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Mesoderm-inducing factors: a small class of molecules.

Mesoderm-inducing factors (MIF's) from chick embryos, XTC cells and WEHI-3 cells were studied using various procedures. The object was to find whether they are similar to heparin-binding growth factors (HBGFs-the only known pure mesoderm-inducing substances) and, if not, whether they are similar to each other. The major active components from all three MIF sources behave as somewhat hydrophobic, acid-stable molecules and do not bind to heparin. They all have relative molecular masses of about 13,000 measured by HPLC size exclusion chromatography. The isoelectric points measured by chromatofocusing were 6.7 (WEHI) and 7.7 (XTC). The chick MIF seemed somewhat heterogeneous by chromatofocusing and a portion of its activity bound to heparin sepharose. All three MIFs have similar effects on explants of Xenopus blastula ectoderm to the heparin-binding growth factors, causing an elongation at the time of gastrulation followed by the development of mesenchyme, mesothelium and muscle cells, the proportion of muscle increasing with dose. Unlike the HBGFs they all also induce notochord if sufficiently high concentrations are used. Our study shows that the MIFs examined here form a small group of potent agents distinct from the HBGFs and from other known growth and differentiations factors. Their occurrence in various tissues and cell lines suggests that they have functions in the adult organism as well as during early development.