The skin separates the inside of the body from the outside world and helps us sense the environment, such as whether it is wet or dry, or hot or cold. As the body’s largest organ, the skin acts as a thermostat to regulate body temperature and protects the underlying ligaments, muscles, bones, and internal organs. The skin also guards against viruses and bacteria that can cause infections. The production of vitamin D and the formation of scar tissue are other important functions of the skin.
In humans, skin pigmentation or color varies among different people. Skin types can range from dry to oily.
The skin has three distinct layers: the epidermis, the dermis, and the subcutaneous layer. The epidermis is the thin, outermost layer. The supportive layer below the epidermis, made mostly of fibrous tissue, is called the dermis. The subcutaneous layer is the innermost layer, which is made up of fat. Each of these layers has special structures and functions.
The epidermis is divided into five layers. The bottom layer, or basal layer, has two types of cells: keratinocytes and melanocytes. Keratinocytes make up most of the epidermis and play an important role in the ongoing renewal of our skin.
They are continuously migrating from the basal layer to the surface, where they flatten out and die, becoming the outermost layer of the epidermis, called the stratum corneum. The skin constantly sheds this outermost layer of dead cells. Keratinocytes also produce a tough and fibrous protein called keratin, the main substance of our hair, nail, and skin cells. When the keratinocytes of the basal layer become cancerous, the tumor is called a basal cell carcinoma. When keratinocytes from other layers of the epidermis become cancerous, it’s called a squamous cell carcinoma.
Unlike keratinocytes, melanocytes stay in the basal layer and produce a pigment called melanin. This pigment is distributed to the keratinocytes and not only gives our skin its color, but can also protect the skin from harmful ultraviolet radiation. One in every 10 cells of the basal layer is a melanocyte. If these cells become cancerous, it’s called a melanoma.
The dermis is made up of several types of fibrous connective tissue allowing it to move, stretch, and contract with the body’s movements. It also contains blood vessels, lymph vessels, nerves, and glands, which provide circulation, sensation, and nutrition to the skin. Some glands make sweat and others produce sebum, an oily substance that helps keep the skin moist. Sweat and sebum reach the skin’s surface through tiny openings called pores.
The subcutaneous layer, made primarily of fat cells, provides storage for energy, a source of insulation, and cushioning to protect underlying structures.