Inductive effects of fibroblast growth factor and lithium ion on Xenopus blastula ectoderm

We have studied the response of Xenopus blastula ectoderm to fibroblast growth factor and to lithium ion. The properties of acidic and basic FGF are very similar showing a 50% induction level at 1-2 ng ml-1 and a progressive increase of muscle formation up to concentrations of 100-200 ng ml-1. The elongation of explants also shows a dose-response relationship. The minimum contact requirement for induction of ectoderm explants is about 90 min and the stage range of ectodermal competence extends from midblastula to early gastrula, both these figures resembling those obtained in embryological experiments with vegetal tissue as the inducer. Lithium chloride concentrations which produce anteriorization of whole embryos have no effect on isolated ectoderms unless accompanied by FGF. Simultaneous treatment with FGF and Li lead to a marked enhancement of both elongation and muscle formation over that produced by FGF alone. By contrast, ventral marginal explants show increased elongation and muscle formation if treated with lithium alone suggesting that they have already received a low-dose FGF treatment within the embryo. It is concluded that endogenous FGF may be solely responsible for inducing the ventral mesoderm and that dorsalization of ventral mesoderm to the level of somitic muscle might be achieved either by a very high local concentration of FGF in the dorsal region, or by the action of a second, synergistic, agent in the dorsal region.