Skin – organ
Skin covers the human body. It consists of the epidermis (epidermis), true skin (dermis), and subcutaneous tissue (hypodermis). The skin weighs 3-4 kg, making up 6-7% of body weight, with the epidermis weighing 0.5 kg. The total skin area is about 2 m2. The thickness of the true skin is 0.5-4.75 mm. The thickness of the subcutaneous tissue is uneven and depends on the thickness of the fat layer. The epidermis is the outer layer of the skin. The lower surface of the epidermis is larger because its projections descend into the true skin. The formed epidermis consists of five layers. The first layer is the basal layer, which reaches the true skin with its lower part (through the basal membrane). It is made up of a single row of cylindrical cells, attached at their base to the basal cell membrane. Cylindrical cells are characterized by maximal activity of mitosis and synthesis processes. This layer also contains melanocytes (the color of the skin depends on their functional properties) – similar to nerve cells, they have branched projections. The second layer is the spinous layer, composed of 6-8 rows of polygonal cells that connect with each other through protoplasmic projections. The upper rows of cells are flattened. Langerhans cells are also present in this layer, capable of phagocytizing and migrating between the skin and lymph nodes. The third layer is the granular layer, consisting of 2 layers of grainy cells with a lot of keratohyaline granules in their protoplasm. The fourth layer is the clear layer, made up of 1-3 layers of flat cells, many of which have lost their nucleus. The protoplasm of the cells contains eleidin. The reaction of the upper part of this layer is acidic, while the lower part is alkaline. It is believed that this layer protects against water and electrolytes (does not allow them to pass through). The fifth layer is the horny layer. This layer is not cellular in structure, consisting of 5-8 rows of horny plates that constantly peel off – desquamate, known as stratum disjunctum. The lower rows are compactly compressed – impregnated with fats and other substances. This layer makes up 0.01 of the skin, in some areas (such as the palms and soles) up to 0.5 of the skin. The epidermis does not have blood vessels, so the cells are nourished by the dermal papillary layer through osmosis (diffusion). Normal epidermal turnover occurs every 25-35 days – during this time, all layers change. The epidermis is separated from the dermis by the basal membrane, composed of 2 layers: lamina lucida and lamina densa.
The dermis consists of 2 layers (papillary and reticular). It contains muscles, skin appendages, lymph nodes, blood vessels, nervous elements, collagen, elastic, pre-collagenous, reticular fibers, and various cellular elements: fibrocytes, histiocytes, plasma cells, leukocytes, lymphocytes, skin structures: hair, nails, sebaceous and sweat glands. The layers of the dermis are: 1) papillary (stratum papillare) – these papillae, of which there are about 300 per 1 mm2, form a unique pattern for each individual; 2) reticular (stratum reticulare) – a network of fibers that gradually transitions into the subcutaneous tissue.
The true skin also contains muscle fibers – smooth muscles: nipple muscle, anal sphincter muscle, arrector pili muscle, and striated muscles: m. platysma, facial mimic muscles.
The hypodermis connects the skin to the underlying tissues. The subcutaneous tissue is made up of a network of connective tissue, which is looser than in the true skin, filled with fat cells, blood vessels, nerves, lymphatic vessels. Sweat gland parts are also located here. The subcutaneous tissue gives shape to the body, protects against cold and injuries, and is a source of nutrients. Its thickness depends on age, sex, and constitution.
Skin appendages – hair, nails, sweat, and sebaceous glands.
The skin and mucous membranes perform many important functions regulated by the nervous and endocrine systems. Skin functions:
1. Protective (barrier) – performed by all layers, especially the intact horny layer. Protects against mechanical damage, chemical, thermal injuries (dermal fibers and subcutaneous tissue absorb mechanical impacts, dry skin is resistant to electricity); microbes, harmful substances (acidic environment), acts as a barrier (buffer), neutralizing the effects of harmful chemicals; the skin surface reflects part of UV radiation, while it is absorbed in the dermis, also the thickened horny layer poorly transmits UV rays, melanin also protects against harmful radiation. The protective skin membrane consists of lipids, an acidic membrane, antibodies.
