The structure of normal skin

Skin is an organ consisting of 3 layers: epidermis, dermis and hypodermis. The thickness of the skin without subcutaneous fat varies from 0.8 (in the eyelid area) to 4-5 mm (in the palms and soles). The thickness of the hypodermis varies from tenths of a millimeter to several centimeters.

The epidermis is an epithelial tissue of ectodermal origin, consisting of 4 layers of keratinocytes: basal, awl-shaped, granular and horny. Each layer, with the exception of the basal, consists of several rows of cells, the number of which depends on the localization of the skin area, the age of the organism, genotype, etc.

The basal or germinal (germinal) layer consists of basal keratinocytes located in one row and being the mother cell for the epidermis. These cells play a leading role in the process of eliminating the consequences of trauma. Their proliferative, synthetic activity, ability to quickly respond to neurohumoral impulses and move to the site of injury are fundamental for optimal healing of skin defects. They are the most active mitotic processes, processes of protein synthesis, polysaccharides, lipids, contain the largest amount of DNA and RNA-containing structures. These same cells are the basis for the creation of cellular compositions in the form of a multilayer layer of keratinocytes for the biotechnological treatment of skin injuries and diseases. Among the cells of the basal layer there are two types of process cells - Langerhans cells and melanocytes. In addition, the basal layer contains special sensitive Merkel cells, Greenstein cells, as well as an insignificant number of lymphocytes.

Melanocytes synthesize melanin pigment contained in melanosomes, which they transmit to keratinocytes of almost all layers, thanks to long processes. The synthetic activity of melanocytes increases under the influence of ultraviolet radiation, inflammatory processes in the skin, which leads to the appearance of foci of hyperpigmentation on the skin.

Langerhans cells. They are considered to be a type of macrophage with all the functions inherent to these cells. However, they also have significant differences from typical macrophages (a different set of receptors on the surface, limited ability to phagocytosis, a lower content of lysosomes, the presence of Birbeck granules, etc.). Their number in the skin is constantly changing due to the migration of their hematogenous precursors into the dermis, due to migration from the epidermis to the dermis and further to the lymph nodes and due to their exfoliation from the skin surface along with keratinocytes. Langerhans cells have an endocrine function, secreting a number of substances necessary for the vital activity of the skin, such as gamma interferon, interleukin-1, prostaglandins, factors regulating protein biosynthesis, factors stimulating cell division, etc. There is also data on the specific antiviral action of the cells. One of the main functions of these cells is associated with immunological reactions of the skin, local immunity. The antigen that enters the skin meets the Langerhans cell, is processed by it, and is subsequently expressed on its surface. In this form, the antigen is presented to lymphocytes (T-helpers), which secrete interleukins-2, inducing the proliferative activity of T-lymphocytes, which forms the basis of the skin's immune response.

Basement membrane. It is a formation that connects the epidermis with the dermis. It has a complex structure, including a plasma membrane with hemidesmosomes of basal keratinocytes, electron-dense and electron-transparent plates, and a subepidermal plexus of fibers. It contains glycoproteins (laminin, fibronectin, etc.), proteoglycans, and collagen types IV and V. The basement membrane performs supporting, barrier, and morphogenetic functions. It is responsible for the penetration of nutrients and water into keratinocytes and the epidermis.

The middle part of the skin or dermis occupies the main volume of the skin. It is delimited from the epidermis by the basal membrane and without a sharp boundary passes into the third layer of the skin - the hypodermis or subcutaneous fat. The dermis is built mainly of collagen, reticulin, elastic fibers and the main amorphous substance. It contains nerves, blood and lymphatic vessels, sweat and sebaceous glands, hair follicles and various types of cells. Among the cells, the bulk is made up of fibroblasts, dermal macrophages (histiocytes), mast cells. There are monocytes, lymphocytes, granular leukocytes, and plasma cells.

It is generally accepted to distinguish the papillary and reticular layers in the dermis.

The basement membrane forms outgrowths on the sides of the epidermis - papillae, which include papillary microvessels of the superficial arterial network, providing nutrition to the skin. In the papillary layer on the border with the epidermis, an area of subepidermal plexus is distinguished, formed by parallel lying reticulin and thin collagen fibers. Collagen fibers of the papillary layer consist mainly of collagen type III. The main amorphous substance is a gel or sol consisting of hyaluronic acid and chondroitin sulfates associated with water, fastening the fibrous framework, cellular elements and fibrillar proteins.

The reticular layer of the dermis occupies most of it and consists of intercellular substance and thick collagen fibers that form a network. The collagen fibers of the reticular layer mainly consist of collagen type I. In the interstitial substance between the fibers there is a small number of mature fibroblasts (fibroclasts).

The intradermal vascular bed consists of 2 networks.

The first superficial vascular network with small-caliber vessels (arterioles, capillaries, venules) is located under the basement membrane in the papillary layer. It mainly performs gas exchange and nutritional functions for the skin.

The second deep vascular network is located on the border with the subcutaneous fat tissue, the so-called vascular subdermal plexus.

It consists of larger arterial-venous vessels, mainly performing the function of heat exchangers of blood with the external environment. This network is poor in capillaries, which excludes the possibility of inappropriate intensive exchange of substances between blood and tissue. The lymphatic network, which performs a drainage function, is closely connected with the circulatory system of the skin.

The superficial lymphatic network begins from the papillary sinuses with blindly opening wide lymphatic capillaries. From these initial structures between the arterial and venous superficial networks, the superficial lymphatic plexus is formed. From the superficial plexus, lymph flows into the subdermal lymphatic plexus, located on the lower border of the skin.

The skin nerves in the form of large trunks together with the vessels enter the subcutaneous tissue through the fascia, where they form an extensive plexus. From it, large branches go to the dermis, where they branch out and form a deep plexus, from which the nerve fibers are directed to the upper part of the dermis, forming its superficial plexus in the papillary layer. From the superficial plexus, branching bundles and fibers depart to all the papillae of the skin, vessels and appendages of the skin.

Afferent nerves serve, on the one hand, as a channel of communication between the skin and the central nervous system via impulse activity, and, on the other hand, as a channel of chemical communication between the central nervous system and the skin, which underlies the trophic influence of the nervous system, which maintains the structure and integrity of the skin.

Skin receptors are divided into encapsulated, corpuscular and non-corpuscular (free nerve endings). All receptors are relatively specialized structures.

Subcutaneous fat tissue (hypodermis).

It is the third and lowest layer of the skin. It consists of fat cells (adipocytes), formed into small and large lobules, surrounded by connective tissue, in which vessels and nerves of different calibers pass.

Subcutaneous fat tissue performs many functions - supporting, protective, trophic, thermoregulatory, endocrinological, aesthetic. In addition, it is one of the depots of stem cells in the body.

Subcutaneous fat tissue is developed unevenly in different parts of the body. So, on the stomach, thighs, mammary glands, it is the thickest and reaches a thickness of more than ten centimeters. On the forehead, temples, back of the feet, hands, shins, its thickness is expressed in millimeters. Therefore, it is in these areas that thin and flat atrophic scars form after injuries, under which large vessels shine through.

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