The skin is the body's largest organ. It protects underlying tissue and internal organs from injury and microorganisms, controls the loss of water and other fluids, and regulates body temperature. The skin also helps the body get rid of excess water and salts, and allows you to feel pain, temperature, and touch.

The skin has three main layers:

• The epidermis is the thin top layer of skin we can see with the naked eye. It functions as an effective, waterproof barrier and protects us from bacteria and other microscopic organisms in the environment. The epidermis does not have its own blood vessels, so must be supplied with nutrients from lower layers of the skin.

  • Keratinocytes are the most abundant cells in the epidermis. The outermost part of the epidermis is made up of dead keratinocytes that are shed from the skin continually. Located beneath this outermost layer are squamous cells, live keratinocytes that make keratin, an important skin protein. Basal cells form the basal layer, the innermost layer of the epidermis. These cells divide to produce new keratinocytes, which move up through the layers of the epidermis to replace older cells as they die and are shed.
  • Melanocytes are located in the basal layer of the epidermis. They produce melanin, the pigment that gives us our skin and hair color and provides protection against the damaging effects of ultraviolet radiation. Clusters of melanocytes often form pigmented spots on the skin. The most common are freckles and moles.

• The dermis is the thicker, second layer of skin located beneath the epidermis. It contains blood and lymph vessels, nerve endings, muscle fibers, oil and sweat glands, and hair follicles. The dermis is divided into two layers:

  • The papillary dermis, the upper layer, is made of loose connective tissue, blood vessels, and nerves. Fingerlike projections called papillae connect the dermis to the epidermis and provide the epidermis with vital nutrients.
  • The reticular dermis, the thicker lower layer, is a network of collagen fibers, dense connective tissue that gives skin its elasticity and strength. This layer contains a rich supply of blood vessels and nerves, as well as lymph vessels, glands, and hair follicles.

• The subcutaneous layer, or subcutis, is a thick layer of fat and connective tissue beneath the dermis. Like the dermis, it contains a rich supply of blood and lymph vessels. The subcutaneous layer conserves body heat and acts as a shock absorber to protect underlying tissues and organs from injury.