I have started culturing a vial of Normal Human Melanocytes adult from ATCC which is already in passage 2 when you first culture it. It is mentioned that they can grow
Article The correlation of TRPM1 (Melastatin) mRNA expression with m...
Please check the gene expression pattern of melanocytes. I speculate that the loss of gene regulatory factors may be due to the telomere length. During the passaging, you may experience a loss of telomere length. So supplementation of telomerase to the medium might restore the length and expression of melanin.
Normally when you culture, there is a limit and it depends on how much it is cultured. For a low number of passages is no more than 3 and if you insist no more than 7 which is the result. 15 corresponds to a different kind of culture