Skin is a specialized boundary tissue which forms the entire external surface of the body and is continuous with mucosa lining the respiratory, gastrointestinal, and urinogenital tracts at their respective openings. Skin is the largest organ in the body but is often overlooked in this respect.
Skin has many functions, some of which are not immediately obvious.
• It minimizes damage from mechanical, thermal, osmotic, chemical, and sunlight insults.
• It forms a barrier against microorganisms.
• It has a major function in thermoregulation.
• It is a sensory surface equipped with touch, pressure, temperature, and pain receptors.
• It has good frictional properties useful in locomotion and handling objects.
• It is waterproof.
• It is the site of vitamin D synthesis.
• It also plays a role in non-verbal communication when we blush, alter our facial expression, or use tactile communication such as touching or kissing.
Skin has two distinct parts when seen under a microscope, the superficial epidermis and the deeper dermis.
The epidermis is a surface epithelium in which the outer cells are keratinized. Keratinization is the deposition of tough mats of keratin which are intracellular fibrous proteins that make the cells tough; keratinization also kills the superficial cells so the outer layers of your skin are dead. The epidermis varies in thickness. The thickest and most heavily keratinized areas are on the soles of the feet and palms of the hands whereas the epidermis on the face and back of the hand is much thinner and less heavily keratinized. Habitual activity, such as holding a pen, digging with a shovel or using scissors, may produce localized thickenings of thick skin by increasing the thickness of keratin to produce calluses. Cells below the keratin layer have a special coating that forms a permeability barrier, preventing water moving between cells, thus preventing water loss from the body and water-logging when exposed to water. Epithelium does not contain blood vessels, which is why you do not bleed when you lightly knock your skin. To bleed, you need to expose the blood vessels that lie in the dermis and supply the overlying epidermis by diffusion of nutrients through fenestrated capillaries.
The dermis is made up of fibrous connective tissue, containing numerous blood and lymphatic vessels and nerves. It is also variable in thickness. In the dermis, the bundles of collagen fibres are mostly arranged in parallel rows whose direction is seen as skin creases on the surface. These may be tension lines which are small, irregular furrows dividing the skin into a series of lozenges, most easily seen on the back of the hand, or flexure lines which are associated with regular movements and are conspicuous in skin overlying joints, especially/>