Fig. 11. Model for pigment pattern metamorphosis in zebrafish. (A) Throughout metamorphosis new pigment cells appear from undifferentiated stem cells (see text for references). These cells (white) may be specified for one or another cell fate, or they may be pluripotent. Recruitment of stem cells to the xanthophore lineage (yellow cells, left) requires fms; in the absence of Fms activity these cells die, fail to advance through stages of xanthophore differentiation, or both. Stem cells also are recruited to melanophore fates (grey cells, right) under the influence of ednrb1, mitfa and kit. Although gene expression analyses reveal fms expression at early stages in some of these cells, a cell autonomous role for fms in promoting the development of early stages in the melanophore lineage has yet to be documented. (B) Terminal differentiation of chromatophores depends on genes encoding pigment synthesis enzymes that are likely to differ between xanthophores (e.g., gch; xdh) and melanophores (e.g., dopachrome tautomerase, dct; tyrosinase, tyr). During these stages, xanthoblasts express and require fms (F). A parallel requirement for kit is observed for fin melanoblasts, and likely body melanoblasts that also express kit (K). (C) During middle stages of pigment pattern metamorphosis and possibly prior to the terminal differentiation of chromatophores, fms-dependent cells of the xanthophore lineage influence kit-dependent cells of the melanophore lineage to form stripes. Although this interaction promotes melanophore competence for stripe formation, the directionality of these stripes depends on additional cues, possibly including initial asymmetries in chromatoblast or stem cell distributions, or other features of the extracellular environment. In the absence of Fms activity, xanthophores are not recruited and do not influence melanophore stripe formation. (D) During late stages of pigment pattern metamorphosis extending through adult life, fms-dependent xanthophores (or their precursors) contribute to maintaining melanophore stripes. In the absence of Fms activity, xanthophores die and melanophore stripes degenerate.

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