Table of Contents
Definition of Skin
Skin is the body’s largest organ, making up the body’s covering and about 15 percent of the total body weight. The skin’s three layers-epidermis, dermis, and subcutaneous layer-help the body maintain its structure; protect against INFECTION; and regulate fluids, electrolytes, and temperature. Numerous health conditions, localized and systemic, can affect the skin and its functions.
The subcutaneous layer, innermost to the body, contains primarily adipose tissue more familiarly called body fat. The dermis, the middle layer, provides the structure of the skin. It contains connective tissue, the SEBACEOUS GLANDS, and an abundant supply of nerves and blood vessels.
The dermis nourishes the epidermis above it and attaches to the subcutaneous layer beneath it, holding the skin in place. HAIR follicles and SWEAT GLANDS extend from the epidermis into the dermis and a bit into the subcutaneous layer.
Cells of the Skin
The primary cells of the skin, melanocytes and keratinocytes, originate in the base, or basal, level of the epidermis. Keratinocytes migrate outward to form the upper epidermis, gradually flattening and hardening. The epidermis varies in thickness and other characteristics, accommodating the needs of different body surfaces. The epidermis of the palms of the hands and the soles of the feet is thick and tough, for example, while that of the eyelids is soft and only two or three cells in depth.
The skin is also the body’s organ of tactile sensory perception, or touch. Millions of nerve endings in the skin continually sense environmental factors such as pressure, temperature, moisture. Other specialized nerve cells, called nociceptors, perceive itching and PAIN. Sweat evaporation on the skin’s surface is the body’s primary cooling mechanism, as well as a secondary mechanism for electrolyte regulation and balance.
Health Conditions that may Affect the Skin
For further discussion of the skin within the context of integumentary structure and function, please see the overview section “The Integumentary System.”