Melanophores normally differentiate in dorsal but not in ventral skin of Xenopus laevis. We have sought factors which might regulate this differentiation pattern, and we have obtained a putative melanization inhibiting factor (MIF) from ventral but not from dorsal skin. Preliminary studies reveal that MIF is destroyed by heat or trypsin treatment, indicating its protein composition, and has a molecular weight in the range of 300 kDa. The effects of MIF on the differentiation of neural crest derivatives to melanophores were examined in vitro in the presence of tyrosine and fetal calf serum (FCS). Tyrosine enhances melanophore differentiation in vitro at concentrations equivalent to those estimated in adult Xenopus blood plasma (20 μM). FCS also stimulates melanization, by way of materials other than the tyrosine contained in FCS. MIF strongly inhibits outgrowth and melanization of neural crest cells from neural tube explants. MIF also inhibits the differentiation of melanoblasts contained in cultured explants of ventral skin. Inhibition of melanization or melanophore differentiation by MIF occurs even in the presence of l-tyrosine and/or FCS. We suggest that MIF plays an important role in the establishment of dorso-ventral pigment patterns in amphibia.
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