human skin human skin the human skin is the outer covering of the body and is the largest organ of the integumentary system. the skin has up to seven layers of ectodermal tissue guarding muscles, bones, ligaments and internal organs. human skin is similar to most of the other mammals' skin, and it is very similar to pig skin. though nearly all human skin is covered with hair follicles, it can appear hairless. there are two general types of skin, hairy and glabrous skin (hairless). the adjective cutaneous literally means "of the skin" (from latin cutis, skin). this article is about skin in humans. for other mammals, see skin. skinepidermis, dermis, and subcutis, showing a hair follicle, gland, and sebaceous glanddetailssystemintegumentary systemidentifierslatincutismeshd012867ta98a16.0.00.002ta27041thh3.12.00.1.00001 fma7163anatomical terminology skin plays an important immunity role in protecting the body against pathogens and excessive water loss. its other functions are insulation, temperature regulation, sensation, synthesis of vitamin d, and the protection of vitamin b folates. severely damaged skin will try to heal by forming scar tissue. this is often discoloured and depigmented. in humans, skin pigmentation (affected by melanin) varies among populations, and skin type can range from dry to non-dry and from oily to non-oily. such skin variety provides a rich and diverse habitat for bacteria that number roughly 1000 species from 19 phyla, present on the human skin. structure human skin shares anatomical, physiological, biochemical and immunological properties with other mammalian lines, especially pig skin. pig skin shares similar epidermal and dermal thickness ratios to human skin; pig and human skin share similar hair follicle and blood vessel patterns; biochemically the dermal collagen and elastic content is similar in pig and human skin; and pig skin and human skin have similar physical responses to various growth factors. skin has mesodermal cells, pigmentation, such as melanin provided by melanocytes, which absorb some of the potentially dangerous ultraviolet radiation (uv) in sunlight. it also contains dna repair enzymes that help reverse uv damage, such that people lacking the genes for these enzymes have high rates of skin cancer. one form predominantly produced by uv light, malignant melanoma, is particularly invasive, causing it to spread quickly, and can often be deadly. human skin pigmentation varies among populations in a striking manner. this has led to the classification of people(s) on the basis of skin colour. in terms of surface area, the skin is the second largest organ in the human body (the inside of the small intestine is 15 to 20 times larger). for the average adult human, the skin has a surface area of from 1.5–2.0 square metres (16–22 sq ft). the thickness of the skin varies considerably over all parts of the body, and between men and women and the young and the old. an example is the skin on the forearm which is on average 1.3 mm in the male and 1.26 mm in the female. one average square inch (6.5 cm2) of skin holds 650 sweat glands, 20 blood vessels, 60,000 melanocytes, and more than 1,000 nerve endings. the average human skin cell is about 30 micrometres (μm) in diameter, but there are variants. a skin cell usually ranges from 25 to 40 μm2, depending on a variety of factors. skin is composed of three primary layers: the epidermis, the dermis and the hypodermis. layers, receptors, and appendages of human skin epidermis main article: epidermis the epidermis, "epi" coming from the greek meaning "over" or "upon", is the outermost layer of the skin. it forms the waterproof, protective wrap over the body's surface which also serves as a barrier to infection and is made up of stratified squamous epithelium with an underlying basal lamina. the epidermis contains no blood vessels, and cells in the deepest layers are nourished almost exclusively by diffused oxygen from the surrounding air and to a far lesser degree by blood capillaries extending to the outer layers of the dermis. the main type of cells that make up the epidermis are merkel cells, keratinocytes, with melanocytes and langerhans cells also present. the epidermis can be further subdivided into the following strata (beginning with the outermost layer): corneum, lucidum (only in palms of hands and bottoms of feet), granulosum, spinosum, and basale. cells are formed through mitosis at the basale layer. the daughter cells (see cell division) move up the strata changing shape and composition as they die due to isolation from their blood source. the cytoplasm is released and the protein keratin is inserted. they eventually reach the corneum and slough off (desquamation). this process is called "keratinization". this keratinized layer of skin is responsible for keeping water in the body and keeping other harmful chemicals and pathogens out, making skin a natural barrier to infection. 2d projection of a 3d oct-tomogram of the skin at the fingertip, depicting the stratum corneum (~500 µm thick) with the stratum disjunctum on top and the stratum lucidum in the middle. at the bottom are the superficial parts of the dermis. the sweat ducts are clearly visible. 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