The skin - Dermatology and aesthetics


Dermatology and aesthetics

Dermatology and aesthetics


What is skin Types of skin
  • What is skin
  • Types of skin

What is skin

The skin, or skin, is a complex organ, consisting on the surface of the epidermis and in depth on the dermis; under the skin there is instead the subcutaneous tissue or hypodermis, rich in fat, which reaches up to the fasciae. The skin represents 5-6% of body weight and covers an area of ​​approximately 1.8 m2; its thickness varies from 0.5 mm (eyelids) to 4 mm (nape).

The skin surface has grooves that delimit lozenge areas which, in correspondence with the palm-plantar surfaces, alternate characteristic reliefs (ridges or skin papillae) that form patterns called dermatoglyphs. The orifices of the hair follicles and glands are also visible to the naked eye; skin folds, physiological, temporary or permanent (expression folds); wrinkles, which are formed in relation to aging, are not physiological and are more accentuated in the photo-exposed areas.

The skin is an organ that performs multiple functions: in the first place it acts as a barrier against mechanical (trauma), chemical (water and solutions), thermal, infectious, physical agents (electromagnetic radiation and electric currents); the dermis also provides valuable mechanical support thanks to the presence of connective fibers, which allow the body surface to cushion trauma and the pressure exerted by one's weight or other objects; the skin also avoids the loss of water by dispersion; the impermeability of the epidermis, in fact, allows neither the entry nor the escape of water and proteins (only the small-fat-soluble molecules manage to be absorbed by the epidermis).

The skin also participates in thermoregulation, providing for the regulated dispersion of heat through the skin circulation and sweating: the hypoderm prevents the thermal release from being too rapid for radiation and conduction, ensuring a sort of insulation for the body; subcutaneous tissue also represents the main nutritional reserve in humans and adipocytes synthesize hormones (liponectins) important in the control of hunger. It should not be forgotten that all sensations (touch, pressure, vibration, heat, pain) are possible thanks to the skin, which is home to numerous sensitive endings. Finally, the skin represents an important "organ of relationship", which has assumed an ever greater importance in our social behaviors.

The epidermis is a paving, stratified and keratinized epithelium, consisting of four layers: basal, spiny, grainy and horny.

The basal layer, the deepest of the epidermis, is made up of cubic or elongated keratinocytes, perpendicular to the basement membrane, to which the hemidesmosomes adhere thanks to special joints; these keratinocytes have a high proliferative activity and always generate new cells, some of which abandon the basal layer to go to the more superficial layers, up to the horny.

At the level of the deepest, basal and grainy layers, keratinocytes appear united by intercellular junctions, called desmosomes, which are responsible for the exchange of nutrients and water.

The keratinocyte cytoskeleton consists of cytokeratins, which are arranged in tonofilaments, organized in turn into tonofibrils, thin cytoplasmic fibrils, which are inserted on the cell membrane at the level of the desmosomes, reinforcing the adhesion between the cells. The basal layer performs three main functions: proliferation, adhesion between the epidermis and dermis and pigmentation. Melanocytes are dendritic cells responsible for the synthesis of melanin, a pigment that defends our skin from damage by ultraviolet rays (it is responsible for tanning): this synthesis takes place starting from an amino acid, tyrosine, and is catalyzed by an enzyme, tyrosinase; the latter is first synthesized in melanocytes and then stored in granules (melanosomes) which gradually fill with melanin and migrate to the dendrites, where they are transferred to the basal keratinocytes by phagocytosis.

In the basal layer there are also Merkel cells, containing small granules whose content is secreted following the tactile stimulation of the epidermis, determining the activation of the adjacent receptor nerve endings.

The spinous layer is made up of one or more groups of polyhedral cells, from whose periphery thin offshoots (just like spines) branch off. Tonofibrils are abundant in the cytoplasm of cells in this layer. At the level of the spines, the cells are joined together by desmosomes, which also have small granules in their cytoplasm, containing lipids and acid hydrolases (the so-called Odland granules), responsible for closing the intercellular spaces, ensuring the impermeability of the epidermis .

The grainy layer is made up of flattened and elongated cells, which have lost their spiny periphery. In the cytoplasm there are keratohyaline granules, a mixture of proteins responsible for the formation of the stratum corneum and the degradation of the desmosomes, guaranteeing the conditions for detachment at the level of the corneum. To increase the impermeability of the tissue, given by the secretion of the lipids contained in the Odland granules, also the occluding intercellular junctions contribute.

The stratum corneum is composed of particular cells, the corneocytes, devoid of nucleus and organelles, not attached to each other.

The dermis is a fibrous and dense connective tissue with intertwined bundles, which can be divided into a superficial portion (the papillary dermis) and a deeper one (the reticular dermis): in the first portion the fibrous bundles are thinner and narrower, while in the other they are coarser. Unlike the epidermis, made up mostly of cells, the extracellular component prevails in the dermis; the matrix is ​​given by a fibrous component and by an amorphous one, called the anist fundamental substance. The fibrous portion is made up of collagen and elastic fibers.

