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) to 4-5 mm (in the palm 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, subulate, granular and horny. Each layer, with the exception of the basal layer, consists of several rows of cells, the number of which depends on the localization of the skin area, the age of the organism, the genotype,

Basal or germinal (germ) layer consists of basal keratinocytes, located in one row and being the mother cell for the epidermis. These cells have a leading role in the process of eliminating the consequences of trauma. Their proliferative, synthetic activity, the ability to react quickly to neurohumoral impulses and to move to the focus of trauma are fundamental for optimal healing of skin defects. In them, the most active processes are mitotic processes, protein synthesis processes, polysaccharides, lipids, contains the greatest amount of DNA and RNA-containing structures. The same cells are the basis for the creation of cellular compositions in the form of a multilayer keratinocyte layer for biotechnological treatment of skin lesions and diseases. Among the cells of the basal layer there are outgrowth cells of two types - Langerhans cells and melanocytes. In addition, in the basal layer there are special sensitive Merkel cells, Grenstein cells. As well as a small number of lymphocytes.

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

Cells of Langerhans. It is believed that they are a kind of macrophage with all the functions inherent in these cells. However, they also have significant differences from typical macrophages (another set of receptors on the surface, a limited capacity for phagocytosis, a lower content of lysosomes, the presence of Beerbeck granules, etc.). Their amount in the skin is constantly changing due to the migration of their hemagogenic precursors to the dermis, by migrating from the epidermis to the dermis and further into the lymph nodes and by sloughing them off the surface of the skin along with keratinocytes. Langerhans cells have an endocrine function, secreting a number of substances necessary for the life of the skin, such as gamma-interferon, interleukin-1, prostaglandins, factors of protein biosynthesis regulation, factors that stimulate cell division, etc. There are also data on the specific antiviral action of cells. One of the main functions of these cells is associated with immunological skin reactions, local immunity. The antigen that enters the skin meets the cell of Langerhans, it is processed by it with the subsequent expression on its surface. In this form, the antigen appears to the lymphocytes (T-helpers). Which secrete interleukins-2, inducing proliferative activity of T-lymphocytes, which is the basis of the immune response of the skin.

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

The middle part of the skin or dermis occupies the bulk of the skin. It is bounded from the epidermis by the basal membrane and without a sharp border passes into the third layer of the skin - hypodermis or subcutaneous fat. Dermis is built mainly of collagen. Reticulin, elastic fibers and a basic amorphous substance. It contains nerves, blood and lymph vessels. Sweat and sebaceous glands, hair follicles and various kinds of cells. Among the cells, the bulk is made up of fibroblasts, dermal macrophages (histiocytes), mast cells. There are monocytes, lymphocytes, granular leukocytes, and also plasma cells.

It is customary to isolate the papillary and reticular layers in the dermis.

The basal membrane forms outgrowths in the sides) of the epidermis - the papillae, which include the papillary microvessels of the surface arterial network that provides nourishment to the skin. In the papillate layer on the border with the epidermis, the subepidermal plexus region is formed, formed parallel to the recurrent retikulinovymi and thin collagen fibers. Collagen fibers of the papillary layer consist mainly of type III collagen. The basic amorphous substance is a gel or sol consisting of hyaluronic acid and chondroitin sulfates bound to water, binding the fibrous framework, cellular elements and fibrillar proteins.

The mesh layer of the dermis occupies most of it and consists of intercellular substance and thick collagen fibers forming a network. The collagen fibers of the mesh layer are mainly composed of type I collagen. In the interstitial matter between the fibers there is a small amount of mature fibroblasts (fibroblasts).

Intradermal vascular bed consists of 2 nets.

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

The second deep vascular network is located on the border with subcutaneous fat, 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 ineffective intensive metabolism between blood and tissue. With the circulatory system of the skin, a lymphatic network is closely associated with the drainage function.

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

Nerves of the skin in the form of large trunks along with the vessels enter through the fascia into the subcutaneous tissue, where they form a vast plexus. From it large branches go to the dermis, where they branch and form a deep plexus, from which the nerve fibers go to the upper part of the dermis, forming in its papillary layer its surface plexus. From the surface plexus branching bundles and fibers to all papillae of the skin, vessels and appendages of the skin.

The afferent nerves serve as a channel of the skin-to-CNS communication with the help of impulsive activity on one side and, on the other hand, the channel of the chemical connection of the CNS with the skin, which underlies the trophic influence of the nervous system supporting the structure and integrity of the skin.

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

Subcutaneous fatty tissue (hypodermis).

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

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

Subcutaneous fat tissue is developed unequally in different parts of the body. So on the abdomen, hips, mammary glands it is the most thick and reaches a thickness of more than a dozen centimeters. On the forehead, temples, rear of the feet, hands, legs, its thickness is expressed in millimeters. Therefore, it is in these areas after injuries that thin and flat atrophic scars form, under which large vessels shine through.

[1], [2], [3], [4], [5], [6], [7]