Identification of Sox10-positive cells at the dorsal fin base of juvenile flounder that are correlated with blind-side skin ectopic pigmentation.

Flounder develop left-right asymmetric body color, with a dark ocular side and white blind side. However, ectopic pigmentation often occurs on the blind side when juveniles are reared in tanks. To examine the developmental mechanism underlying ectopic pigmentation, we first examined the pigmentation process on the blind side and the localization of chromatoblasts during spontaneous and regeneration-stimulated ectopic pigmentation. Wild-caught juveniles that had completed metamorphosis in a natural environment were reared in tanks, where they exhibited ectopic pigmentation on the blind side that was initiated at the base of the dorsal and anal fins, with chromatoblasts appearing at the edges of scales and melanophores spreading on the scale papilla beneath the epidermis. During tissue regeneration at the base of the dorsal fin in juvenile before ectopic pigmentation, melanophores and chromatoblasts newly appeared on regenerated blind-side skin, resulting in rapid pigmentation at the wounded site. During regeneration-stimulated pigmentation, gch2-positive chromatoblasts were detected only under the regenerated epidermis. Next, we found that Sox10-positive cells were localized in connective tissue at the base of the dorsal fin and that when connective tissue was labeled with DiO, DiO-labeled melanophores appeared in regenerated skin of the blind side after wounding. Therefore, we conclude that in flounder juveniles, Sox10-positive progenitors of pigment cell lineage reside at the base of the dorsal fin and start migrating to the blind-side skin in response to specific stimuli, resulting in ectopic pigmentation. Ectopic pigmentation in flounder could be a good model for examining the flexibility of pigment cell differentiation.