Tumors in tadpoles: the Xenopus embryo as a model system for the study of tumorigenesis

Abstract

One very useful aspect of the Xenopus embryo for the study of gene function is the rapidity with which experiments can be performed. Xenopus embryos are fertilized in vitro and subsequently microinjected with in vitro-synthesized mRNA encoding the protein to be tested1 (Fig. i). Hundreds of Xenopus embryos can easily be injected in one afternoon, allowing several different constructs to be examined in parallel. These externally developing embryos live happily in a dish of simple saline and develop to swimming tadpoles in just three days. Furthermore, these tadpoles have fully developed nervous systems, eyes, kidneys, hearts and muscles; thanks to accurate fate maps, each of these tissues can easily be targeted by injecting different cells at the 32-cell stage2. For example, the colored arrows and text in the accompanying figure indicate the predominant fates of four different cells in a normal, 32-cell Xenopus embryo. By injecting mRNA into only one of these cells in an embryo, expression can be targeted to a specific tissue. Finally, injections made into one side of the embryo usually have no effect on the other side, providing an effective internal control in each embryo. So, in one set of experiments, the effects of several different constructs can be tested in several different cell types in vivo with effective internal controls; all in ∼1 week. Coupled with the simple β-galactosidase assay for tumor formation (see Fig. 1), the Xenopus embryo becomes a powerful model system indeed.