Melanophore lineage and clonal organization of the epidermis in Xenopus embryos as revealed by expression of a biogenic marker, GFP.

Melanophore lineage during embryogenesis of Xenopus laevis was traced using the overexpression of a biogenic marker, green fluorescent protein (GFP). Two different approaches were applied after injection of GFP mRNA (hence a marker construct) into each blastomere at the 16-cell stage. In in vivo experiments, the embryos injected with a marker construct were grown until stage 45, in which melanophores were distributed over the whole body and were good enough for checking GFP expression at their migratory destination. In in vitro experiments, neural tubes of the embryos injected with a marker construct were isolated and cultured at stage 21 to examine by virtue of GFP expression how neural crest cells differentiate into melanophores. The results obtained from both in vivo and in vitro experiments indicated the following: 1) selected animal blastomeres vastly contribute to the development of melanophores, whereas other animal blastomeres do so slightly at a limited pace; and 2) vegetal blastomeres never contribute to melanophores in normal development, whereas certain vegetal blastomeres have a potential to give rise to melanophores in vitro. The analyses using GFP also disclosed that the dorsal and ventral epidermis derive from the restricted animal blastomeres in the normal development. Since the dorso-ventrality of the epidermis has been inseparably coupled with integumental pigmentation, the clonal organization of the epidermis observed in the present study is discussed in the light of pigment pattern formation attributed by melanophores.