A ventrally localized melanization-inhibiting factor (MIF) may play an important role in the expression of dorsal-ventral pigment patternsof amphibians. In efforts to purify this putativeMIF, ventral skin conditioned medium (VCM) from Rana forreri was partially fractionated and used to immunize mice. A monoclonal antibody that has the ability to block the activity of MIF was isolated, and an immunoaffinity matrix was prepared bycross-linking the antibody to protein G-Sepharose. The fraction of VCM that bound to the affinity matrix decreased the number of melanized cells inthe Xenopus laevis neural tube explant assay, butdid not reduce significantly the number of cells that emigrated. The monoclonal antibody was used for immunohistochemical studies on R. pipiens skin. Strong staining with the antibody was observedbeneath the basement membrane, in mucous glands, and in the subcutaneous tissue of the ventral skin. A weak staining was also observed in the ground substances of both ventral and dorsal skin. These results confirm that a monoclonal antibody hasbeen secured against at least one of the MIF constituents and that it is useful as a probe in detecting the distribution of MIF in tissues. The results of its use in this study support the hypothesis that MIF plays a role in the expression, development, and maintenance of the dorsal-ventral pigmentation patterns of frogs.
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