The main functional units of the skin that take part in skin defect healing and scarring

There are many adhesive molecules - they all create a support network along which cells move, binding to certain receptors on the surface of cell membranes, transmitting information to each other using mediators: cytokines, growth factors, nitric oxide, etc.

Basal keratinocyte

Basal keratinocyte is not only the mother cell of the epidermis, which gives rise to all overlying cells, but also a mobile and powerful bioenergetic system. It produces a lot of biologically active molecules, such as epidermal growth factor (EGF), insulin-like growth factors (IGF, fibroblast growth factors (FGF), platelet growth factor (PDGF), macrophage growth factor (MDGF), vascular endothelial growth factor (VEGF), transforming growth factor alpha (TGF-a), etc. Having learned about the damage to the epidermis through information molecules, basal keratinocytes and cambial cells of sweat glands and hair follicles begin to actively proliferate and move along the bottom of the wound for its epithelialization. Stimulated by wound detritus, inflammation mediators and fragments of destroyed cells, they actively synthesize growth factors that promote accelerated wound healing.

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Collagen

The main structural component of connective and scar tissue is collagen. Collagen is the most common protein in mammals. It is synthesized in the skin by fibroblasts from free amino acids in the presence of a cofactor - ascorbic acid and makes up almost a third of the total mass of human proteins. It contains proline, lysine, methionine, tyrosine in small amounts. Glycine accounts for 35%, and hydroxyproline and hydroxylysine account for 22% each. About 40% of it is found in the skin, where it is represented by collagen types I, III, IV, V and VII. Each type of collagen has its own structural features, preferential localization and, accordingly, performs different functions. Collagen type III consists of thin fibrils, in the skin it is called reticular protein. It is present in greater quantities in the upper part of the dermis. Collagen type I is the most common human collagen, it forms thicker fibrils of the deep layers of the dermis. Collagen type IV is a component of the basal membrane. Collagen type V is part of the blood vessels and all layers of the dermis, collagen type VII forms “anchoring” fibrils that connect the basal membranes with the papillary layer of the dermis.

The basic structure of collagen is a triplet polypeptide chain, forming a triple helix structure, which consists of alpha chains of different types. There are 4 types of alpha chains, their combination determines the type of collagen. Each chain has a molecular weight of about 120,000 kDa. The ends of the chains are free and do not participate in the formation of the helix, so these points are sensitive to proteolytic enzymes, in particular, to collagenase, which specifically breaks the bonds between glycine and hydroxyproline. In fibroblasts, collagen is in the form of triplet helices of procollagen. After expression in the intercellular matrix, procollagen is converted into tropocollagen. Tropocollagen molecules are connected to each other with a shift of 1/4 of the length, fixed by disulfide bridges and thus acquire a strip-like striation visible in an electron microscope. After the release of collagen molecules (tropocollagen) into the extracellular environment, they gather into collagen fibers and bundles that form dense networks, creating a strong framework in the dermis and hypodermis.

Subfibrils should be considered the smallest structural unit of mature collagen of the human skin dermis. They have a diameter of 3-5 μm and are spirally arranged along the fibril, which is considered a structural element of collagen of the 2nd order. Fibrils have a diameter of 60 to 110 μm. Collagen fibrils, grouped into bundles, form collagen fibers. The diameter of a collagen fiber is from 5-7 μm to 30 μm. Closely located collagen fibers are formed into collagen bundles. Due to the complexity of the collagen structure, the presence of spiral triplet structures connected by cross-links of various orders, the synthesis and catabolism of collagen takes a long period, up to 60 days.

In conditions of skin trauma, which is always accompanied by hypoxia, accumulation of decay products and free radicals in the wound, the proliferative and synthetic activity of fibroblasts increases, and they respond with increased collagen synthesis. It is known that the formation of collagen fibers requires certain conditions. Thus, a slightly acidic environment, some electrolytes, chondroitin sulfate and other polysaccharides accelerate fibrillogenesis. Vitamin C, catecholamines, unsaturated fatty acids, especially linoleic, inhibit collagen polymerization. Self-regulation of collagen synthesis and degradation is also regulated by amino acids found in the intercellular environment. Thus, the polycation poly-L lysine inhibits collagen biosynthesis, and the polyanion poly-L glutamate stimulates it. Due to the fact that the time of collagen synthesis prevails over the time of its degradation, a significant accumulation of collagen occurs in the wound, which becomes the basis for the future scar. The breakdown of collagen is carried out with the help of fibrinolytic activity of special cells and specific enzymes.

