Hair

Hair

Young cells rise to the surface as they age, die, and are sloughed off by friction. That friction stimulates the growth of new cells and causes the epidermis to thicken, which is why skin is generally thicker on the bottoms of your feet and the palms of your hands. It is also why a callus -- rough, thickened skin on any part of the body -- forms on areas that are rubbed excessively. The skin you see on your body today may look familiar, but it has been completely renewed over the course of a month.

Cross Section of Skin and Hair

The pigment in the skin that helps protect the body against ultraviolet rays is called melanin. This protein is made primarily by cells called melanocytes but is also present in the keratinocytes. In general, skin color is a hereditary characteristic determined not by the number of melanocytes but by their degree of activity.

People with darker skin have more active melanocytes, resulting in more melanin. The ultraviolet rays in sunlight stimulate the activity of melanocytes to produce more melanin, causing skin to darken, thereby defending the body against the damaging effects of the sun.

If the top layer of skin is totally replaced every month, why does skin age? Skin aging occurs, in part, because the layer of skin beneath the epidermis, called the dermis, gradually loses its supply of two substances -- elastin and collagen. Long fibers of elastin give the skin its elasticity, while collagen fibers provide strength. Over time and with repeated exposure to ultraviolet light (mainly sunlight), these fibers deteriorate.

The dermis also contains four sets of glands:

Sebaceous (oil) glands, primarily located in the ducts containing shafts of hair, secrete an oil called sebum. The sebaceous glands are highly active during adolescence because of the hormone changes that occur at this time, which contribute to the higher likelihood of acne.

Ceruminous glands are the ear’s version of sebaceous glands and are the source of earwax (cerumen).

Sweat glands called eccrine or exocrine glands are found all over the body’s surface. They are most numerous on the soles, palms, upper lip, and forehead. Sweat is important in maintaining a comfortable and healthy body temperature. When the body gets heated or is under stress, it produces sweat, which evaporates and cools the body.

Copyright 1999 © by President and Fellows of Harvard College
Reprinted with permission from the Harvard Medical School Family Health Guide, Simon & Schuster 1999. Art copyright © Harriet R. Greenfield, Newton, Mass.