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. The adjective cutaneous literally means "of the skin".
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 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 the approximately one thousand species of bacteria from nineteen phyla which have been found on human skin.
Structure
Human skin shares anatomical, physiological, biochemical and immunological properties with other mammalian lines. Pig skin especially 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 elastin 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 which produce pigmentation, such as melanin provided by melanocytes, which absorb some of the potentially dangerous ultraviolet radiation in sunlight. It contains DNA repair enzymes that help reverse UV damage. People lacking the genes for these enzymes have high rates of skin cancer. One form predominantly produced by UV light, melanoma, is particularly invasive, causing it to spread quickly, and can often be deadly. Human skin pigmentation varies substantially between populations; this has led to the classification of people on the basis of skin colour.
In terms of surface area, the skin is the second largest organ in the human body. For the average adult human, the skin has a surface area of. The thickness of the skin varies considerably over all parts of the body, and between men and women, and young and old. An example is the skin on the forearm, which is on average in males and in females. 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 in diameter, but there are variants. A skin cell usually ranges from, depending on a variety of factors.
Skin is composed of three primary layers: the epidermis, the dermis and the hypodermis.
Epidermis
The epidermis, "epi" coming from the Greek language 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.File:HautFingerspitzeOCT nonanimated.gif|thumb |209px|
2D projection of a 3D OCT-tomogram of the skin at the fingertip, depicting the stratum corneum 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.
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 : corneum, lucidum, granulosum, spinosum, and basale. Cells are formed through mitosis at the basale layer. The daughter cells 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. 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.
Sublayers
The epidermis is divided into the following 5 sublayers or strata:- Stratum corneum
- Stratum lucidum
- Stratum granulosum
- Stratum spinosum
- Stratum basale
Genes and proteins expressed in the epidermis
About 70% of all human protein-coding genes are expressed in the skin. Almost 500 genes have an elevated pattern of expression in the skin. There are fewer than 100 genes that are specific for the skin, and these are expressed in the epidermis. An analysis of the corresponding proteins show that these are mainly expressed in keratinocytes and have functions related to squamous differentiation and cornification.Dermis
The dermis is the layer of skin beneath the epidermis that consists of connective tissue and cushions the body from stress and strain. The dermis is tightly connected to the epidermis by a basement membrane. It also harbours many nerve endings that provide the sense of touch and heat. It contains the hair follicles, sweat glands, sebaceous glands, apocrine glands, lymphatic vessels and blood vessels. The blood vessels in the dermis provide nourishment and waste removal from its own cells as well as from the stratum basale of the epidermis.The dermis is structurally divided into two areas: a superficial area adjacent to the epidermis, called the papillary region, and a deep thicker area known as the reticular region.
Papillary region
The papillary region is composed of loose areolar connective tissue. It is named for its finger-like projections called papillae, which extend toward the epidermis. The papillae provide the dermis with a "bumpy" surface that interdigitates with the epidermis, strengthening the connection between the two layers of skin.In the palms, fingers, soles, and toes, the influence of the papillae projecting into the epidermis forms contours in the skin's surface. These epidermal ridges occur in patterns that are genetically and epigenetically determined and are therefore unique to the individual, making it possible to use fingerprints or footprints as a means of identification.
Reticular region
The reticular region lies deep in the papillary region and is usually much thicker. It is composed of dense irregular connective tissue, and receives its name from the dense concentration of collagenous, elastic, and reticular fibres that weave throughout it. These protein fibres give the dermis its properties of strength, extensibility, and elasticity.Also located within the reticular region are the roots of the hairs, sebaceous glands, sweat glands, receptors, nails, and blood vessels.
Tattoo ink is held in the dermis. Stretch marks, often from adolescent growth spurts, weight gain, pregnancy and obesity, are also located in the dermis.
Subcutaneous tissue
The subcutaneous tissue is not part of the skin, but lies below the dermis of the cutis. Its purpose is to attach the skin to underlying bone and muscle as well as supplying it with blood vessels and nerves. It consists of loose connective tissue, adipose tissue and elastin. The main cell types are fibroblasts, macrophages and adipocytes. Fat serves as padding and insulation for the body.Cross-section
Cell count and cell mass
Skin cell table
The below table identifies the skin cell count and aggregate cell mass estimates for a 70 kg adult male.Tissue mass is defined at 3.3 kg and addresses the skin's epidermis, dermis, hair follicles, and glands. The cell data is extracted from 'The Human Cell Count and Cell Size Distribution', Tissue-Table tab in the Supporting Information SO1 Dataset. The 1200 record Dataset is supported by extensive references for cell size, cell count, and aggregate cell mass.
Detailed data for below cell groups are further subdivided into all the cell types listed in the above sections and categorized by epidermal, dermal, hair follicle, and glandular subcategories in the dataset and on the dataset's graphical website interface. While adipocytes in the hypodermal adipose tissue are treated separately in the ICRP tissue categories, fat content resident in the dermal layer is addressed by the below interstitial-adipocytes in the dermal layer.
| Named tissue and associated cell groups | Cell count | Aggregate cell mass | Percent of total mass |
| Skin total | 6.1E+11 | 846.7 | 100% |
| Adipocyte | 7.3E+08 | 291.9 | 34.5% |
| Endothelial cell | 1.5E+10 | 6.16 | 0.7% |
| Epithelial cells | 4.1E+11 | 313.9 | 37.1% |
| Eccrine gland | 1.7E+11 | 105 | 12.4% |
| Epidermal keratinocytes | 1.1E+11 | 85.5 | 10.1% |
| Hair follicle | 1.3E+11 | 119.9 | 14.2% |
| Mechanoreceptors | 4.9E+09 | 3.6 | 0.4% |
| Epithelial cells ; non-nucleated | 7.2E+10 | 28.2 | 3.3% |
| Fibroblasts | 4.3E+10 | 94.6 | 11.2% |
| Myocytes | 2.6E+07 | 0.08 | 0.01% |
| Neuroglia | 8.5E+09 | 12.8 | 1.5% |
| Perivascular cells / Pericytes / Mural | 1.5E+09 | 0.56 | 0.07% |
| Stem cells; epithelial | 3.6E+09 | 1.50 | 0.2% |
| White blood cells | 5.4E+10 | 97.1 | 11.5% |
| Granulocytes | 2.2E+10 | 32.6 | 3.8% |
| Lymphoid | 1.3E+10 | 1.6 | 0.2% |
| Monocyte-macrophage series | 1.9E+10 | 62.9 | 7.4% |