2. Immunological – involved in increased sensitivity to plasma cells and active biological substances (histamine, serotonin, etc.) and immune reactions (along with lymph nodes, T and B lymphocytes, Ig). Immunodepressive factors disrupt the body’s protective reactions and lead to pathological processes of bacterial, viral, fungal diseases in the skin, mucous membranes, and other systems. Certain lymphocytes, with affinity for the skin and the ability to recirculate, can protect the skin from them. A factor produced in the epidermis promotes the proliferation of T lymphocytes, while TH helps B lymphocytes proliferate and become plasma cells, producing Ig. Plasma cells with histamine and other mediators are involved in hypersensitivity reactions. The skin has a rich network of blood vessels and lymphatic vessels.
3. Thermoregulatory – maintains a constant body temperature. Heat can be dissipated through conduction, radiation, and evaporation. The main mechanisms for heat dissipation include stimulation of the thermoregulatory center, reflexive dilation of capillaries, and sweating. There are about 4 million sweat glands in the skin, with the majority located in the palms and soles.
4. The excretory function of the skin is performed through sweat and sebaceous glands (also mammary glands in women). Sweat glands secrete sweat, which is weakly acidic (composed of 99% water, inorganic and organic substances). Sweat eliminates: excess sugar, various drugs, heavy metals. About 20% of the total body water content is removed through the skin. Apocrine sweat glands secrete a secretion containing glycogen, cholesterol and its esters, iron. Skin fats contain neutral fats, free fatty acids, nitrogen and phosphorus compounds, steroid hormones, etc. 2 grams of fats are secreted per day, which mix with sweat and form a protective film on the skin surface. Some drugs and toxic endogenous substances can also be excreted with fats. The secretory function of the epidermis is desquamation (keratohyalin, eleidin, keratin in cells).
5. The respiratory function of the skin is insignificant. It absorbs about 4g of O2 and releases about 8g of CO2 per day. This accounts for 2% of the body’s total gas exchange.
6. The absorptive function of the skin depends on the buffer properties of the skin surface and glands, epidermal maceration, skin hyperemia, the duration of contact with substances, and the chemical properties of substances. Various substances can be absorbed into the body through hair follicles and sebaceous gland ducts. The horny layer greatly hinders absorption, thus protecting against intoxication. Substances dissolved in lipids are most easily absorbed through the skin, and absorption is further increased by using ultrasound. Gaseous and liquid substances also pass through easily.
7. The sensory function is performed through nerve endings and corpuscles. There are many receptors in the skin. The skin senses: 1) warmth – through Ruffini corpuscles in the dermis and hypodermis; 2) cold – through Krause corpuscles in the subcutaneous layers; 3) pressure – through Pacinian corpuscles in the subcutaneous tissue; 4) touch – through Meissner corpuscles in the subcutaneous layers; 5) pain – through bare nerve endings: hyperalgesia, hypoalgesia, analgesia; 6) specific sensations – tickling, itching. The skin can express certain emotions and feelings: anger (turning red and swelling), fear (sweating and shivering), etc.
8. Aesthetic. All skin functions are important, but the aesthetic function of the skin is no less important. Human appearance depends on skin color, hair quantity and color, and other individual characteristics.
9. Other functions – the skin produces vitamin D2, stores fats, water, trace elements, enzymes, blood. Amino acid metabolism occurs in the skin – collagen synthesis. In diabetes mellitus, sugar accumulates in the skin. Various chemical substances and drugs are easily absorbed through the skin, when treating skin diseases, and such phenomena are often harmful, as they can cause toxic and allergic reactions.
Source | Glossary of Most Commonly Used Biomedical Terms and Concepts | Lithuanian University of Health Sciences | Academician Professor Antanas Praškevičius, Professor Laima Ivanovienė