Target Health Blog

Human Skin

July 1, 2019


Anatomy of the human skinGraphic credit: by US-Gov - Anatomy of the Skin (as last archived 2008-06-12 09:37:35 at Anatomy of the Skin), Public Domain,

The human skin is the outer covering of the 1) ___ and is the largest organ of the integumentary system. The skin has up to seven layers of ectodermal tissue and guards the underlying 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). Because it interfaces with the environment, 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 2) ___. This is often discolored and depigmented.

In humans, skin pigmentation varies among populations, and skin type can range from dry to 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. Skin has mesodermal cells, pigmentation, such as melanin provided by melanocytes, which absorb some of the potentially dangerous ultraviolet 3) ___ (UV) in sunlight. It also contains DNA repair enzymes that help reverse UV damage, such that people lacking the genes for these enzymes suffer 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. In terms of surface area, the skin is the second largest organ in the human body (the inside of the small 4) ___ is 15 to 20 times larger). For the average adult human, the skin has a surface area of between 1.5-2.0 square meters, or 16.1-21.5 sq ft.

Skin is composed of three primary layers: the epidermis, the 5) ___ and the hypodermis. 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 which make up the epidermis are Merkel cells, keratinocytes, with melanocytes and Langerhans cells also present. Blood capillaries are found beneath the epidermis, and are linked to an arteriole and a venule. Arterial shunt vessels may bypass the network in ears, the nose and fingertips. About 70% of all human protein-coding genes are expressed in the skin.

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 harbors many nerve endings that provide the sense of 6) ___ 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. The papillary region is composed of loose areolar connective tissue. It is named for its fingerlike projections called papillae, that 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 (like a fingerprint) 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.

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 fibers that weave throughout it. These protein fibers 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.

The subcutaneous tissue (also hypodermis and subcutis) is not part of the skin, and 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 (subcutaneous tissue contains 50% of body fat). Fat serves as padding and insulation for the body. Human skin shows high skin color variety from the darkest brown to the lightest pinkish-white hues. Human skin shows higher variation in color than any other single mammalian species and is the result of natural selection. Skin pigmentation in humans evolved to primarily regulate the amount of ultraviolet radiation (UVR) penetrating the skin, controlling its biochemical effects. The single most important substance determining human skin color is the pigment melanin. Melanin is produced within the skin in cells called melanocytes and it is the main determinant of the skin color of darker-skinned humans. The skin color of people with light skin is determined mainly by the bluish-white connective tissue under the dermis and by the hemoglobin circulating in the veins of the dermis. The red color underlying the skin becomes more visible, especially in the face, when, as consequence of physical exercise or the stimulation of the nervous system (anger, fear), arterioles dilate.

There is a correlation between the geographic distribution of UV radiation (UVR) and the distribution of indigenous skin pigmentation around the world. Areas that highlight higher amounts of UVR reflect darker-skinned populations, generally located nearer to the equator. Areas that are far from the tropics and closer to the poles have lower concentration of UVR, which is reflected in lighter-skinned populations. In the same population it has been observed that adult human females are considerably lighter in skin pigmentation than males. Females need more calcium during pregnancy and lactation, and vitamin D which is synthesized from sunlight helps in absorbing calcium. For this reason, it is thought that females may have evolved to have lighter skin in order to help their bodies absorb more calcium. As skin ages, it becomes thinner and more easily damaged. Intensifying this effect is the decreasing ability of skin to heal itself as a person ages. Among other things, skin aging is noted by a decrease in volume and elasticity. There are many internal and external causes to skin aging. For example, aging skin receives less blood flow and lower glandular activity. Photoaging has two main concerns: an increased risk for skin cancer and the appearance of damaged skin.

1.     Skin provides an anatomical barrier from pathogens and damage between the internal and external environment in bodily defense; Langerhans cells in the skin are part of the adaptive immune system. Perspiration contains lysozyme that break the bonds within the cell walls of bacteria.

2.     Skin provides sensation, containing a variety of nerve endings that react to heat and cold, touch, pressure, vibration, and tissue injury; see somatosensory system and haptics.

3.     Skin regulates heat, and to that end, contains a blood supply far greater than its requirements which allows precise control of energy loss by radiation, convection and conduction. Dilated blood vessels increase perfusion and heat loss, while constricted vessels greatly reduce cutaneous blood flow and conserve heat.

4.     Skin controls evaporation of fluids by providing a relatively dry and semi-impermeable barrier to fluid loss. Loss of this function contributes to the massive fluid loss in burns.

Regarding excretion, sweat contains urea, however its concentration is 1/130th that of urine, hence excretion by sweating is at most a secondary function to temperature regulation. Absorption: the cells comprising the outermost 0.25-0.40 mm of the skin are “almost exclusively supplied by external oxygen“, although the “contribution to total respiration is negligible“. In addition, medicine can be administered through the skin, by ointments or by means of adhesive patch, such as the nicotine patch or iontophoresis. The skin is an important site of transport in many other organisms.

Water resistance: The skin acts as a water-resistant barrier so essential nutrients are not washed out of the body.

The human skin is a rich environment for microbes. Around 1000 species of flora bacteria from 19 bacterial phyla have been found. Most come from only four phyla. The NIH launched the Human Microbiome Project to characterize the human microbiota which includes that on the skin and the role of this microbiome in health and disease. The skin supports its own ecosystems of microorganisms, including yeasts and bacteria, which cannot be removed by any amount of cleaning. Estimates place the number of individual bacteria on the surface of one square inch (6.5 square cm) of human skin at 50 million, though this figure varies greatly over the average 20 square feet (1.9 m2) of human skin. Oily surfaces, such as the face, may contain over 500 million bacteria per square inch (6.5 cm?). Despite these vast quantities, all of the bacteria found on the skin's surface would fit into a volume the size of a pea. In general, the microorganisms keep one another in check and are part of a healthy skin. When the balance is disturbed, there may be an overgrowth and infection, such as when antibiotics kill microbes, resulting in an overgrowth of 7) ___. The skin is continuous with the inner epithelial lining of the body at the orifices, each of which supports its own complement of microbes.

