The cytoplasm is released and the protein keratin is inserted. The daughter cells (see cell division) move up the strata changing shape and composition as they die due to isolation from their blood source. Cells are formed through mitosis at the basale layer.
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. The main type of cells that make up the epidermis are Merkel cells, keratinocytes, with melanocytes and Langerhans cells also present. 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. 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, "epi" coming from the Greek meaning "over" or "upon", is the outermost layer of the skin.
#Skin cell turnover rate by age skin#
Skin is composed of three primary layers: the epidermis, the dermis and the hypodermis. A skin cell usually ranges from 25 to 40 μm 2, depending on a variety of factors. The average human skin cell is about 30 micrometres (μm) in diameter, but there are variants.
One average square inch (6.5 cm 2) of skin holds 650 sweat glands, 20 blood vessels, 60,000 melanocytes, and more than 1,000 nerve endings. 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. The thickness of the skin varies considerably over all parts of the body, and between men and women and the young and the old. For the average adult human, the skin has a surface area of from 1.5–2.0 square metres (16–22 sq ft). 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). This has led to the classification of people(s) on the basis of skin colour. Human skin pigmentation varies among populations in a striking manner. One form predominantly produced by UV light, malignant melanoma, is particularly invasive, causing it to spread quickly, and can often be deadly. 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. Skin has mesodermal cells, pigmentation, such as melanin provided by melanocytes, which absorb some of the potentially dangerous ultraviolet radiation (UV) in sunlight. 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. Human skin shares anatomical, physiological, biochemical and immunological properties with other mammalian lines, especially pig skin. Such skin variety provides a rich and diverse habitat for bacteria that number roughly 1000 species from 19 phyla, present on the human skin. 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. This is often discoloured and depigmented. Severely damaged skin will try to heal by forming scar tissue. Its other functions are insulation, temperature regulation, sensation, synthesis of vitamin D, and the protection of vitamin B folates. Skin plays an important immunity role in protecting the body against pathogens and excessive water loss. The adjective cutaneous literally means "of the skin" (from Latin cutis, skin). There are two general types of skin, hairy and glabrous skin (hairless). Though nearly all human skin is covered with hair follicles, it can appear hairless. Human skin is similar to most of the other mammals' skin, and it is very similar to pig skin. The skin has up to seven layers of ectodermal tissue guarding muscles, bones, ligaments and internal organs. The human skin is the outer covering of the body and is the largest organ of the integumentary system.
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