CH.YBW.CTS.C07.Final.q 6/21/03 1:10 PM Page 98
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CELLS, TISSUES, AND SKIN
The deeper and thicker reticular layer is composed of dense connective tissue. It contains a combination of collagen and elastic fibers in the extracellular matrix, which allows skin to stretch and then return to its original shape. However, substantial body weight gain, as with pregnancy or obesity, can tear the dermis, resulting in visible lines called stretch marks. Also, the resilience of skin decreases with age, as collagen fibers stiffen and elastic fibers lose their elasticity. These effects, along with a reduction in the ability of the dermis to hold moisture, produce wrinkles and sagging skin, which usually first become apparent by the late forties. The reticular layer also contains blood vessels, sweat and oil glands, and receptors for the sensation of deep pressure. SKIN COLOR
Two main factors contribute to skin color: the quantity and distribution of pigments (melanin and carotene) in the skin, and blood flow. Melanin is a skin pigment made of amino acids. It comes in two forms: yellow to red (pheomelanin) and the more common brown to black (eumelanin). Although melanin is only produced by melanocytes, it is continually released by exocytosis from these cells. Surrounding cells subsequently accumulate the pigment by endocytosis. Interestingly, all people have roughly the same number of melanocytes. That means variations in skin color are due to differences in the form and amount of melanin produced, and in the way it is dispersed. (Figure 7.2) The most important factor in determining melanin production is a person’s genetic predisposition: that is, the particular characteristics inherited from the parents. Also, melanocytes are stimulated by exposure of skin to sunlight, causing them to increase their production of melanin. This response helps protect DNA when there is an increased exposure to ultraviolet radiation. It also is responsible for the development of a tan. However, excessive exposure to