1st layer - horny
Formally, the stratum corneum is the uppermost part of the layer called the epidermis. But in cosmetology it is usually considered separately, since it is the action of most cosmetics that is directed at it. The stratum corneum is the thinnest film on the surface of the skin, which can be raised with a needle and which forms a wall of blisters in the burn. If you put it under a microscope, you can see a lot of translucent scales (horny scales, or corneocytes), which are built from a special protein - keratin. Once horny scales were living cells, but in the process of development, the nucleus and cellular organelles were lost. Since the moment the cell loses its nucleus, it formally becomes dead. The main task of these dead cells is to protect what is under them. In other words, they perform the same role as scales of lizards or feathers of birds. Unless they look less impressive.
Horny scales closely adjoin each other, connecting with special outgrowths on the membrane. And the entire space between the layers of horny flakes is filled with a substance that is a mixture of lipids (fats). Intercellular substance performs the same role that cement masonry in a brick wall, i.e. Binds horny flakes among themselves and ensures the integrity of the entire structure. Possessing water-repellent properties, the intercellular substance of the stratum corneum does not let water and water-soluble substances enter the skin, nor does it allow excessive loss of water from the depth of the skin. It is thanks to the horny layer that the skin is a reliable barrier that protects us from the external environment and foreign substances (we will talk more about the skin barrier later).
Note that substances that make up cosmetic products are foreign to the skin, since they do not belong to the body. Carrying out its main task - to protect the body from any external influences, the skin does not rush to "take" the stranger and tries to prevent the penetration of cosmetic ingredients inside. Some cosmetics can destroy or weaken the protective layer of the skin, and then it will begin to lose moisture, and its sensitivity to environmental factors will increase.
No matter how strong the scales are and how well the cement is cemented, the tests that the skin undergoes daily are so great that the horny layer wears out very quickly (just as clothes wear out). The way out, which nature has found from this situation, begs for itself - if the clothing has worn out, it needs to be replaced. Therefore dilapidated horny scales fly off the surface of the skin and turn into ordinary household dust that accumulates on bookshelves and under the sofas (of course, not only our skin contributes to the formation of dust, but the contribution of the skin is very large).
The stratum corneum is what we see when we look at the skin, and it is also the main point of influence for cosmetic products. However, its formation begins in the depth of the epidermis, and it is there that the processes that affect its appearance occur. Working from the outside, we can decorate the stratum corneum, improve the properties of the surface (make it more smooth and plastic), and protect from damage. And yet, if we want to significantly change its structure, the impact must begin from within.
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2nd layer - epidermis
The main task of the epidermis is the production of the stratum corneum. This goal is devoted to the life of the main cells of the epidermis, which are called keratinocytes. As maturation keratinocytes move to the surface of the skin. Moreover, this process is organized so well that the cells move upwards in a single layer, "shoulder to shoulder".
The lowest layer of the epidermis, where cells are continuously dividing, is called basal. The rate of skin renewal depends on how intensively the cells of the basal layer are divided. Although many cosmetics promise to stimulate the division of cells in the basal layer, in reality, only a few are capable of this. And this is good, since under certain conditions of the skin, stimulation of cell division of the basal layer is undesirable.
The basal membrane between the basal keratinocytes is occupied by the cells responsible for the formation of the pigment (melanocytes), the immune cells responsible for the recognition of foreign substances and microorganisms (Langerhans cells) are located a little higher. Obviously, the drugs penetrating deeper than the stratum corneum will affect not only the keratinocytes, but also the cells of the immune system and the pigment cells. Another type of cells found in the epidermis - Merkel cells - responsible for tactile sensitivity.
3rd layer - dermis
The dermis is a sort of soft mattress on which the epidermis rests. From the epidermis of the dermis is delimited by the basal membrane. It contains blood and lymph vessels that nourish the skin, while the epidermis is devoid of blood vessels and completely depends on the dermis. The basis of the dermis, as the basis of most mattresses, is "springs". Only in this case it is special fibers, built from proteins. Fibers made up of collagen proteins (collagen fibers) are responsible for the elasticity and stiffness of the dermis, and the fibers consisting of the elastin protein (elastin fibers) allow the skin to stretch and return to its original state. The space between the "springs" is filled with "packing". It is formed by gel-like substances (mainly hyaluronic acid) that hold water. Although the dermis is partly protected from external influences by the epidermis and the stratum corneum, damage is gradually accumulated in it. But it happens quite slowly, as all the structures of the dermis are constantly updated. If the renewal process went equally well throughout life, the skin would always remain fresh and young. However, as the body ages, all the renewal processes in it slow down, which leads to the accumulation of damaged molecules, reducing the elasticity and elasticity of the skin, the appearance of wrinkles.
Between the fibers are the main cells of the dermis - fibroblasts. Fibroblasts are biosynthetic factories that produce various compounds (components of the intercellular matrix of the dermis, enzymes, signal molecules, etc.).
Derma is not visible from the outside. But on the condition of its structure, it depends on whether the skin looks elastic or lethargic, whether it is smooth or wrinkled. Even the color of the skin depends partly on the dermis, since the blush gives the skin blood that runs through the vessels of the dermis. At an atrophy of a derma and epidermis the skin gets yellowish color because of translucent subcutaneous fatty tissue.
4th layer - fatty tissue
Fatty tissue, as its name suggests, contains fat. And he should be where he is. Everyone, probably, had to hear the admired appreciation of a slender girl - "she does not have a gram of fat." However, if this were true, the girl would represent a miserable sight. In fact, there is no beauty without fat, as it is the fatty tissue that gives the forms a roundness, and the skin - freshness and smoothness. In addition, it softens the blows, keeps the heat and at certain periods of life a woman helps in the synthesis of female sex hormones. Fatty tissue consists of lobules separated by fibrous tissue. Inside the lobule lie fat cells, similar to pouches with fat, and also pass the blood vessels. Any violation of the quality of fat tissue - the accumulation of excess fat in the cells, thickening of the partitions between the lobules, swelling, inflammation, etc., have a catastrophic effect on the appearance.
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