Melanin: Aging of the Skin and Skin Cancer
By Diana Clarke
"Ultraviolet (UV) radiation is responsible for 90%
of the visible signs of aging on the skin of whites," says Dr. Michael
J. Martin, former Assistant Clinical Professor in the Dept. of Epidemiology
and Biostatistics at University of California, San Francisco.
Blacks' skin, however, ages much slower.
Why are most dark-skinned blacks protected from harmful
UV rays? Because compared to whites, blacks possess more melanin,
the pigment that gives skin its color.
Melanin
Melanin offers protection against UV rays for blacks
and other dark-skinned people. Conversely, fair-skinned people are
much less protected and more susceptible to skin cancer. Furthermore,
albinos' skin offers no protection.
Although blacks' skin produces more melanin than whites',
all skin has the same number of melanocytes, the cells that manufacture
the melanin.
Melanocytes manufacture melanin from an amino acid,
tyrosin, with the help of an enzyme, tyrosinase. In the bottom layer
of the epidermis above the dermis, UV light stimulates the production
of melanin in the form of insoluble melanosomes. These surround the
epidermal cells, which move up to the surface of the skin. The result
is a tan.
Blacks' skin produce more melanin, even in the absence
of sunlight, and their type of melanin, eumelanin, is more effective
at blocking solar rays. However, white skin produces melanin only
in the presence of sunlight and after the UV rays have penetrated
the lower portion of the epidermis and have caused skin damage.
"Melanin also functions as an excellent free radical
scavenger. It affects the delicately designed lipids that hold moisture
in the stratum corneum (the outermost layer of the epidermis). If
the skin loses its moisture, it becomes rigid and cracks," says Sergio
Nacht, PhD., Senior Vice-President of Enhanced Derm Technologies,
Inc. in Redwood City.
UV Radiation and Skin
UV-A has the longest wavelength, is not filtered by
the ozone and passes through glass. It reaches the earth all year
long and the amount is comparatively stable. It can penetrate the
skin down to the dermis, beneath the four layers of epidermis.
It is responsible for most of the visible signs of
aging, due to damage to collagen and elastic fibers of the connective
tissue of the dermis.
UV-A radiation also plays a role in the development
of sunburns and skin cancer. Tanning salon lamps emit a large amount
of UV-A rays to generate tans, so the American Academy of Dermatology
does not recommend their use.
UV-B radiation, which is partially filtered by the
ozone, penetrates the skin to the bottom layer of the epidermis where
the basal cells are produced. UV-B can break the molecular bonds,
disturbing the dividing cells and altering their structure.
Compared with UV-A, UV-B is responsible for most of
DNAs damage. It also causes most sunburns. During a sunburn the reddening
of the skin, erythema, is caused by dilation of capillaries.
More UV-B is present during summer months between
10 a.m. and 4 p.m. and at latitudes closer to the equator. Furthermore,
at high altitudes the air is thinner and cleaner, so UV-B radiation
is more abundant.
UV-C, which is generally filtered by the ozone, has
the shortest wavelength and the most energy, or intensity. It can
sterilize hospital equipment and kill bacteria.
In addition, UV light that reaches the earth is scattered
in all directions, and up to 85% is reflected from surfaces.
The Theory of Melanin for Environmental Adaptation
Originally, people of a particular race resided in
a particular area. As time went on, their skin adapted to the environment.
For instance, people who lived geographically close to the equator
had darker skin, and people who lived far from the equator had lighter
skin.
In Scotland, which lies at a northern latitude, descendants
of the Britons have white skin. When their skin is exposed to the
meager sunlight, the scant amount of melanin their skin produces is
unable to block the sunlight.
Therefore, their bodies are able to make Vitamin D
with the help of sunlight. Vitamin D, a vitamin found in fish oil,
is necessary to prevent rickets, a bone disease caused by too little
calcium.
In contrast, in Africa, which is near the equator,
blacks require intense sunlight to penetrate their dark skin to make
Vitamin D. This is all well and good. However, when blacks lived in
England during the Industrial Revolution, they were the first to develop
symptoms of rickets, such as retarded growth, bowed legs and fractures
because not enough sunlight was available.
Fortunately, in 1930, Vitamin D was discovered and
dispensed as a supplement to add to the diet.
On the other hand, the skin of whites in Australia
are in complete opposition to their climate. Consequently, intense
UV radiation has been the major cause of skin damage and skin cancer
Down Under.
About The Author: Diana Clarke
is a teacher, freelance writer and founder of The Sun and Your Skin,
a website on life and light at http://www.yourskinandsun.com.
dianaclarke2001@yahoo.com
