The skin (from Latin pellis) constitutes the barrier between the external environment and the interior of the body.

Overall structure. The skin is a part of the integument which corresponds to the whole of the skin and the appendages, that is to say the hairs and the nails. The skin is made up of a superficial part, the epidermis, and an internal part, the dermis and the hypodermis. (1)

Structure of the epidermis. It constitutes the outer envelope of the skin and corresponds to the thinnest layer, the thickness of which varies from 1 to 4 mm depending on the different parts of the body. The epidermis is made up of two layers:

• the basal layer, located deepest in the epidermis. It contains several cell layers, including keratinocytes and melanocytes, and is responsible for cell renewal in the skin.
• the horny layer, located on the surface and being in direct contact with the outside. It contains aged keratinocyte cells from the basal layer. These cells rise to the surface and then shed, that is to say that the dead cells come off in horny shreds. This phenomenon allows the continual renewal of the epidermis. (1)

The epidermis is not vascularized, which is why the dermis, located just below the latter, plays an important role in its functioning.

Structure of the dermis. This deep layer corresponds to the thickest layer. It contains many elements (2):

• hair follicles with their appendages
• the arrector smooth muscles, responsible for the bristling of the hairs
• the sebaceous glands, which secrete sebum
• sweat glands, secreting sweat
• sensitive nerve endings
• collagen and elastin fibers
• blood and lymphatic vessels

Structure of the hypodermis. Also called subcutaneous tissue, the hypodermis is located under the dermis and constitutes a layer of fatty tissue or adipose tissue.

Protective role. The skin protects the body from external aggressions such as sun rays, heat, shocks or even infections. (3)

Sensory role. The different nerve endings in the skin are used to transmit thermal, tactile, and pain sensations. (4)

Maintain temperature. The body temperature is maintained at 37 ° C thanks in particular to the blood vessels, as well as the sweat glands.

Waste disposal. Some body wastes are eliminated through sweat through the skin.

Diseases of the skin. Pathologies can affect certain constituents of the skin. Their causes and symptoms vary widely.

• Acne. This chronic pathology affects the hair follicles as well as the sebaceous glands, the secretion of which is impaired. It manifests itself on different scales by the retention of sebum, inflammation and infection. Symptoms are the appearance of blackheads, pimples, and other skin changes.
• Psoriasis. It corresponds to a chronic inflammation of the skin and is due to an alteration in the renewal of keratinocytes.

Skin cancer. Malignant tumors can develop in the skin. There are two types: carcinomas and melanomas. (6)

Medical treatment. Depending on the pathology diagnosed, different treatments may be prescribed to reduce pain, inflammation and other skin problems.

Surgical treatment. Depending on the type of pathology, a surgical operation can be carried out, in particular a skin graft.

Chemotherapy, immunotherapy, radiotherapy. Depending on the stage of the cancer, these therapies may be prescribed.

Physical examination. Carried out by a dermatologist, this examination makes it possible to identify and assess a skin condition.

Medical imaging examination. Depending on the proven or suspected pathology, additional examinations may be performed such as an X-ray, a CT scan or an MRI.

The first total skin transplant was performed in 1870 by Georges Lawson in London. (7)