SKIN PHYSIOLOGY

As the body’s largest organ, the skin performs a series of key functions resulting from multiple chemical and physical reactions taking place within it. The most common and obvious of these functions is to protect the body against injury, heat and light radiation; the penetration of chemical agents; and the invasion of microorganisms. The skin also acts as a temperature regulator, enabling the body to adapt to different ambient temperature and atmospheric by regulating moisture loss.

The surface of the skin, the part visible to naked eye and on which cosmetic products are applied, is made of a conglomeration of dead cells. Underneath the surface, the skin’s multiple functions take place over three very thin and distinct layers made of different skin tissues: the epidermis, the dermis, and the hypodermis.

The epidermis is very thin, Its thickness varies from 1.6mm on the soles of the feet to 0.04mm on the eyelids. The Langerhans cells, responsible for the immunology of the skin, and the melanocytes and tyrosinase enzyme, responsible for the production of melanin and skin color, are located in the epidermis. This is the layer of skin to which products are applied, and the one with which an individual comes most in contact when caring for it, whether through cleasing, exfoliating, healing, or hydrating.

The second skin layer, or dermis, lies below the epidermis and is connected to it by the basement membrane. Normally, the dermis is approximately five to seven times thicker than the epidermis. It id made of connective tissues, collagen, and elastin, and contains most of the nerve endings, blood vessels, and immune and lymphatic systems of the skin.

The third skin layer, the hypodermis, is the deepest of the three layer. Comprised primarily of connective and fatty tissues, the hypodermis is much thicker than the epidermis. Its measured thickness, however, depends on the part of the body being evaluated and the fat content of the indinidual.

A close examination of each layer, including composition and function, is important for further understanding of the impact a cosmetic product may have on the skin.

Reference: 

Skin Physiology- Delmar Cengage Learning