The National Cancer Institute (NCI) reports that most people have at least 10 moles, and some persons have as many as 40. Moles are usually circular or oval in shape and may be either flush with the skin or raised above the surrounding skin layers. Most moles develop during the first 20 years of life, but some may not appear until later in life, according to the American Academy of Dermatology (AAD). New mole creation typically occurs until about the age of 40.
A mole is known in medical terms as a “nevus” (“nevi” is the term for more than one mole). A nevus is created when clusters of skin cells called melanocytes attach to the surrounding tissue cells. The NCI states that these clusters are “spread evenly throughout the skin and produce the pigment that gives skin its natural color.” When the skin has additional layers of melanocytes, a mole is created. The AAD maintains that moles “probably are determined before a person is born,” a product of genetic configuration. Only one baby in 100 will be born with a mole (called a congenital nevus).
A typical mole is flesh-colored, but when it is exposed to the sun, additional pigments produced by the melanocytes darken the mole. The AAD states that the life cycle of a normal mole is approximately 50 years. Moles are typically smaller than a pencil eraser but may undergo numerous alterations.
As a person ages, moles created over the decades "may flatten again, become flesh-colored and go away,” according to the NCI. It is important to document moles by taking photographs every year to determine if the moles (and melanocytes) have undergone any recent changes. Moles disappear from the body by fading into the surrounding skin, developing a “stalk” and falling off, or being eroded by rubbing, according to the AAD.
The NCI states that 10 percent of the population has at least one atypical (abnormal) mole. These are called dysplastic nevi and they may be identified by the irregularity of their size, color and shape. These atypical moles may at any time develop into skin cancer (melanoma). Melanomas are created by abnormal expansion of the melanocytes. The mutant melanocytes begin destroying nearby cells and eventually invade other cells, creating a tumor, according to the NCI. Not all atypical moles develop melanoma; in fact, the NCI reports that only half of the cases of melanoma are caused by atypical moles.
Ultraviolet Rays and Moles
The AAD states that the number of moles on a person’s body is increased by one's amount of sun exposure over the years. The AAD and the NCI agree that the more moles on the body, the higher the risk that the person will develop melanoma when the body is regularly exposed to the sun’s rays. Cancer may not develop directly from a specific mole, but a person with a high percentage of moles is in a higher category of skin cancer risk. The reasons for this increased risk are unknown.