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What Causes Wrinkles?เหตุที่ทำให้เกิดริ้วรอยเหี่ยวย่นของผิวหนัง      

This article describes how skin typically ages and outlines factors - such as sun exposure and smoking - which can accelerate the formation of wrinkles.

Aging Process

As a person ages, skin cells divide more slowly, and the inner skin, or dermis, starts to thin. Fat cells beneath the dermis begin to atrophy, and the underlying network of elastin and collagen fibers, which provides scaffolding for the surface layers, loosens and unravels. Skin loses its elasticity; when pressed, it no longer springs back to its initial position but instead sags and forms furrows. The skin's ability to retain moisture diminishes; the sweat- and oil-secreting glands atrophy, depriving the skin of their protective water-lipid emulsions. As a consequence, the skin becomes dry and scaly. In addition, the ability of the skin to repair itself diminishes with age, so wounds are slower to heal. Frown lines (those between the eyebrows) and crow's feet (lines that radiate from the corners of the eyes) appear to develop because of permanent small muscle contractions. Habitual facial expressions also form characteristic lines, and gravity exacerbates the situation, contributing to the formation of jowls and drooping eyelids. (Eyebrows, surprisingly, move up as a person ages, possibly because of forehead wrinkles.)

Sun Damage (Photoaging)

The skin can also age prematurely as a result of prolonged exposure to ultraviolet (UV) radiation emanating from the sun (called intrinsic or photoaging). The role of the sun cannot be overestimated as the most important cause of aging skin and skin cancers. Overall, exposure to ultraviolet (referred to as UVA or UVB) radiation from sunlight accounts for about 90% of the symptoms of premature skin aging, and most of these effects occur by age 20. Both UVA and UVB rays cause damage leading to wrinkles, lower immunity against infection, aging skin disorders, and cancer. Even small amounts of UV radiation damage collagen fibers (the major structural protein in the skin) and cause accumulation of abnormal elastin (the protein that causes tissue to stretch). During the process, large amounts of enzymes called metalloproteinases are produced. The normal function of these enzymes is to remodel the sun-injured tissue by synthesizing and reforming collagen. This is an imperfect, process, however, and to achieve it, some of these enzymes actually degrade collagen. The result is an uneven formation (matrix) of disorganized collagen fibers called solar scars. If this process of imperfect skin rebuilding occurs over and over, wrinkles result. One study indicated that when people with light to moderate skin color are exposed to sunlight for just five to 15 minutes metalloproteinases remain elevated for about a week. It should be noted, however, that some studies indicate that metalloproteinases may also have factors that protect against melanoma. Researchers have recently identified a molecule called trans-urocanic acid that reacts to UVA radiation by releasing oxygen-free radicals. These are unstable particles that, in excess, can damage cells, and in skin, contribute to photoaging.

Other Factors

Other environmental factors, including cigarette smoke and pollution, particularly ozone, may hasten chronologic aging by producing oxygen-free radicals. These are particles produced by many of the body's normal chemical processes; in excessive amounts they can damage cell membranes and interact with genetic material, possibly contributing to the development of a number of skin disorders, including wrinkles and, more importantly, cancer. Rapid weight loss can also cause wrinkles by reducing the volume of fat cells that cushion the face. This not only makes a person look gaunt, but can cause the skin to sag.

Who Is Most Likely to Have Wrinkles?

The vast majority of undesirable consequences of aging skin occur in individuals who are repetitively exposed to the sun, notably farmers, fishermen, construction workers, lifeguards, outdoor enthusiasts, and sun-worshippers. People who live in areas where the earth's protective ozone layer is thinning, namely in southern climates and at high altitudes, are more likely to have sun-damaged skin.

Ethnicity also influences a person's susceptibility to aging skin. Individuals with fair complexions, and those who have blue, green, or gray eyes, and red or blond hair, are more susceptible to photoaging than are darker-skinned persons. Overall, African Americans develop fewer wrinkles, have fewer lesions, and experience less sagging of facial skin than do whites, presumably because the larger amount of melanin present in their skin helps block the penetration of UV radiation. Nonetheless, premature aging from sunlight can affect all ethnic groups.

Cigarette smokers are more prone to wrinkles and skin cancers. According to one study, heavy smokers are almost five times as likely to have wrinkled facial skin than nonsmokers. In fact, heavy smokers in their 40s often have facial wrinkles more like those of nonsmokers in their 60s. A recent study of 25 sets of twins found smokers to have thinner skin than non-smokers, in some cases by as much as 40%.

Well-Connected Board of Editors

Harvey Simon, M.D., Editor-in-Chief
Massachusetts Institute of Technology; Physician, Massachusetts General Hospital

Masha J. Etkin, M.D., Gynecology
Harvard Medical School; Physician, Massachusetts General Hospital

John E. Godine, M.D., Ph.D., Metabolism
Harvard Medical School; Associate Physician, Massachusetts General Hospital

Daniel Heller, M.D., Pediatrics
Harvard Medical School; Associate Pediatrician, Massachusetts General Hospital; Active Staff, Children's Hospital

Irene Kuter, M.D., D. Phil., Oncology
Harvard Medical School; Assistant Physician, Massachusetts General Hospital

Paul C. Shellito, M.D., Surgery
Harvard Medical School; Associate Visiting Surgeon, Massachusetts General Hospital

Theodore A. Stern, M.D., Psychiatry
Harvard Medical School; Psychiatrist and Chief, Psychiatric Consultation Service, Massachusetts General Hospital