Human skin has a low permeability; that is, most foreign substances are unable to penetrate and diffuse through the skin. Skin's outermost layer, the stratum corneum, is an effective barrier to most inorganic nanosized particles. This protects the body from external particles such as toxins by not allowing them to come into contact with internal tissues. However, in some cases it is desirable to allow particles entry to the body through the skin. Potential medical applications of such particle transfer has prompted developments in nanomedicine and biology to increase skin permeability. One application of transcutaneous particle delivery could be to locate and treat cancer. Nanomedical researchers seek to target the epidermis and other layers of active cell division where nanoparticles can interact directly with cells that have lost their growth-control mechanisms (cancer cells). Such direct interaction could be used to more accurately diagnose properties of specific tumors or to treat them by delivering drugs with cellular specificity. Skin and Immune System Work Together to Keep out unhealthy substances.

Langerhans cells (LCs) protect by keeping dangerous antigens (any substance that causes the immune system to produce antibodies against it) from entering the body. Langerhans cells (LCs) are located in the skin (the epidermis and the dermis) of the respiratory, digestive, and urogenital tracts. They can also be found in other tissues such as lymph nodes, particularly when the condition Langerhans cell histiocytosis (LCH) is involved. Langerhans cells are present in all layers of the epidermis and are members of the dendritic family. Dendritic cells are immune cells that present antigens to the immune system and are found all throughout the body. LCs are dendritic cells in the skin and since dendritic cells are extremely efficient at alerting the immune system to the presence of pathogens and other foreign materials, the skin is an important barrier to infection. LCs were originally thought to be a part of the nervous system and only alert the immune system to antigens. Instead, research has found that LC's dampen the skin's reaction to infection and inflammation in 2 very different ways: By protecting your skin from infection; and by stimulating allergic reactions. Langerhans cells send out special agents - immune cells such as T cells and B cells - immediately after sensing any kind of danger in the skin. The immune cells capture trespassers such as bacteria and viruses and fight off injuries like cuts and scrapes.

The environment of the skin is constantly monitored by LCs for unsafe situations and the immune cells are sent to bring back information about any foreign invaders. The body then amasses a great force of inflammatory cells to fight off invaders by creating an allergic reaction or forming scar tissue to protect from infection. Research suggests that Langerhans cells are able to identify and attack viruses from the surrounding environment, thereby preventing infection. This finding that LCs are involved in 8) ___ responses against various diseases could make them potential targets for immunotherapy. This finding also has the potential to significantly change the understanding of the mechanisms underlying many skin disorders such as skin cancer, which is the most common form of 9) ___ in the US, characterized by the abnormal growth of skin cells.

There may also be the development of topical vaccines administered through the skin (Epicutaneous immunization) that focus on loading LCs directly into the skin with antigens. Research is already looking at vaccines given through barrier-disrupted skin in order to inhibit the growth of melanoma, a serious type of skin cancer.

Swimming in the ocean alters the skin microbiome and may increase the likelihood of infection, according to research presented at ASM Microbe 2019, the annual meeting of the American Society for Microbiology. Data demonstrate for the first time that ocean water exposure can alter the diversity and composition of the human skin microbiome. While swimming normal resident bacteria were washed off while ocean bacteria were deposited onto the skin.“ The researchers detected ocean bacteria on all participants after air drying and at six and 24 hours post-swim, but some participants had acquired more ocean bacteria and/or had them persist for longer. The research was motivated by previous studies which have shown associations between ocean swimming and infections, and by the high prevalence of poor water quality at many beaches, due to wastewater and storm water runoff. Recent research has demonstrated that changes in the microbiome can leave the host susceptible to infection, and influence disease states. Exposure to these waters can cause gastrointestinal and respiratory illness, ear infections, and skin infections. The investigators sought 9 volunteers at a beach who met criteria of no sunscreen use, infrequent exposure to the ocean, no bathing within the last 12 hours, and no antibiotics during the previous six months. The researchers swabbed the participants on the back of the calf before they entered the water, and again after subjects had air dried completely following a ten-minute swim and at six and 24 hours post swim. Before swimming, all individuals had different communities from one-another, but after swimming, they all had similar communities on their skin, which were completely different from the “before swim“ communities. At six hours post swim, the microbiomes had begun to revert to their pre-swim composition, and at 24 hours, they were far along in that process. One very interesting finding was that Vibrio species -- only identified to the genus level -- were detected on every participant after swimming in the ocean, and air drying (The Vibrio genus includes the bacterium that causes cholera.) At six hours post swim, they were still present on most of the volunteers, but by 24 hours, they were present only on one individual. While many Vibrio are not pathogenic, the fact that we recovered them on the skin after swimming demonstrates that pathogenic Vibrio species could potentially persist on the skin after swimming. The fraction of Vibrio species detected on human skin was more than 10 times greater than the fraction in the ocean water sample, suggesting a specific affinity for attachment to human skin.

Skin is the body's first line of defense, both physically and immunologically, during exposure to contaminated water. Recent studies have shown that human skin microbiome plays an important role in immune system function, localized and systemic diseases, and infection. A healthy 10) ___ protects the host from colonization and infection by opportunistic and pathogenic microbes. Sources: American Society for Microbiology; ScienceDaily; Wikipedia;

ANSWERS: 1) body; 2) tissue; 3) radiation; 4) intestine; 5) dermis; 6) touch; 7) yeast; 8) immune; 9) cancer; 10) microbiome

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