The former are inextensible, inelastic and resistant to tension; the latter, less abundant than collagen, are made up of an inextensible tubular microfibrillar component and an amorphous matrix made up of a protein, elastin, responsible for elastic behavior.

The fundamental anist substance consists mainly of water, glycoproteins and proteoglycans. There is an amorphous intrafibre matrix, present inside the collagen fibers to cement them, and an inter-fiber, which has the function of promoting the passage between the water, solutes and macromolecules fibers, keeping the dermis turgid and guaranteeing skin resistance and elasticity .

The cellular component of the dermis is mainly given by fibroblasts, responsible for the synthesis and renewal of the extracellular matrix, and macrophages. Some fibroblasts, the myofibroblasts, adhere to the collagen fibers and contract causing a retraction of the dermis; these cells are involved in wound healing processes.

Between the epidermis and the dermis there is a specialized layer of extracellular matrix, the basement membrane, which constitutes a connecting area between the different tissues; the basement membrane is also an obstacle to the spread of macromolecular complexes and a source of signals for adjacent cells.

It consists, from the outermost layer to the innermost one, of three laminae: the rare lamina, attached through hemidesmosomes to the basal layer of the epidermis; the dense lamina, an intertwining of type IV collagen molecules; the reticular lamina, made up of different fibrous structures, which on one side are inserted on the dense lamina and on the other continue in the dermis.

The hypodermis is organized into lobules separated by fibrous septa, which can be superficially rounded (areolar layer) or more flattened in depth (lamellar layer). The deepest, lamellar layer is the one that allows the sliding of the overlying layers with respect to the deep planes.

Cutaneous arterial vascularization is provided by two plexuses, one deep and one superficial, which form a rich capillary network, which can be, so to speak, "short-circuited" when the body needs to retain heat.

The skin is also rich in nerve endings, such as those close to Merkel's cells, but also other sensory structures: Meissner's corpuscles, which are responsible for detecting surface pressure; the corpuscles of Pacini, which incorporate the deep vibratory and pressure stimuli; the corpuscles of Ruffini, which respond to relaxation; the clubs of Krause and the corpuscles of Golgi-Mazzoni. Any sensory skin termination, if stimulated excessively, can cause painful sensations.

The skin has its own immune system, which includes antigen-presenting cells (APCS), which are found not only in the dermis but also in the epidermis, where they are called Langerhans cells. These are dendritic cells that absorb the antigenic molecules, hydrolyze them and re-expose them on their surface, triggering a specific immune response in the T lymphocytes. The morphological markers of these cells are the Birbeck granules.

Always to the cutaneous immune system belong macrophages, mast cells, lymphocytes.

The skin also includes hair, nails, sweat glands and sebaceous glands. The hair of our body can be thin (pelo vellus) or thick and pigmented (terminal hair), according to the age, the sex of the subject and the different body location.

Various parts are distinguished: the stem, which is the protruding portion; the root, immersed in the skin, which can in turn be divided into a deep portion, the bulbopiliferous, where the proliferative activity takes place, and a more superficial one, the hair follicle; the latter is divided into an upper funnel-shaped area, the infundibulum, and a deeper area, the collar, which continues even deeper with the body. The different life stages of the hair are divided into anagen (growth), catagen (stasis) and telogen (fall).

The hair erector muscle, whose contraction has a thermogenic function, and the sebaceous gland, whose excretory duct is given by the follicular infundibulum itself, also adheres to the basal membrane surrounding the hair follicle.

The sebaceous glands (which are not present on the palms of the hands and soles of the feet) are branched glands that produce sebum, which is responsible for protecting the skin. The sweat glands are instead tubular glands of the glomerular type which secrete a hydrosaline liquid which, by evaporating, allows the dispersion of heat when the external temperature is higher than the body temperature.

Through sweating, toxic substances (urea, metals) are also eliminated; sweat, according to the different chemical composition, is also responsible for the smell. Glands similar to the sweat glands, but deeper, are the apocrine glands, present at the perineum and armpit level, which secrete a characteristic dense and whitish substance; the ceruminous glands of the external auditory canal and the ciliary glands of the eyelids are also apocrine.

The last distal phalanx of the fingers and toes is covered dorsally by a hard plate, the nail. The nail is made up of cornified cells, not desquamating and thickly packed and cohesive with each other.

The proximal end of the nail is called the root; the portion of epidermis on which it rests, devoid of a stratum corneum and adhering to the nail plate, is called hyponichium and forms, on a proximal level, the so-called nail matrix, a paved epithelium compound used to renew the nail.

Distally the nail is separated from the fingertip by the subungual groove. The presence of the nails improves the grip in man, as well as being a defense weapon.

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