The skin is composed of three parts, the epidermis, the most superficial part, the dermis, the intermediary part and the hypodermis, the deepest part.
The epidermis is very thin and composed of several layers.
The epidermis renews itself in approximately 28 days. The keratinocytes are formed in the basal layer and certain keratinocytes then divide themselves into two by mitosis. One will stay in the basal layer for keratinocyte reproduction, and the other will move to the surface of the epidermis, pushing aside the older kératinocytes. During the rising towards the surface of the epidermis, the keratinocytes change form, losing their core and loading themselves with very resistant keratin filaments. They go through the basal layer, the spinous layer, the granular layer (where keratin is produced) and finally the corneum layer. Thus, first-degree wounds heal due to the natural renewal of the epidermis.
The corneum layer, the most superficial layer of the skin is composed of dead keratinocytes cells which form a solid membrane due to keratin. It plays the role of the epidermis' « barrier ». It blocks the entrance of toxic substances and also prevents the vital liquids from escaping. Once at the surface, the cells fall off, taking away the eventual germs and foreign substances with them. This is the desquamation process. This mechanism of cellular programmed death is called apoptosis.
The epidermis is composed principally of keratinocytes. Between the keratinocytes are found the Langerhans cells, which destroy the chemical substances found in the dermis, as well as melanocytes, which make an ultraviolet-absorbing pigment (melanin) and transmits this to the neighbouring keranocytes. They decide of the skin's colour, of its tan.
The dermis nourishes and sustains the epidermis. It is a connective tissue, four times thicker than the epidermis. It is formed by an interwoven network of collagen fibers which make the scar tissue to repair cuts or scrapes, and of elastic fibers which allow the skin to get back into place after stretching. Besides being strongly vascularized, the dermis contains numerous vessels, blood-transporting and lymphatic, nerves, nerve endings and sweat glands which insure perspiration and are the basis of the pilosebaceous system.
It is a subcutaneous adipose layer, strongly vascularized. It plays a protective role as a shock-absorber between the dermis and the bones, a thermic, morphologic and energetic isolator by the stocking of fats.
Ageing, a natural process
The epidermis renews itself in approximately 28 days. But with age, the renewal of cells slows down and can take more than 30 days. The older cells can then accumulate, and give the impression of a blurred complexion and thicker skin, translating as well into skin dryness, which is generally the first cause of complaint of ageing people. If the dryness isn't remedied, wrinkles will more easily leave their print on the skin, especially since the skin also has a tendency to thin. Simultaneously, modifications of the dermis appear, responsible for the loss of tonus and for the sagging also underpinned by the melting of the fat and muscular tissues which no longer play their sustaining role. The volume and aspect of the face progressively change while wrinkles deepen.
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