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Collagenase

The specific enzyme for breaking down the most common collagen types I and III in the skin is collagenase. Such enzymes as elastase, plasminogen and other enzymes play an auxiliary role. Collagenase regulates the amount of collagen in the skin and scar tissue. It is believed that the size of the scar that remains on the skin after the wound has healed depends mainly on the activity of collagenase. It is produced by epidermal cells, fibroblasts, macrophages, eosinophils and is a metalloproteinase. Fibroblasts that participate in the destruction of collagen-containing structures are called fibroclasts. Some fibroclasts not only secrete collagenase, but also absorb and utilize collagen. Depending on the specific situation in the wound, the state of the macroorganism, the rationality of treatment measures, the presence of concomitant flora, either fibrinogenesis or fibroclasis processes, i.e. synthesis or destruction of collagen-containing structures, prevail in the injury zone. If fresh cells producing collagenase stop entering the inflammation site, and old ones lose this ability, a prerequisite for collagen accumulation arises. In addition, high collagenase activity in the inflammation site does not mean that this is a guarantee of optimization of reparative processes and the wound is insured against fibrous transformations. Activation of fibrolytic processes is often regarded as an exacerbation of inflammation and its chronization, while the predominance of fibrogenesis is regarded as its attenuation. Fibrogenesis, or the formation of scar tissue at the site of skin injury, is carried out mainly with the participation of mast cells, lymphocytes, macrophages and fibroblasts. The triggering vasoactive moment is carried out with the help of mast cells, biologically active substances, which help attract lymphocytes to the lesion. Tissue decay products activate T-lymphocytes, which via lymphokines connect macrophages to the fibroblastic process or directly stimulate macrophages with proteases (necrohormones). Mononuclear cells not only stimulate fibroblast function, but also inhibit them, acting as true regulators of fibrogenesis, releasing inflammatory mediators and other proteases.

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Mast cells

Mast cells are cells characterized by pleomorphism with large round or oval nuclei and hyperchromically stained basophilic granules in the cytoplasm. They are found in large quantities in the upper dermis and around blood vessels. They are a source of biologically active substances (histamine, prostaglandin E2, chemotactic factors, heparin, serotonin, platelet growth factor, etc.). When the skin is damaged, mast cells release them into the extracellular environment, triggering an initial short-term vasodilator reaction in response to injury. Histamine is a potent vasoactive drug that leads to vasodilation and increased permeability of the vascular wall, especially postcapillary venules. In 1891, I.I. Mechnikov assessed this reaction as protective in order to facilitate the access of leukocytes and other immunocompetent cells to the lesion. In addition, it stimulates the synthetic activity of melanocytes, which is associated with the frequently occurring post-traumatic pigmentation. It also causes stimulation of mitosis of epidermal cells, which is one of the key moments in wound healing. Heparin, in turn, reduces the permeability of the intercellular substance. Thus, mast cells are not only regulators of vascular reactions in the injury zone, but also of intercellular interactions, and therefore immunological, protective and reparative processes in the wound.

Macrophages

In the process of fibrogenesis, in wound repair, lymphocytes, macrophages and fibroblasts play a decisive role. Other cells play an auxiliary role, since they can influence the function of the triad (lymphocytes, macrophages, fibroblasts) through histamine and biogenic amines. Cells interact with each other and with the extracellular matrix through membrane receptors, adhesive intercellular and cellular-matrix molecules, mediators. The activity of lymphocytes, macrophages and fibroblasts is also stimulated by tissue decay products, T-lymphocytes through lymphokines connect macrophages to the fibroblastic process or directly stimulate macrophages with proteases (necrohormones). Macrophages, in turn, not only stimulate the functions of fibroblasts, but also inhibit them by releasing inflammatory mediators and other proteases. Thus, at the stage of wound healing, the main active cells are macrophages, which take an active part in cleansing the wound from cellular detritus, bacterial infection and promote wound healing.

