Synonyms in a broader sense
Birthmark, mole Medical: nevus, nevus cell nevus, nevus pigmentosus, junctional nevus, compound nevus, dermal nevus
A “mole” is generally referred to in medicine as a nevus (= mal, plural nevi) and describes a localized malformation of the skin, which is characterized by an increase in pigment cells, so-called nevus cells. These are probably genetically modified pigment cells (melanocytes). A distinction is made between congenital (occurring at birth or within weeks) and acquired liver spots. Each of these forms can be divided into junctional, compound and dermal nevi.
EpidemiologyOccurrenceFrequency
The mole is one of the most common changes in the skin. Congenital forms are rarer than acquired ones. The incidence in newborns is about 1:100; larger moles in babies are even less common (1:10 000-500 000).
A familial accumulation of congenital moles has been described. Men and women are equally affected. Acquired moles are more common among different ethnic groups, but are more common in the white population than in more pigmented populations.
On average, each person possesses about 30 acquired nevus cell nevi, with men showing slightly more than women. The liver spot forms especially on the light-exposed parts of the body and reaches its greatest expansion in the 3rd and 4th decade of life. It then recedes. In principle, they all represent precursors of skin cancer (malignant melanoma), but degeneration is very rare and the exception. Only the congenital moles and the “Clark nevi”, a special form of acquired nevi, show an increased risk of degeneration.
Causes
Genes play a role in acquired liver spots. Also sex hormones and UV light. According to current theory, it is assumed that the nevus cells forming the liver spot develop from defective pigment cell precursor cells, the nevomelanoblasts.
These colonize the skin during embryonic development. In the upper layer of skin, they then either multiply in the womb and thus form the congenital nevus cell nevi, or they rest there first and multiply during life, which in turn leads to the acquired form. Both forms of liver spot follow a stereotypical developmental path.
First they are present as a junctional nevus, then they develop into a compound nevus and finally into a dermal nevus. These forms reflect the gradual migration of the firmly attached nevus cells from the skin surface to the depths. Usually, in the congenital mole, the nevus cells reach deeper layers than in the acquired mole.