Biologie de la peau

Normal human skin

Friday 28 January 2011 by Michel Démarchez

The skin is the envelope of our body and is the most visible and the largest organ, with a surface area of 1.8 m2 and a weight of 3 kg.

The skin fulfils many functions, the main one being a barrier function. Being in direct contact with the outside environment, skin is a dynamic barrier that allows and limits inward and outward passage of water, electrolytes and various other substances and that protects against microorganism invasion, toxic agents, ultraviolet radiation   and mechanical insults. Skin is also a temperature regulator, an immune organ that can detect infections, and a sensory organ to detect temperature, touch, pain, itch and mechanical stimuli at every point of the body. Healthy skin being a major component of our physical appearance, skin play a major role in our social and sexual communication.

The skin is a complex organ organised in three layers, the epidermis   (and its associated appendages, pilosebaceous follicles and sweat glands), the dermis and the subcutis or hypodermis  . The dermo-epidermal junction is often the subject of specific studies; it is composed of an extracellular matrix, the constituents of which are produced by dermal and epidermal cells.

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Schematic representation of human skin and of its main components

 1. The different compartments of the skin

The epidermis, is the outer layer of the skin in contact with the external environment; it is a pluristratified keratinised squamous non vascularised epithelium and is mainly composed of layers of keratinocytes (90 to 95%). The epidermis also contains melanocytes involved in skin pigmentation  , Langerhans cells from the immune system, and Merkel cells considered as nervous cells. It is a dynamic and regenerating organ as keratinocytes of the outer layers are continuously shed and replaced by inner cells moving up to the surface. In this tissue, the extracellular compartment is reduced to the inter-cellular cement.

On the opposite, the dermis is composed of a relatively small number of cells and a well-developed extracellular matrix. The fibroblasts are the main resident cells of the dermis. The dermis receives a rich blood supply, the epidermis being non-vascular.

The hypodermis is the deepest part of the skin and the thickest (up to 3 cm thick on the abdomen); it is an important depot of fat. It separates skin from the underlying tissues, aponevroses or the periosteum.

The skin vascularisation is made through a well-developed network of blood vessels that exceed the only local nutitional needs since it is also involved in the thermoregulation  , the wound healing, the immune responses and the blood pressure control of the whole body.

In direct contact with the external environment, the skin is a major sensory organ and the skin innervation is rich and complex with an afferent and an efferent limb, the three cutaneous compartments, namely, hypodermis, dermis and epidermis (with the exception of the horny layer) being innervated. Close contacts between nerve fibers and the other skin cells have been observed. The skin nerve fibers produce locally neuromediators and neurohormones. Epidermal and dermal cells and the immune cells in transit in the skin are also able to produce neuromediators and enzymes able to degrade them; they also express receptors specific for these neuromediators and are therefore sensitive to them. All these observations led to the concept of a neuro-immune-cutaneous-endocrine system (NICE) which would include the skin cells and the skin components of the nervous system and of the immune system. The main functions of the skin would be controlled by the NICE. This concept would allow to explain the influence of the psychism in the maintenance of the skin hoemeostasis and in the triggering of various dermatological disorders, such as psoriasis or eczema.

Deeply embedded into the dermis are the epidermal appendages, such as the pilo-sebaceous follicles and the sweat glands. Nails are other derivatives of the epidermis.

 2. Regional variations of the skin

Human skin is glabrous or hairy. Glabrous skin is observed on the palms, the soles, and glans penis and is characterised by a thick epidermis with dermatoglyphics (fingerprints). On the rest of the body, skin is hairy with a density and a structure of hairs that can vary from site to site.

There are approximately 2000 or more melanocytes per square millimeter in the exposed skin of the head, in the skin of the scrotum or in the foreskin and 1,000 to 1,500 melanocytes per square millimeter on the rest of the body in Caucasoids, Negroids and Mongoloids.

Skin is 0.6mm thick on the body, 0.12mm thick on the facial skin, thinnest on the lips and around the eyes (0.3mm on the eyelid) and thickest on the palms and soles (1.2mm to 4.7mm).

 3. The functions of the skin

Skin fulfills several functions, the major one being a barrier function. Being in contact with the external environment, skin constitutes a dynamic barrier which regulates the body loss of water and electrolytes and protecting the organism against the entrance of toxic agents and microorganisms, against the damages of ultraviolet radiation   or of mechanical forces. The skin is also a thermoregulator, an immune organ which detects and fights against infections, and a sensory organ which transmits information about temperature, touch, pain, itch and mechanical stimuli at every point of the body. Moreover, an healthy skin plays a major role in our social and sexual relationships.

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