Development 102, 555-566 (1988) Printed in Great Britain © The Company of Biologists Limited 1988 555 Mesoderm-inducing factors: a small class of molecules S. F. GODSAVE, H. V. ISAACS and J. M. W. SLACK Imperial Cancer Research Fund, Developmental Biology Unit, Department of Zoology, University of Oxford, South Parks Road, Oxford OX1 3PS, UK Summary Mesoderm-inducing factors (MIFs) from chick em- bryos, 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 mesen- chyme, 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 differen- tiation factors. Their occurrence in various tissues and cell lines suggests that they have functions in the adult organism as well as during early development. Key words: mesoderm induction, mesoderm-inducing factor, MIF, heparin-binding growth factor, HBGF, Xenopus laevis. Introduction In the Xenopus embryo, mesoderm is thought to be formed in the blastula as a result of inductive interac- tions between the vegetal tissue which is destined to become endoderm and an equatorial region extend- ing into the animal hemisphere, the presumptive mesoderm. In simple salt solutions, explants from the animal hemisphere will develop only into spheroids of epidermal cells. However, if they are placed in contact with explants from the vegetal pole region they will also give rise to mesodermal tissues (Sudar- wati & Nieuwkoop, 1971; Dale et al. 1985; Gurdon et al. 1985). It is now known that this interaction can proceed across a small-pore Nucleopore filter sugges- ting that cell contact is not necessary and one or more diffusible factors are involved (Grunz & Tacke, 1986). It has been known for many years that the effect of vegetal pole cells can be mimicked by various prep- arations from adult or late embryonic tissues of which the best known is Tiedemann's vegetalizing factor extracted from 10-day chick embryos (Tiedemann & Tiedemann, 1959; Born et al. 1972a; Schwarz et al. 1981), but also guinea-pig bone marrow (Toivonen, 1953), HeLa cells (Saxen & Toivonen, 1958), carp swim bladder (Kawakami, 1976) and others. Re- cently, a convenient Xenopus source has been dis- covered: conditioned medium from cultures of the cell line XTC (Smith, 1987). The diversity of sources for mesoderm-inducing factors (MIFs) has made it necessary to consider whether mesoderm induction, like neural induction, can be triggered by a number of nonspecific stimuli, although it is equally likely that the same specific factor is found in a variety of tissues. Recently, we identified the heparin-binding growth factors (HBGFs), acidic fibroblast GF (aFGF), basic