The function of macrophages in the epidermis is also performed by Langerhans cells, which are also found in the dermis. When the skin is damaged, Langerhans cells are also damaged, releasing inflammation mediators, such as lysosomal enzymes. Tissue macrophages or histiocytes make up about 25% of the cellular elements of connective tissue. They synthesize a number of mediators, enzymes, interferons, growth factors, complement proteins, tumor necrosis factor, have high phagocytic and bactericidal activity, etc. When the skin is injured, the metabolism in histiocytes increases sharply, they increase in size, their bactericidal, phagocytic and synthetic activity increases, due to which a large number of biologically active molecules enter the wound.

It has been established that fibroblast growth factor, epidermal growth factor and insulin-like factor secreted by macrophages accelerate wound healing, transforming growth factor - beta (TGF-B) stimulates the formation of scar tissue, Activating the activity of macrophages or blocking certain receptors of cell membranes can regulate the process of skin reparation. For example, using immunostimulants, it is possible to activate macrophages, increasing non-specific immunity. It is known that the macrophage has receptors that recognize mannose-containing and glucose-containing polysaccharides (mannans and glucans), which are contained in Aloe Vera, hence the mechanism of action of aloe preparations used for long-term non-healing wounds, ulcers and acne is clear.

Fibroblasts

The basis and most widespread cellular form of connective tissue is fibroblast. The function of fibroblasts includes production of carbohydrate-protein complexes (proteoglycans and glycoproteins), formation of collagen, reticulin, elastic fibers. Fibroblasts regulate metabolism and structural stability of these elements, including their catabolism, modeling of their "microenvironment" and epithelial-mesenchymal interaction. Fibroblasts produce glycosaminoglycans, of which hyaluronic acid is the most important. In combination with fibrous components of fibroblasts, they also determine the spatial structure (architectonics) of connective tissue. The population of fibroblasts is heterogeneous. Fibroblasts of different degrees of maturity are divided into poorly differentiated, young, mature and inactive. Mature forms include fibroclasts, in which the process of collagen lysis prevails over the function of its production.

In recent years, the heterogeneity of the "fibroblast system" has been specified. Three mitotic active precursors of fibroblasts have been found - cell types MFI, MFII, MFIII and three postmitotic fibrocytes - PMFIV, PMFV, PMFVI. By cell divisions, MFI successively differentiates into MFII, MFIII and PMMV, PMFV, PMFVI, PMFVI is characterized by the ability to synthesize collagen I, III and V types, progeoglycans and other components of the intercellular matrix. After a period of high metabolic activity, PMFVI degenerates and undergoes apoptosis. The optimal ratio between fibroblasts and fibrocytes is 2:1. As fibroblasts accumulate, their growth slows down as a result of the cessation of division of mature cells that have switched to collagen biosynthesis. Collagen breakdown products stimulate its synthesis according to the feedback principle. New cells cease to form from precursors due to the depletion of growth factors, as well as due to the production of growth inhibitors by the fibroblasts themselves - chalones.

Connective tissue is rich in cellular elements, but the range of cellular forms is especially wide in chronic inflammation and fibrosing processes. Thus, atypical, giant, pathological fibroblasts appear in keloid scars. in size (from 10x45 to 12x65 μm), which are a pathognomonic sign of keloid. Fibroblasts obtained from hypertrophic scars are called myofibroblasts by some authors due to highly developed bundles of actinic filaments, the formation of which is associated with the elongation of the fibroblast shape. However, this statement can be objected to, since all fibroblasts in vivo, especially in scars, have an elongated shape, and their processes sometimes have a length exceeding more than 10 times the size of the cell body. This is explained by the density of scar tissue and the mobility of fibroblasts. Moving along the bundles of collagen fibers in the dense mass of the scar in an insignificant amount of interstitial substance. They stretch along their axis and sometimes turn into thin spindle-shaped cells with very long processes.

