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The skin

Skin is the largest organ of the human body, a protective barrier that insulates the body from the surrounding environment, protecting it and maintaining their structures, while serving as the communication system with the environment.

The largest organ of the human body

Other functions that the skin has are: maintaining body temperature, feeling the sense of touch, pain and temperature or UV absorption for the synthesis of vitamin D. There are skin appendages such as hair, nails, sweat glands and sebaceous glands; the secretions os these glands keeps the skin hydrated. The skin is responsible for receiving external stimuli through the nerve endings that lie therein, which travel to the cerebellum that tell us how we should react.

Every square inch of skin contains about five thousand sensory receptors.  The skin is the organ which allows us to feel a caress or to notice the fire heat or the coldness of snow. But at the same time the skin is the mirror of the inner feelings and emotions. For example when we blush because of something embarrassing, having “Goosebumps” and sweating when we are afraid are some of the many emotional responses revealed through the skin. For this reason, is not a surprise that skin is a key element in the external image of a person. Moreover, the skin serves as other social functions. It is necessary that skin, especially the one that everybody can see, looks healthy and in best conditions.

SKIN STRUCTURE

The skin consists of three layers: epidermis, dermis and hypodermis.

Epidermis: is the outermost layer of the skin and serves as a barrier to protect the body against microbial pathogens, oxidant stress  (UV light), chemical compounds  and provides mechanical resistance. The epidermis is avascular, nourished  almost exclusively by diffused oxygen from surrounding air. It is mainly composed of keratinocytes (95%) but it also contains melanocytes, Langerhans cells, Merkel cells and inflammatory cells.

The epidermis is composed of 5 different layers:

The cornified layer (Stratum corneum) is the outermost layer of the epidermis. Composed of 10 to 30 layers of corneocytes.  Most of the barrier functions of the epidermis are localized in this layer, since it works as a permeable protective layer (the acid pH of the skin plays an essential role in the formation of the permeability barrier), against external factors and endogenous water loss. Clear/translucent layer (stratum lucidum, only in palms and soles), granular layer(stratum granulosum) and spinous layer (stratum spinosum) are where some polar lipids are converted into non-polar lipids and arranged parallel to the cell surface, then the keratinocytes become connected through desmosomes and start producing lamellar bodies.The basal/germinal layer (epiderm stratum basale/ germinativum) is the lowest layer of the epidermis. Basal cells are directly in the basement membrane, forming a well-defined boundary between the dermis and the epidermis. Basal cells act as stem cells, ensuring continuous skin regeneration by cell division (proliferation). Because of cell division, the new cells are slowly moved towards the outer layers where they pass through several stages of development. Melanocytes, the cells responsible for pigmentation, are also found in the basal layer.

 

Dermis: The dermis is a layer of skin between the epidermis and subcutaneous tissues, that consists of connective tissue, which cushions the body from stress and strain. The dermis is composed by fibroblasts, collagen and elastin fibers and other structural molecules. The thickness of the dermis increases throughout childhood and adolescence, then remains stable and slowly starts to decline after 50 years old. 

The dermis has 2 layers: the superficial dermis (stratum papillare) and deep (or stratum reticular) dermis. The dermis is highly vascularized and in addition to its main role, as support (provided by the fibers of collagen and elastin), it plays an important nutritional role too. It is involved in thermoregulation, cicatrization and the disposal of waste (through sweat containing urea).

Hypodermis: also called the subcutaneous tissue is the lowermost layer of the integumentary system in vertebrates. The types of cells found in the hypodermis are fibroblasts, adipose cells, and macrophages. The hypodermis has a variable thickness depending on their location (fine in the forehead, thick on the hips). Represents between 15 and 30% of body weight (approximately 8 to 20 kg).

 

The main role of the hypodermis is to store energy reserves for the body. It acts as an interface between the dermis and the mobile structure below it (muscles, tendons, ...).It also protects the body from impact and constitutes a heat protection. With age, the disappearance of existing conjunctive walls between the lobes of adipocytes leads to the weakening of the tissues and, therefore, loss of skin density.