Module 4: Skin Physiology - The Epidermis

THE EPIDERMIS: The epidermis, which is several layers thick, differentiates to form the outer stratum corneum. This process, known as cornefication, is outlined below. The dermis consists of collagen, elastin and ground substance that provide the structural framework for all the components of the skin.

The diagram above is an illustration of a vertical section through the skin. The epidermis is separated from the dermis by the basement membrane (b.m.).

The stem cells of the epidermis are found along the basement membrane. These cells have the ability to divide and reproduce themselves. They can divide and move up into the epidermis or they move along the basement membrane (level I).

Once the cells move upward into the epidermis towards the air interface they start to differentiate into the corneocytes that eventually form the stratum corneum. They loose their nucleus and start to flatten. Although unable to divide, the cells in these layers still maintain multiple essential metabolic functions (levels II and III).

The fully differentiated keratinocyte is found as the corneocyte in the outermost skin layer, the stratum corneum (level IV). These cells are gradually lost into the environment in an active process known as desquamation.

Dendritic melanocytes (A), that occur predominantly at the dermo-epidermal junction produce melanin, the pigment that accounts for skin colour and radiation protection.

Dendritic Langerhans cells that patrol the epidermis are the resident antigen-presenting cells of the skin associated lymphoid tissue (SALT) system.

Merkel cells (B) also occur at the dermo-epidermal junction and are thought to have a role in sensory perception.

The main chemical and water barrier of the skin is found in a thin layer of the stratum corneum adjacent to the epidermis, which is approximately 1 micron thick.