Control of melanoblast differentiation in amphibia by alpha-melanocyte stimulating hormone, a serum melanization factor, and a melanization inhibiting factor.

A ventrally localized melanization inhibiting factor (MIF) has been suggested to play an important role in the establishment of the dorsal-ventral pigment pattern in Xenopus laevis [Fukuzawa and Ide:Dev. Biol., 129:25-36, 1988]. To examine the possibility that melanoblast expression might be controlled by local putative MIF and melanogenic factors, the effects of alpha-melanocyte stimulating hormone (alpha-MSH), a serum melanization factor (SMF) from X. laevis or Rana pipiens, and MIF on the "outgrowth" and "melanization" of Xenopus neural crest cells were studied. Outgrowth represents the number of neural crest cells emigrating from cultured neural tubes, and melanization concerns the percentage of differentiated melanophores among the emigrated cells. MSH or SMF stimulate both outgrowth and melanization. The melanogenic effect of Xenopus serum in this system is more than twice that of Rana serum. The actions of MSH and Xenopus serum on melanization seem to be different: 1) Stronger melanization is induced by Xenopus serum than by MSH, and the onset of melanization occurs earlier with Xenopus serum; 2) MSH stimulates melanization only in the presence of added tyrosine; and 3) MSH causes young melanophores to assume a prominent state of melanophore dispersion during culture, while Xenopus serum (10%) had only a slight dispersing effect and not until day 3. A fraction of Xenopus serum presumably containing molecules of a smaller molecular weight (MW less than 30 kDa) than that of a pigment promoting factor reported in calf serum [Jerdan et al.: J. Cell Biol., 100:1493-1498, 1985] produces the same remarkable melanogenic effects as does intact serum. While this fraction stimulates outgrowth, another fraction presumably containing larger molecules (MW greater than 100 kDa) does not. MIF contained in Xenopus ventral skin conditioned medium (VCM) inhibits both outgrowth and melanization dose dependently. When VCM is used in combination with MSH, the stimulating effects of MSH on both outgrowth and melanization are completely inhibited. In contrast, the stimulatory effects of Xenopus serum are not completely inhibited when combined with VCM, although melanization is reduced to approximately 40% that of controls. MIF activity was also found to be present in ventral, but not in dorsal, skin conditioned media of R. pipiens when tested in the Xenopus neural crest system.(ABSTRACT TRUNCATED AT 400 WORDS)