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Biology      Skin biology

Skin biology

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The skin is the second largest organ in the body after the skeleton and of primary importance, integral to the survival of mammalian life, but often overlooked when examining the health and welfare of an individual. The average adult has over 3,000 square inches of skin surface area and the fat-free skin accounts for at least 6 percent of an individual’s total weight. In other words, the fat free skin of a 150 pound person weighs at least 9 pounds. Skin thickness in different body locations may vary: the thinnest is an eye lid skin (up to 0.05 mm) and the thickest is the skin of the feet soles (up to 0.6 mm). With exception of lips and certain parts of legs and hands the rest of the body is covered with hair. Hairy skin of the head is not very different in structure from other parts of the body. Except head hair follicles located deeper in the skin in comparison to hair follicles of other body parts. Skin is a living organ, covering entire body, which is being constanly renewed from inside and changing accordingly. It consists of three main layers: Epidermis (1 - outer layer that protects from the harmful environmental impacts), Dermis (2 - this layer has a large nervous and capillary network) and Hypodermis or Subcutaneous tissue (3 - lower connective tissue layer). The heterogeneous skin structure is derived from the ectoderm and mesoderm of an embryo giving rise to the epidermis and dermis respectively. Within these generalized layers are specialized appendages also derived from the ectoderm and/or mesoderm including sensory nerves, sweat glands and hair follicles. The skin as a whole rests on subcutaneous tissue largely composed of a loose mesh of collagen fiber, fat cells and muscle tissue. The density of structures in the skin varies considerably depending on the body location, but on average one square centimeter of skin contains about 10 hair follicles and 15 sebaceous (oil) glands, 100 sweat glands, half a meter of blood vessels, 2 meters of nerves, with 3,000 sensory cells at the ends of nerve fibers, 200 nerve endings to record pain, 25 pressure receptors for the perception of tactile stimulation, 2 sensory receptors for cold and 12 sensory receptors for heat.

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