Epidermis

The outermost layer of the skin is called the stratum corneum. It is composed of dead keratinocytes that are continually sloughing as new ones replace them. In addition to dead cells, the corneum also contains keratin, surface lipids, and dirt. The epidermis does not have any blood vessels.

Dead cells are shed at a fairly constant rate with a process called desquamation. The epidermis also has keratinizing and glandular appendages. Keratinizing appendages develop into hair and nails; glandular appendages include the sweat, scent, and sebaceous glands. The pigment that determines skin color (i.e., melanin) is also found in the epidermis in structures called melanosomes. Manufactured by melanocytes, melanosomes are transported to the keratinocytes and surround them. When skin is exposed to the sun or ultraviolet radiation, the quantity of melanosomes increases, causing a change in skin color.

The innermost layer of epidermis is the stratum basale/germinativum, which contains squamous cells, and is the site of origin for squamous cell carcinoma. Squamous cell cancer is more aggressive than basal cell carcinoma, and often invades surrounding tissues and lymph glands. It is often found on surfaces of the skin which have the most exposure to the sun.

The cells of the lowest, or basal, layer of the epidermis are constantly dividing and producing epidermal cells. Basal cell cancer develops from this layer. It tends to be slow growing, is the most common skin cancer (8 out of 10 cases are basal cell cancer), and has a high rate of recurrence. It is usually found in areas of the body with the most sun exposure.

The granular layer, or stratum granulosum, contains three or four layers of cells. It is composed of cells called keratinocytes, which undergo a maturation process called keratinization. This process produces lipid granules that form waterproof structures and helps in preventing fluid loss and evaporation through the skin into the environment.

Dermis

The dermis, or corium, lies below the epidermis and consists of collagen, elastic, and reticular fibers. It also contains blood vessels, nerves, lymphatics, and smooth muscle.

Hypodermis

The hypodermis functions as a shock absorber and heat insulator. Located under the dermis, it comprises fat, smooth muscle, and adipocytes. These cells store and accumulate necessary fats. The hypodermis acts as a site of energy production, and insulation. This area of the body stores fat in characteristic locations: hips on women, abdomen in men.²

Thermoregulation

Skin, subcutaneous tissue, and fat in the subcutaneous tissue provide heat insulation for the body. Heat is lost from the body to the surroundings by radiation, conduction, convection, and evaporation (Fig. 17-2). Radiation of heat from the body accounts for approximately 60% of the total heat loss. In this mechanism, heat is lost in the form of infrared heat waves. Conduction of heat to objects represents approximately 3% of the total heat loss, whereas conduction of heat to the air represents approximately 15% of the total heat loss. When water is carried away from the skin by air currents, convection of heat occurs. Evaporation constitutes approximately 22% of the heat loss. Even without sweating, water still evaporates from the skin and the lungs; this is called insensible loss and totals approximately 600 mL/day.

FIG. 17-2 Major mechanisms of heat loss from body.

(From Hall JE: Guyton and Hall textbook of medical physiology, ed 12, St. Louis, 2011, Saunders.)

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