic cytoplasm and dark nucleus. In addition to being anchored to the dermis, the basal cells are attached to neighbouring basal cells and prickle cells by desmosomes. Melanocytes are present in between basal cells in ratio of about 1:10. They are clear cells with dark nuclei which produce melanin. The prickle cells are polygonal cells with basophilic cytoplasm and dark nuclei. They are bound together by desmosomes and tonofilaments. The prickle cells arise from basal cells and pass on to granular layer (stratum germinativum). These cells contain deeply stained keratohyalin granules. At the end of granular layer the epidermal cells are converted from nucleated cells to flattened cell remnant, lacking nucleus and consist of almost entirely of tough protein called keratin and this layer is known as keratin layer. This scaly or horny layer serves as a mechanical and chemical barrier between body and environment. Two other cells namely Langerhans’ cells and Markel cells are present in epidermis. Langerhans’ cells are possibly macrophages which take up antigens that are present or enter epidermis and transfer them to immune system. Markel cells are considered to be epidermal nerve cells.

Hair, sebaceous glands, apocrine and eccrine sweat glands, and eccrine glands arise from the epidermis in embryonic life as primary epithelial germ cells and free sebaceous glands arise from sebaceous gland germ cells (Fig. 1.1).

A variety of mucopolysaccharides, hyaluronic acid and dermatan sulphate are synthesized by fibroblasts and occupy the spaces between dermal fibres adding bulk of the dermis.

Cutaneous muscles are of two types:

A complex network of somatic and autonomic nerves (arise from neural crest) serves the skin. Sensory nerves are responsible for cutaneous sensation, autonomic nerves for control of vessels, appendages and erector pili muscles.

The main networks of cutaneous blood vessels, deep plexus at dermal—subcutaneous fat junction and the superficial at papillary dermis are formed by arteries and veins. Lymphatic drainage starts from lower layer of epidermis as well as entire dermis.

Functions of skin are related to its unique position between the environment and body. Out of the main functions, sensations and protection are the two important ones.

Heat, cold, pain, light, touch, pressure, itch and tickle are different types of interpretation of nerves of skin consequent to various external stimuli.

Physical protection is offered by skin at various levels, namely stratum corneum (physical barrier), prickle cell layer (elasticity and support), dermoepidermal junction (mechanical), dermis and subcutaneous fat (bulk, cushion and mobilizing), radiation damage is protected by melanin.

Cutaneous mechanisms like insulation (subcutaneous fat), regulation of cutaneous blood flow, eccrine sweat activity (cools through evaporation) and muscle activity (shivering—warmth).