The skin draws the line between the end of the organism and the beginning of the world outside. Internally, the skin shelters and protects all the physiochemical phenomenon necessary for life, externally it is a barrier against mechanical forces, both physical and chemical, which can be hostile to life. The most important role of the skin both for man and for every other organism, vertebrate or invertebrate, unicellular or multicellular, is to create an obstacle for all those things outside the organism: the rest of the world.
If life were hermetically sealed, like a pod, survival would be impossible for a being that depended on the outside world. An organism must, therefore, develop in relation to the environment in which it must live and with which it must communicate. So, the barrier which protects the organism from the outside, must at the same time inform the interior of all that is occurring outside itself. It is the perfect balance of these two barrier functions that determines survival, Furthermore, human skin acts as an organ of attraction between individuals. The appearance of the skin and hair is the first image that others have of us. Personal expression changes with variations in the condition of our hair and skin, whose appearance is derived from their intrinsic well-being. Modern cosmetology has the task of interacting with physiology in maintaining its good condition.
The skin is the largest, most extensive organ of our body. In fact, the average adult has about 170-200 square cm of skin with a weight that varies between 15 kg and 17 kg (obviously varying according to the subject's height and dimensions). The thickness of the epidermis, the outermost layer of the skin, can be from 0.5 mm in the thinnest areas (the eyelids, for example) to 4-6 mm at its thickest points (as on the palm of the hand and the sole of the foot). This thickness parameter becomes especially important when a substance is applied to the skin, be it a pharmaceutical or cosmetic product. In fact, once in contact with the skin any substance can penetrate the cutaneous barrier in a way directly proportional to the skin's thickness at that point.
The skin, even if it appears smooth and compact to our eyes, is in reality marked over its entire surface by grooves, some shallow, others deeper, which by their layout mark many small polygons. On the palm of the hand and the sole of the foot these grooves (dermatoglyphics) have become so evident as to characterize each individual and so unique that they are a distinct identification for each person. These apparently unimportant grooves are, however, necessary to accomplish an essential function-that of permitting the skin to stretch; if the skin were completely smooth, many of our movements would be impossible.
The skin tissue houses within its structure other important constituents: hairs, nails, etc. (the skin's annexes). Even with the naked eye one can see that (with the exception of the palm and sole) the whole of the skin is covered with hairs. In some areas the hairs are more developed and more coloured, as on the scalp, in the pubic region, and in the armpit. In other areas they are finer and much paler. These characteristics vary above all according to sex but also with individual biology and in the presence of certain pathologies. Furthermore, tiny, invisible openings are found over the entire skin surface. These are the outlets of the eccrine sudoriparous glands, which, together with the apocrine sudoriparous glands and the sebaceous glands.
When we consider the skin in its entirety, three different superimposed tissues can be identified:
1. Epidermis: the most external layer in contact with the environment
2. Dermis: below the epidermis it is the structural component of the skin and the underlying organs
3. Hypodermis: immediately below the dermis, composed of a layer of adipose cells and representing a cushion of fat between the skin and the organs underneath
The boundary between epidermis and dermis, the dermo-epidermal junction, is an undulating area resulting from many introflexions of the dermis and extroversions of the epidermis, dermal papillae, and epidermic crests, respectively. Along the entire dermo-epidermal junction there is a thin membrane, the basal membrane. The interconnection of the dermic papillae and epidermic crests is made functional by the presence of the basal membrane; this junction is a true structure fundamental for the relationship that exists between exchange and semipermeable barrier between the epidermis and the dermis and, consequently, also between the external environment and the internal organs. This boundary changes from zone to zone: it is flatter in the area of the forehead and highly accentuated on the back and on the soles of the feet.
The basal membrane (dermo-epidermal junction) is really a complex structure formed of many components: the cytoplasmic membrane of the basal keratinocytes, two thin layers (the lucid layer and basal lamina), and finally a fibrous structure in contact with the dermis. Toward the innermost side ofthe basal membrane there are special structures called emidesmosomes, which have an anchoring function. The fibrous structures below the basal lamina are of dermal origin and ensure the correct adhesion between epidermis and dermis.
We will now examine the epidermis, dermis, and hypodermis individually as each one contributes to the physiology of the cutaneous organ through its specific functions.
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