A mesoderm-inducing factor is produced by a Xenopus cell line

Inductive interactions play a major role in the diversification of cell types during vertebrate development. These interactions have been extensively studied in amphibian embryos (usually Xenopus laevis) where the earliest is mesoderm induction, in which an equatorial mesodermal rudiment is induced from the animal hemisphere under the influence of a signal from the vegetal hemisphere. The molecular basis of mesoderm induction is unknown, although Tiedemann has isolated a protein from 9- to 13-day chick embryos that has the properties one would expect of a mesoderm-inducing factor. However, the relevance of this molecule to the events of early amphibian development is unclear, and it is a matter of some importance to discover a Xenopus mesoderm-inducing factor. In this paper I show that the Xenopus XTC cell line secretes mesoderm-inducing activity into the culture medium. Isolated animal pole regions cultured in XTC- conditioned medium differentiate into muscle and notochord, while controls form 'atypical epidermis'. Three different cell lines - XL, XL177 and KR - secrete no such activity. Preliminary characterization of the XTC mesoderm-inducing activity indicates that the active principle is heat stable, trypsin sensitive, nondialysable, and has an apparent relative molecular mass of about 16000. Work is in progress to characterize the activity further and to discover whether the mesoderm-inducing factor is also present in normal embryos.

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