A Nervous Origin for Fish Stripes

The current excitement about adult stem and progenitor cells is an opportunity for developmental biologists to shed new light on old problems. Where do differentiated cells in the adult come from, and what else can their progenitor cells do? These questions are especially accessible in the zebrafish, where there is a deep experimental and genetic toolbox, and a developmental pathway with a well-defined metamorphic transition from larvae to adult. One of the premier model systems in this premier-league model organism is pigmentation, since pigment cells in fish—including black melanocytes (known as melanophores in fish), yellow xanthophores, and reflective iridophores—are organized into an array of beautiful, accessible, and evolutionarily diverse patterns that have helped motivate their study [1], [2]. Adult zebrafish show a longitudinally striped pigment pattern, distinct from the embryonic one. The adult pigment cells derive primarily from dormant stem cells, not embryonic pigment cells. But the cellular identity and location of these adult pigment cell progenitors has remained mysterious. In this issue of PLoS Genetics, Budi et al. [3] provide an answer to the cryptic origin of adult pigment cells, concluding that they originate from cells associated with the peripheral nervous system (Figure 1). The findings of this study are remarkably similar to recent work on the developmental origin of mammalian melanocytes [4], demonstrating (once again) the conservation of developmental principles across large evolutionary distances. Figure 1 Model for neural origins of metamorphic melanocytes and iridophores.