| 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
|
