Figure 2.

Schematic depiction of the single-origin hypothesis for pigment cells originally proposed by Bagnara et al. [8]. The three primary pigment cell types arise directly from a neural-crest-derived progenitor cell (chromoblast) with a tripotent pigment organelle. Fate specification of the chromoblast is determined by genetic and environmental factors that affect the formation of pigment-producing organelles. According to this hypothesis, melanophores permissively differentiate in melanoid axolotls because multipotent pigment progenitor cells have reduced XDH activity and lower levels of purine and pterin pigments that are synthesized by organelles of iridophores and xanthophores, respectively.