In zootomy and dermatology, skin is the largest organ of the integumentary system made up of multiple layers of epithelial tissues that guard underlying muscles and organs. Skin pigmentation varies among populations, and skin type can range from dry skin to oily skin.
The adjective cutaneous literally means "of the skin" (from Latin cutis, skin).
As ash the interface with the surroundings, skin plays the most important role in protecting (the body) against pathogens. Its other main functions are insulation and temperature regulation, sensation, and synthesis of vitamin D and the protection of vitamin B folates.
Severely damaged skin will try to heal by forming scar tissue, often giving rise to discoloration and de-pigmentation of the skin.
The use of natural or synthetic cosmetics to treat the appearance of the face and condition of the skin (such as pore control and black head cleansing) is common among many cultures. Oily skin is caused by hormonal fluctuations in the body, which lead to a DHT sensitivity. This sensitivity means that the skin begins to lose moisture and essential fatty acids (linoleic acid in particular), causing thousands of skin cells to die, so the skin compensates for this loss of moisture by producing higher levels of oil. Oily skin can be cleaned quickly with a mild solution of detergent, when pure bath soaps fail. Afterward, body lotions could be used to recondition cleansed skin, as would be used to treat dry skin.
Hair is an outgrowth of protein, found only on mammals. It projects from the epidermis, though it grows from hair follicles deep in the dermis. Although many other organisms, especially insects, show filamentous outgrowths, these are not considered "hair". So-called "hairs" (trichomes) are also found on plants. The projections on arthropods, such as insects and spiders are actually insect bristles, composed of a polysaccharide called chitin. The hair of non-human mammal species is commonly referred to as fur. There are varieties of cats, dogs, and mice bred to have little or no visible fur. In some species, hair is absent at certain stages of life.
The primary component of hair fibre is keratin. Keratins are proteins: long chains (polymers) of amino acids.
Nails grow at an average rate of 3 millimetres a month. Fingernails require 3 to 6 months to regrow completely. Toenails require 12 to 18 months. Actual growth rate is dependent upon age, season, exercise level, and hereditary factors. Contrary to popular belief, nails do not continue to grow after death; the skin dehydrates and tightens, making the nails (and hair) appear to grow.
This growth record can show the history of recent health and physiological imbalances, and has been used as a diagnostic tool since ancient times. Major illness will cause a deep groove to form across the nails. Discoloration, thinning, thickening, brittleness, splitting, grooves, Mees' lines, small white spots, receded lunula, clubbing (convex), flatness, spooning (concave) can indicate illness in other areas of the body, nutrient deficiencies, drug reaction or poisoning, or merely local injury. Nails can also become thickened (onychogryphosis), loosened (onycholysis), infected with fungus (onychomycosis) or degenerative (onychodystrophy); for further information see nail diseases.
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