The skin or skin, although varying according to the age, sex, and size of each, is the thinnest organ (0.5 to 4 mm thick), the largest (about 2 m2 of the surface) and simultaneously more heavy (between 8 and 10 kg) of the human body. The function of the skin is to coat the other organs of the body, protecting them; it also has the function of thermoregulation and reception of external stimuli (pain, pressure, etc.).
It is a mechanism through which the temperature of the body is regulated. The skin disperses heat through sweating and spares it through horripilation ('' goosebumps '').
The skin color comes from the sum of three colors: the gray, typical of the keratin of the stratum corneum, the brown of melanin, and the red of the blood that circulates in the dermis. The surface of the skin has folds and reliefs, and many small holes. The latter are of two types, the follicular ostium, visible to the naked eye, which constitutes the outlet to the outside of the pilosebaceous follicles, and the sweat pore, much smaller and not visible, the outlet of the sweat glands. The skin is made up of a set of three tissues, arranged one on top of the other, with different characteristics and functions:
- epidermis
- dermis
- hypodermis
The so-called "skin appendages" complete the skin structure, which includes the glands, the circulatory system, and the nerve endings.
The epidermis acts as a barrier: on the one hand, it prevents the penetration from the outside of water, foreign substances, and microorganisms and, on the other hand, the loss of water and electrolytes from the body. It is a fabric approximately 0.2 mm thick, made up of several layers, the main component of which is keratin. The stratum corneum, the outermost one, is formed by dead keratinized cells, which are continuously renewed and eliminated according to a 3-4 week cycle.
It is divided into 5 layers:
1. Stratum basale,
2. Stratum spinosum,
3. Stratum granulosum,
4. Stratum lucidum,
5. Stratum corneum
It is located in inferior contact with the dermis, it is formed by a single row of cylindrical cells and represents the area in which keratinocytes are produced. It starts from stem cells characterized by an intense proliferative capacity; these cells undergo mitotic divisions approximately every 19 days. One of the two daughter cells will be destined to become a new cell of the basal layer, subsequently meeting a new cell division while the other cells will be pushed towards the external surface of the epidermis. The time required for a cell to reach the epidermal surface is approximately 40-56 days.
This layer, placed above the basal layer, is made up of 8-10 polyhedral-shaped keratinocyte layers. It is more voluminous than that of the basal layer and characterized by numerous small spine-like overturns through which they adhere to each other. During their relocation to the surface, these cells flatten out.
The cytoplasm of these cells gradually becomes acidophilic and contains melanosomal complexes, keratin filaments. When they go up towards the upper rows, the / lamellar bodies are also called keratinosomes. These are secretory organelles containing lipids and glycoproteins, which, once reached the granular layer, pour their contents into the extracellular environment, where they give rise to epidermal barrier lipids.
It is made up of tapered and flattened keratinocytes, with a small nucleus, and arranged in 2-5 rows. The cytoplasm of these cells is rich in keratohyalin granules. Keratohyalin is an important protein for the synthesis of keratin and filaggrin and is subsequently degraded, and its elementary constituents become part of the natural moisturizing factor (NMF). In this layer, as already mentioned, the lipid material released by Odland's bodies is arranged between the cells of the upper layers, forming the corneocytes cohesion, a mixture of substances.
The shiny layer is present only in some areas of the body (in the so-called thick skin present in the areas of greatest friction of the body such as the palms of the hands, the soles of the feet and the tips of the fingers. It is composed of flattened and nucleus-free keratinocytes, arranged in 2-3 rows and equipped with a cytoplasm rich in eleidin, an amorphous substance rich in lipids and sulfur which refracts light and prevents tanning in these locations.
It is composed of 25 or more layers of corneocytes, which are flat anucleated keratinocytes equipped with a cytoplasm rich in keratin fibers immersed in a matrix of filaggrin. These cells are extremely flattened and represent a large surface (45 times greater than that of a keratinocyte of the basal layer). Consequently, it tends to further increase with advancing age due to the slowdown of epidermal turnover and their greater permanence at the level of the surface layers.
The dermis is a connective tissue, 3-4 mm thick, underlying the epidermis, mainly characterized by elastin fibers, which ensure the right elasticity to the skin, by collagen fibers, with a function of support and resistance and by the fundamental substance that has a cementing function. As it is rich in blood and lymphatic vessels, the dermis also has a nutrition function. Several skin appendages pass through the dermis, such as the sweat glands, hair follicles, and sebaceous glands.
Connective tissue
It is a type of tissue that fulfills nourishing and supporting functions.
The hypodermis is the third and deepest skin layer, directly in contact with the dermis on one side and with the fat and subcutaneous muscle tissues on the other.
Like the dermis, the hypodermis is made up of connective tissue and is particularly rich in adipocytes, the cells responsible for the biosynthesis of fats. Due to the presence of this cell type, this tissue acts as an energy reserve and, at the same time, as thermal insulation and as a pad. In the hypodermis
, the follicles and the sweat glands originate: it is here in the fact that they receive nourishment and give up their waste products.
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