Skin – organ

Skin covers the human body. It consists of the 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 reaches about 2 m². The thickness of the true skin ranges from 0.5 to 4.75 mm, while the subcutaneous tissue thickness depends on the fat layer’s thickness.

The epidermis forms the skin’s outer layer. Its lower surface appears larger because its projections descend into the true skin. The formed epidermis includes five layers:

1. Basal Layer: This layer reaches the true skin through the basal membrane. It comprises a row of cylindrical cells that exhibit maximal mitosis and synthesis activity. This layer also contains melanocytes, which influence skin color with their functional properties and have branched projections similar to nerve cells.
2. Spinous Layer: Contains 6-8 rows of polygonal cells connected through protoplasmic projections. The upper rows of cells are flattened. Langerhans cells, which can phagocytize and migrate between the skin and lymph nodes, are present in this layer.
3. Granular Layer: Consists of 2 layers of grainy cells with keratohyaline granules in their protoplasm.
4. 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 protects against water and electrolytes.
5. Horny Layer: Composed of 5-8 rows of horny plates that desquamate, known as stratum disjunctum. The lower rows are compactly compressed and impregnated with fats and other substances. This layer makes up 0.01 of the skin, up to 0.5 of the skin in areas like the palms and soles. The epidermis lacks blood vessels, so cells get nourishment from the dermal papillary layer through osmosis. Normal epidermal turnover occurs every 25-35 days, during which all layers change. The basal membrane, composed of lamina lucida and lamina densa, separates the epidermis from the dermis.

The dermis contains two layers (papillary and reticular). It houses muscles, skin appendages, lymph nodes, blood vessels, nervous elements, collagen, elastic fibers, various cellular elements like fibrocytes and leukocytes, and skin structures such as hair, nails, sebaceous and sweat glands.

The layers of the dermis include:

1. Papillary Layer (Stratum Papillare): These papillae (about 300 per mm²) form a unique pattern for each individual.
2. Reticular Layer (Stratum Reticulare): A network of fibers that gradually transitions into the subcutaneous tissue.

Muscle fibers in the true skin include smooth muscles like nipple muscle, anal sphincter muscle, arrector pili muscle, and striated muscles such as m. platysma and facial mimic muscles.

The hypodermis connects the skin to underlying tissues. The subcutaneous tissue comprises a looser network of connective tissue filled with fat cells, blood vessels, nerves, and lymphatic vessels. Sweat gland parts are also located here. The subcutaneous tissue gives shape to the body, protects against cold and injuries, and provides a source of nutrients. Its thickness depends on age, sex, and constitution.

Skin appendages include 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): All layers, especially the intact horny layer, protect against mechanical damage, chemical, and thermal injuries. Dermal fibers and subcutaneous tissue absorb mechanical impacts. Dry skin resists electricity. The skin protects against microbes and harmful substances because of an acidic environment, acting as a barrier that neutralizes harmful chemicals. Reflecting part of UV radiation, the dermis absorbs UV rays, while a thickened horny layer and melanin act as protection.
2. Immunological: The skin participates in increased sensitivity to plasma cells and immune reactions (alongside lymph nodes, T and B lymphocytes, Ig). Immunodepressive factors disrupt protective reactions, leading to pathological processes of bacterial, viral, and fungal diseases. Certain lymphocytes recirculate and protect the skin. The epidermis promotes T lymphocyte proliferation, while TH helps B lymphocytes become plasma cells that produce Ig. Plasma cells with histamine and other mediators are involved in hypersensitivity reactions. The skin has a rich network of blood and lymphatic vessels.
3. Thermoregulatory: It maintains a constant body temperature through conduction, radiation, and evaporation. Heat dissipation involves the stimulation of the thermoregulatory center, reflexive dilation of capillaries, and sweating. The skin has about 4 million sweat glands, mostly in the palms and soles.
4. Excretory: Sweat and sebaceous glands perform excretion. Sweat glands eliminate excess sugar, drugs, and heavy metals through weakly acidic sweat (composed of 99% water and various substances). About 20% of total body water content gets removed through the skin. Apocrine sweat glands secrete a mixture containing glycogen, cholesterol, and iron. Skin fats, secreted at about 2 grams per day, contain neutral fats, free fatty acids, nitrogen and phosphorus compounds, steroid hormones, and more. This mixes with sweat to form a protective film on the skin surface, aiding in the secretion of drugs and toxic endogenous substances.
5. Respiratory: Although insignificant, the skin absorbs about 4g of O2 and releases about 8g of CO2 per day, accounting for 2% of total gas exchange.
6. Absorptive: Depends on the buffer properties of the skin surface and glands, epidermal maceration, skin hyperemia, the duration of contact with substances, and their chemical properties. Various substances can absorb through hair follicles and sebaceous gland ducts, although the horny layer significantly hinders absorption, protecting against intoxication. Substances dissolved in lipids are more easily absorbed, and ultrasound can further increase absorption. Gaseous and liquid substances also pass through easily.
7. Sensory: Performed by nerve endings and corpuscles. The skin senses warmth (through Ruffini corpuscles in the dermis and hypodermis), cold (through Krause corpuscles in subcutaneous layers), pressure (through Pacinian corpuscles in subcutaneous tissue), touch (through Meissner corpuscles in subcutaneous layers), and pain (through bare nerve endings: hyperalgesia, hypoalgesia, analgesia). It also detects specific sensations like tickling and itching. The skin expresses emotions and feelings: anger (turning red and swelling), fear (sweating and shivering), etc.
8. Aesthetic: Human appearance depends on skin color, hair quantity and color, and other individual characteristics.
9. Other Functions: Produces vitamin D2, stores fats, water, trace elements, enzymes, and blood. It participates in amino acid metabolism, like collagen synthesis. In diabetes mellitus, sugar accumulates in the skin. The skin easily absorbs various chemical substances and drugs, and when treating skin diseases, such absorption 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