Increased mitotic and synthetic activity of fibroblasts after skin trauma is stimulated first by tissue breakdown products, free radicals, then by growth factors: (PDGF)-platelet-derived growth factor, fibroblast growth factor (FGF), then iMDGF-macrophage growth factor. Fibroblasts themselves synthesize proteases (collagenase, hyaluronidase, elastase), platelet-derived growth factor, transforming growth factor-beta. epidermal growth factor, collagen, elastin, etc. Reorganization of granulation tissue into scar tissue is a complex process based on a constantly changing balance between collagen synthesis and its destruction by collagenase. Depending on the specific situation, fibroblasts either produce collagen or secrete collagenase under the influence of proteases and, above all, plasminogen activator. The presence of young, undifferentiated forms of fibroblasts; giant, pathological, functionally active fibroblasts, together with excessive collagen biosynthesis, ensure the constant growth of keloid scars.

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Hyaluronic acid

It is a natural polysaccharide, of high molecular weight (1,000,000 daltons), which is contained in the interstitial substance. Hyaluronic acid is non-species-specific, hydrophilic. An important physical property of hyaluronic acid is its high viscosity, due to which it plays the role of a cementing substance, binding collagen bundles and fibrils to each other and to cells. The space between collagen fibrils, small vessels, cells is occupied by a solution of hyaluronic acid. Hyaluronic acid, enveloping small vessels, strengthens their wall, prevents the exudation of the liquid part of the blood into the surrounding tissues. It largely performs a supporting function, maintaining the resistance of tissues and skin to mechanical factors. Hyaluronic acid is a strong cation that actively binds anions in the interstitial space, thus, exchange processes between the cellular and extracellular space, proliferative processes in the skin depend on the state of glycosaminoglycans and hyaluronic acid. One molecule of hyaluronic acid has the ability to hold about 500 water molecules near itself, which is the basis for the hydrophilicity and moisture capacity of the interstitial space.

Hyaluronic acid is found in greater quantities in the papillary layer of the dermis, the granular layer of the epidermis, as well as along the vessels and appendages of the skin. Due to numerous carboxyl groups, the hyaluronic acid molecule is negatively charged and can move in an electric field. Depolymerization of the acid is carried out by the enzyme hyaluronidase (lidase), which acts in two stages. First, the enzyme depolymerizes the molecule, and then splits it into small fragments. As a result, the viscosity of the gels formed by the acid decreases sharply, and the permeability of the skin structures increases. Due to these properties, bacteria synthesizing hyaluronidase can easily overcome the skin barrier. Hyaluronic acid has a stimulating effect on fibroblasts, enhancing their migration and activating the synthesis of collagen, has a disinfectant, anti-inflammatory and wound-healing effect. In addition, it has antioxidant, immunostimulating properties, does not form complexes with proteins. Being in the intercellular space of connective tissue in the form of a stable gel with water, it ensures the removal of metabolic products through the skin.

Fibronectin

In the process of stopping the inflammatory reaction, the connective tissue matrix is restored. One of the main structural components of the extracellular matrix is the glycoprotein fibronectin. Fibroblasts and macrophages of the wound actively secrete fibronectin to accelerate wound contraction and restore the basement membrane. Electron microscopic examination of wound fibroblasts reveals a large number of parallel bundles of cellular fibronectin filaments, which allowed a number of researchers to call wound fibroblasts myofibroblasts. Being an adhesive molecule and existing in two forms - cellular and plasmatic, fibronectin in the intercellular matrix acts as "rafters" and provides strong adhesion of fibroblasts to the connective tissue matrix. Cellular fibronectin molecules bind to each other via disulfide bonds and, together with collagen, elastin, and glycosaminoglycans, fill the intercellular matrix. During wound healing, fibronectin acts as a primary framework that creates a certain orientation of fibroblasts and collagen fibers in the repair zone. It binds collagen fibers to fibroblasts via actinic bundles of fibroblast filaments. Thus, fibronectin can act as a regulator of the balance of fibroblastic processes, causing fibroblast attraction, binding to collagen fibrils, and inhibiting their growth. It can be said that due to fibronectin, the phase of inflammatory infiltration in the wound itself passes to the granulomatous-fibrous stage.

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