Structures and Functions of The Skin and Physiological Functions of the Skin
The skin is the largest organ in our body. It comprises about 15% of the body weight. It is composed of three layers: epidermis, dermis, and subcutaneous tissue (fat). The epidermis, the outer most layer is directly contiguous with the environment. It is formed by an ordered arrangement of cells called keratinocytes, the basic function of which is to synthesize keratin, a filamentous protein that serves a protective function.
The dermis is the middle layer, composed of collagen, tough and resilient part of the skin lies on the subcutaneous tissue which is principally composed of lobules of fat cells.
All skin is made up of these three layers. Although there is a considerable regional variation in their relative thickness: the epidermis is thickest on the palms and soles and very thin on the eyelids. The dermis is thickest on the back. The amount of fat is generous on the abdomen and buttock compared with the nose and sternum.
Cells of the epidermis
Keratinocyte - produces keratin which forms the outer most skin layer covered by thin lipids to give the skin protective capacity from water and heat loss, penetration of microbial agents, and other trauma by physical mechanisms.
Melanocytes - they are the melanin (pigment) producing cell of the epidermis. Melanin prevents the skin from. The number of melanocytes in the epidermis is the same, regardless of the person’s race or skin color; it is the number , shape and size of melanosomes (melanin containing granules) and the type of melanin that determine difference in skin color.
Langerhans' cells - these are cells with dendrite processes specialized in antigen processing and presentation (building immunity to infection). They are found in the epidermis but they constantly move as a result, they transport antigens to the regional lymph nodes and present them to naïve T lymphocytes in the regional lymph nodes and consequently the naive T lymphocytes become recruited to the specific antigen and the resultant immunologic response occurs. ("They take the offenders to the police station for investigation and appropriate response "). E.g. when a child receives
BCG vaccination and develops a scar. In this way, the skin is very crucial part of the immune system because of the large surface area that it spans. Countless varieties of external antigens can be sensed by the immune system via the Langerhans' cells in the epidermis.
Physiological Functions of the Skin
1. Display: the skin as a display enables us to assume our own identity and to recognize among our selves and with out the skin emotional expressions wouldn't be possible.
2. Protection: it protects the body from many environmentally unfavorable factors; such as, thermal, chemical, ultra violet radiation and different disease-causing microorganisms. It also protects from unnecessary entry and egress of fluids into and from the body.
3. Thermoregulation: because it bears receptors to detect temperature, it conveys sensory input to the CNS so that the thermoregulatory centre can respond appropriately. The skin is a peripheral thermoregulatory organ through sweating, vasodilation, and shivering.
4. Immunologic: the skin is an end organ for many immunologically mediated disorders as well as a tool for immunologic research. Because it bears immunologic cells (lymphocytes, langerhans' cells, and mast cells) it has an active role in immunologic field of action. The skin can be viewed as a peripheral arm of the immune system involved in normal homeostasis and host defense.
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5. Synthetic function: the skin synthesizes vitamin D, different hormones, melanin, and other substances
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