Dynamic visualization of melanosome endo/phagocytosis during melanin transfer using melanosomes pre-stained with carbocyanine dyes

Melanogenesis in melanocytes and the transfer of melanin into the surrounding keratinocytes mainly contribute to skin color and pigmentation. Melanocytes in the skin and hair transfer their melanin to surrounding keratinocytes via a mechanism called melanin transfer. After melanin is internalized into keratinocytes, it forms a supra-nuclear cap, which acts as a ‘sunscreen’, and protects the genetic material by absorbing and scattering ultraviolet (UV) radiation. Several models for the melanin transfer mechanism have been proposed, and two of them have drawn attention recently [1]: the shedding of melanosome-loaded vesicles by melanocytes that are subsequently phagocytosed by keratinocytes [2–4] and the exocytosis by melanocytes of naked melanin (melanocores), which are then endocytosed by keratinocytes [5,6].