Freckles (cont.)
How do freckles develop?
Freckles are thought to develop as a result of a combination of genetic tendency (inheritance) and sun exposure. Two people receiving the same sun exposure may not have an equal chance of developing freckles. Natural sunlight and artificial suntanning lights emit ultraviolet (UV) rays. After exposure to ultraviolet rays, the outer layer of the skin (the epidermis) thickens and the pigment-producing cells (the melanocytes) produce the pigment melanin at an increased rate. (This production of melanin may in fact give some protection against future sun exposure.)
Of course, people differ a great deal in their reaction to sunlight. To take an extreme example, there is no pigmentation in the skin of an albino because of a defect in melanin metabolism. On the other hand, people with dark complexions are relatively less sensitive to sun exposure than fair-skinned people. However, people with dark skin are not entirely resistant to the effects of the sun, and they, too, can become sunburned with prolonged exposure. People with blond or red hair, light-colored eyes, and fair skin are especially susceptible to the damaging effect of UV rays.
Irrespective of skin color, freckling is caused by the uneven distribution of the melanin pigment in the skin.. A freckle is essentially nothing more than an unusually heavy deposit of melanin at one spot in the skin.
How important is heredity with freckles?
Heredity and skin type are very important factors for the tendency to develop freckles. Freckles tend to be inherited genetically and are most common in individuals with fair skin and/or with blond or red hair.
Research in twin siblings, including pairs of identical twins and pairs of fraternal (nonidentical) twins, have found a striking similarity in the total number of freckles found on each pair of identical twins. Such similarities were considerably less common in fraternal twins. These studies strongly suggest that the occurrence of freckles is influenced by genetic factors. In fact, the variations in freckle counts appear to be due largely to heredity.
Ongoing research in a rare disease called xeroderma pigmentosum has also confirmed the genetic tendency of freckles. Excessive freckles in dark-haired individuals are quite common in this disease.
Further, freckles are also found in skin folds like the underarms in another uncommon genetic disease called neurofibromatosis.
Next: What is the medical meaning of freckles? »
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