Skin
How is the skin, the largest organ in the body, constructed? The skin has two layers: the upper layer is the epidermis, and the lower layer is the der­mis. Below the dermis is the hypodermis, or subcutaneous layer, composed of fat or other connective tissue.
The epidermis itself is an epithelium made up of sublayers. The out­ermost portion consists of many layers of flat, dead, dry epithelial cells called keratinocytes. Clearly this barrier of dead cells needs no blood supply. The waxy surface coating of these cells allows the skin to be waterproof and dry. In effect, the body surrounds itself with a hostile desert where few germs can live.
Living keratinocytes in the deepest layer of the epidermis undergo rapid cell division and push the overlying cells toward the surface. Melanocytes are also deep and produce the dark pigment in the skin, melanin. Upon ex­posure to ultraviolet light, melanin production is increased, a process called tanning. The melanin helps protect other cells from the damaging effects of ultraviolet light. Ultraviolet light from any source can harm the skin by causing skin cancer and wrinkling.
The dermis is composed of fibrous connective tissue. The upper part of the dermis exhibits many hills, called the dermal papillae, which prevent slippage between the dermis and the epidermis and increase surface area. There are blood capillaries and small organs of fine touch inside the papil­lae. In the fingertips, the papillae occur in ridges and help to form the fin­gerprints. The lower parts of the dermis are home to larger blood vessels, nerves, hair follicles, oil glands, sweat glands, and fibrous connective tissues.
Hair follicles contain the root of a hair and have a bulb at the deep end. A small muscle, the arrector pili, attaches to the hair and raises it when the body is cold or frightened. In hairier mammals, the raised hair creates an insulating layer of air to preserve the animal’s warmth, but in humans this reaction merely causes goose bumps. There is a sebaceous (oil) gland asso­ciated with the follicle. Parasites called follicle mites are found in the hair follicles of many people, especially on the face.
Other organs related to the skin are the finger- and toenails. These are made of plates of hardened keratin and are dead and dry, like the upper layer of the epidermis. The nails begin as new cells added in the nail ma­trix, under the skin.
Two types of glands also start out in the dermis: merocrine sweat glands and apocrine sweat glands. Merocrine sweat glands are those that increase their watery secretions when the body starts to overheat. The evapora­tion of the secretions off the skin cools the body off. Sweat is only re­sponsible for about one-fifth of the cooling in a resting person; most is due to radiation, in which heat is given off as infrared rays. Apocrine sweat glands are found around the breasts, armpits, and genitalia and produce sex-attracting chemicals called pheromones. Other glands include the ceruminous (earwax) glands, sebaceous glands, and mammary glands.
References
Diamond, J. “Pearl Harbor and the Emperor’s Physiologists.” Natural History 91, no. 12 (1991): 2, 4, 6-7.
Jarrett, A. Science and the Skin. London: English Universities Press, 1964.
Saladin, Kenneth S. Anatomy and Physiology: The Unity of Form and Function, 2nd ed. Dubuque, IA: McGraw-Hill. 2000.
Zinsser, H. Rats, Lice and History. Boston: Little Brown and Co., 1963.