The skin is the largest organ of the body. Skin is composed of inner "living" and outer "dead" layers. The dead layers include the surface scale, outside hair and exposed nail plate. The living layers include the epidermis, dermis, subcutaneous and all the structures found within, which include blood vessels, sweat glands, etc.

The information needed to understand cancer of the skin is somewhat different from the information necessary to understand the many thousands of other skin diseases. For the purposes of the Web site, this section's discussion will be restricted to the anatomy of the skin and focus less on body chemistry and immune system facts regarding the formation of skin cancer. Because skin cancers only rarely (one in one-hundred to one in one-thousand) penetrate into the bone or muscle, for simplicity let us review the layers of the skin that lie between the scale of the epidermis and the deep fat that overlies the muscle and bone. Please look at the diagram of the skin and its appendages (specialized structures). Note the location of the principle layers and how some structures, such as hair pores, penetrate the layers of the skin. Click to the glossary for definitions and explanations of the importance of each layer.

The principle layers that dermatologists work in and study are the epidermis, dermis and subcutaneous. Usually the uppermost layer of the epidermis on most exposed tissue, the STRATUM CORNEUM, also referred to as the horny layer, is made up of tightly packed, scale-like, dead keratinocyte cells, that are constantly being replaced by new cells from the lower epidermis underneath. These dead keratinocyte cells consist mostly of the protein keratin and contribute greatly to the outer physical barrier of the skin. The outermost stratum corneum is relatively acidic and is therefore also called the acid mantle.

The EPIDERMIS, the outermost living layer of the skin, contains no nerves or blood vessels, so this layer alone is incapable of sensation or bleeding. The epidermis is composed of a protective outer layer of nonliving keratin-derived scale cells. The epidermis acts an envelope or seal from the environment and exerts some control on the amount of moisture lost from the body. The epidermis contains mainly keratinocytes (cells that make keratin) and melanocytes (cells that make pigment), as well as other specialized cells.

The DERMIS, the layer of the skin that lies just below the epidermis on most of the body, is largely made up of collagen (fibrous or connective) tissue. The dermis, as a layer, makes up the bulk of the skin and is usually thickest on the back and the back of the neck. The dermis may best be thought of as the "leather layer" of the skin. The dermis protects the body from mechanical injury, binds and stores water, maintains temperature and carries nerves to detect sensation and feeling.

The SUBCUTANEOUS layer of tissue lying below or under the cutaneous (dermis, leather) tissue is usually composed of fat cells and the structures that run through it (i.e. blood vessels, nerves, etc.). Fat is a soft, shock-absorbing barrier that acts as a conduit for the passage of nerves and blood vessels. Subcutaneous tissue is also an energy storage device. (See Fat in glossary.) The sebaceous glands of the hair follicle are said to arise in the subcutaneous layer. Sebaceous glands are in high concentration on the face and scalp. There are no sebaceous glands on the palms or soles.

If you require further information on the structure and function of the skin, please seek a good dermatologic textbook, such as Fitzpatrick or Rook.