Structure of the skin

The skin is the largest and heaviest organ of our body. In adults, it measures about 2 m² and weighs around 5 kg. This protective covering is divided into three main layers: the epidermis, dermis and hypodermis.

Submitted on 20/07/2012
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Epidermis


The outermost layer of the skin, the epidermis is a veritable shield that protects our body from harmful external factors. Yet it is very thin: depending on the region of the body, its thickness varies from half a millimetre (on the eyelids, for example) to 2 or 3 millimetres (on the soles of the feet or palms of hands). Irrigated by diffusion from the dermis, it contains no blood vessels. However, it has many nerve endings that make our skin very sensitive to touch.

The epidermis is pierced with hundreds of pores per square centimetre, through which the sweat and sebum flow. These two substances are mixed to form the hydrolipidic film on the surface that constantly moisturises and protects the skin.

The epidermis is composed of 85% of keratinocytes. These cells filled with keratin and lipids are generated by cell division in the depths of the epidermis, in the "basal layer". They are then subjected to a double movement, flattening and then gradually moving outwards. Once they are at the surface, they lose their nucleus and die, forming the "horny layer": a semi-permeable shield composed of degraded keratinocytes (corneocytes) bound together by lipids and by protein complexes that resemble staples (the corneodesmosomes). Finally, they are removed by desquamation and are replaced by the next generation of keratinocytes. Hence, the skin regenerates constantly: it is completely renewed every 4 to 6 weeks.


The remaining 15% are divided between:
 
  • Melanocytes: the function of these cells is to synthesize melanin, the natural pigment responsible for the colour of our skin that provides some protection against UV rays.
  • Langerhans' cells: True sentinels of the skin, these cells originating from the bone marrow detect and trap foreign bodies that enter the epidermis (chemical products, bacteria, viruses, etc.).
  • Merkel cells: these sensory cells are involved in the sense of touch.


Dermis


Located beneath the epidermis, the dermis is 10 to 40 times thicker. It is irrigated by numerous blood vessels that allow it to supply the skin with the nutrients it needs. It also houses the sebaceous glands and sweat glands which secrete sebum and sweat, as well as the hair follicles.

The main cells of the dermis are the fibroblasts which synthesize two types of protein fibres: collagen and elastin. Elastin makes the dermis supple and extensible, while collagen makes it resistant and enables healing of damaged tissue in case of injury to the skin.

The space between cells is called the "extracellular matrix": collagen and elastin are immersed in a gel composed of complex carbohydrates that retain water (glycoaminoglycans), such as hyaluronic acid.

Over the years, the fibres of collagen and elastin become scarce and degraded, resulting in a decrease in skin firmness and the appearance of wrinkles.


Hypodermis


The deepest and thickest layer of the skin, the hypodermis is predominantly composed of fat cells, adipocytes, that insulate body temperature changes and form a protective layer against the pressures to which the skin is subjected. It is thus very thick in areas that have to withstand frequent impacts (buttocks and heels, for example), and much thinner in other parts of the body.

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