Fig. 1. Four possible modes for melanocyte regeneration distinguished by roles for cell division and the differentiation state of precursors. (A) Regeneration involves direct proliferation of pre-existing, differentiated melanocytes and requires no further role for differentiation (compensatory regeneration). (B) Regeneration is achieved through the recruitment of quiescent precursors or stem cells to proliferate and differentiate (epimorphosis). (C) Regeneration involves transdifferentiation of other differentiated cells without cell division to generate the new melanocytes (morphallaxis). (D) Regeneration occurs by direct differentiation of set-aside late-stage undifferentiated precursors (for instance, dct⁺ melanoblasts) into melanocytes without cell division. Note that unlike stem cell-based models, regeneration by this last mechanism would tend to exhaust the precursor pool.