Development 108, 173-183 (1990) Printed in Great Britain © T h e Company of Biologists Limited 1990 173 The biological effects of XTC-MIF: quantitative comparison with Xenopus bFGF J. B. A. GREEN, G. HOWES, K. SYMES*, J. COOKE and J. C. SMITH Laboratory of Embryogenesis, National Institute for Medical Research, The Ridgeway, Mill Hill, London NW71AA, UK * Present address: Department of Cell and Molecular Biology, 385 LSA, University of California, Berkeley, CA 94720, USA Summary Mesoderm in Xenopus and other amphibian embryos is induced by signals from the vegetal hemisphere acting on equatorial or animal hemisphere cells. These signals are diffusible and two classes of candidate signal mol- ecule have been identified: the fibroblast growth factor (FGF) and transforming growth factor /? (TGF-/?) types. In this paper, we compare the effects of cloned Xenopus basic FGF (A1>FGF) and electophoretically homo- geneous XTC-MIF (a TGF-0-like factor obtained from a Xenopus cell line) on animal pole explants. We find that they have a similar minimum active concentration (0.1-0.2ngml"') but that, nonetheless, XTC-MIF is at least 40 times more active in inducing muscle. In general, we find that the two factors cause inductions of significantly different characters in terms of tissue type, morphology, gene expression and timing. At low concentrations (0.1-1.0 ng ml" 1 ) both factors induce the differentiation of 'mesenchyme' and 'meso- thelium' as well as blood-like cells. These latter cells do not, however, react with an antibody to Xenopus globin. This raised the possibility that the identification of red blood cells in other studies on mesoderm induction might have been mistaken, but combinations of animal pole regions with ventral vegetal pole regions confirmed that genuine erythrocytes are formed. The identity of the blood-like cells formed in response to the inducing factors remains unknown. At higher concentrations XTC-MIF induces neural tissue, notochord, pronephros and substantial and often segmented muscle. By con- trast, -YbFGF only induces significant amounts of muscle above 24ngml~ 1 and even then this is much less than that induced by XTC-MIF. For both factors an exposure of less than 30min is effective. Competence of animal pole cells to respond to A1)FGF is completely lost by the beginning of gastru- lation (stage 10) while competence to XTC-MIF is detectable until somewhat later (stage 11). Since animal pole tissue is known to be able to respond to the natural inducer at least until stage 10, and perhaps until stage 10.5, this suggests that bFGF cannot be the sole inducer of mesoderm in vivo. Taken together, these results are consistent with XTC- MIF being a dorsoanterior inducer and AT)FGF a ventroposterior inducer, suggesting that body pattern is established by the interaction of two types of inducing signal. This model is discussed in view of the qualitative and quantitative differences between the factors. Key words: mesoderm induction, mesoderm-inducing factors, bFGF, XTC-MIF, amphibian embryo, Xenopus laevis. Introduction The mesoderm of amphibian embryos arises through an inductive interaction during which a signal from the vegetal hemisphere of the embryo acts on overlying animal pole cells (reviewed by Dawid et al. 1989; Gurdon et al. 1989; Smith, 1989). Recently, two meso- derm-inducing factors (MIFs) have been derived from homologous sources. XTC-MIF is produced by the Xenopus XTC cell line and may be related to TGF-/J (Smith, 1987; Rosa et al. 1988; Smith et al. 1988). Basic fibroblast growth factor (bFGF) has been cloned from a Xenopus oocyte cDNA library and the protein is known to be present in the egg and early embryo (Slack et al. 1987; Kimelman and Kirschner, 1987; Kimelman et al. 1988; Slack and Isaacs, 1989). Both XTC-MIF and bFGF induce mesodermal cell types from Xenopus blastula ectoderm but until now most work has been done only with partially purified XTC-MIF and with bFGF from bovine brain. The limited information that is available suggests that XTC-MIF induces dorsal, intermediate and ventral mesodermal cell types while bFGF induces only intermediate and ventral tissue (see Smith, 1987; Godsave et al. 1988; Smith et al. 1988). In this paper, we undertake a direct quantitative comparison of the effects of the two Xenopus-dtrived MIFs in their pure forms. While they display a similar potency for mesoderm induction, the two factors are