Clonal separation of mature melanocytes from premelanocytes in a diploid human cell strain: spontaneous and induced pigmentation of premelanocytes

Strains of pigmented melanocytes can be derived reproducibly from normal human skin. Published procedures have been modified here to yield a strain, 'Nohm-1', comprising many unpigmented cells as well as cells with various degrees of pigmentation observable by light microscopy. The unpigmented cells contain early melanosomes (pigment organelles) and the specific enzyme tyrosinase. They are on average smaller and less dendritic than the pigmented cells. Nohm-1 cells show normal chromosomal banding patterns and normal proliferative behaviour, including senescence. They form no tumours in immunodeficient (nude) mice. Nohm-1 cells have been cloned and yield two distinct types of colony, depending on the progenitor cell. Well-pigmented melanocytes engender pure colonies of pigmented cells, but cells with little or no pigment can produce both unpigmented and pigmented progeny. Thus there is a separate cell type, or premelanocyte, which can differentiate spontaneously and stably in culture; this cell type includes both unpigmented and faintly pigmented cells. Usefully, most premelanocytes are viable after frozen storage, unlike well-pigmented melanocytes. Some components of the culture medium affect the proportion of pigmented cells in Nohm-1 cultures, and hence probably the ratio of mature melanocytes to premelanocytes. Rapid pigmentation can be induced artificially and simply, by using a medium with increased extracellular pH and tyrosine concentration.