Why do some individuals have more moles than others?

The moles, or naevi, are small pigmented spots that appear on the skin. They result from a localised accumulation of melanocytes, the cells responsible for the production of melanin, the pigment that colours the skin. The moles can be present from birth - in which case we speak of congenital naevus - but, most often, they appear during childhood or adolescence. Most naevi are benign but they still need to be closely monitored to ensure they do not transform into melanoma. The number, shape, and colour of moles can vary among individuals.

The variability in the number of moles observed between different individuals can be explained by genetic and environmental factors.

Firstly, heredity plays a significant role in an individual's predisposition to the development of moles. Having a parent with numerous moles considerably increases the likelihood of having many nevi oneself. This genetic transmission is linked to several genes involved in the regulation of melanocyte proliferation and the skin's response to UV rays. Among them, the MC1R gene, coding for the type 1 melanocortin receptor, also responsible for hair colour and sun sensitivity, has been identified as a factor influencing the number and pigmentation of moles. This is also why individuals with light skin, eyes, or hair often have more moles than those with a darker phototype.

However, while genetics play a significant role in the occurrence of moles, it alone cannot explain why some individuals have more dark spots than others. Indeed, if we take the example of identical twins, we find that the number of moles between them is not necessarily uniform and can sometimes be very different. This is where lifestyle comes into play, and more specifically the habits of exposure to the sun's UV rays. These significantly influence the quantity of moles found on a person.

Indeed, under the effect of the sun, melanocytes are activated to protect the skin against UV-induced damage. However, if the sun has benefits, excessive exposure can disrupt the process of melanogenesis and lead to uncontrolled proliferation of melanocytes and their clustering into groups, giving rise to moles. This is why moles are mainly found on areas often exposed to the sun, such as the face, shoulders, arms or legs. It's also worth noting that UV rays can cause genetic mutations in skin cells, particularly in the BRAF gene, which is involved in the regulation of melanocyte growth. These alterations are not necessarily malignant, but they promote the appearance of moles and can, in some cases, increase the risk of transformation into melanoma.

Intense and repeated sun exposure, particularly during childhood and adolescence, is associated with a higher number of moles in adulthood.