Brachyury correlates with expression of Pax-3-inducing activity. Second, the ability of leukemia inhibitory factor (LIF) to block mesoderm differentiation at two different points is demonstrated and correlated with the inhibition of Pax-3-inducing activity. Finally, two mesodermal cell

Many of the experimental observations on which the understanding of inductive interactions during vertebrate development are based were originally made using amphibian embryos (reviewed by Slack and Tannahill, 1992). In amphibians, induction of dorsal mesoderm and specification of the anteriorposterior axis begin at the time of gastrulation, in Spemann’s organizer region, in response to signals from dorsovegetal blastomeres. Initially, the organizer has a posterior specification; however, the first invaginating mesodermal cells acquire an extreme anterior specification. As the dorsal mesoderm invaginates, the invaginating cells progressively acquire more posterior specifications. A property of even the most anterior dorsal mesoderm is the expression of neural-inducing activity. Several genes have been isolated that are expressed specifically in dorsal mesodermal lineages of both amphibian and mouse gastrula and are likely to have a role in defining the properties of this tissue. The Brachyury gene (Herrmann et al., 1990) is expressed throughout the primitive streak mesoderm in the mouse starting at approximately 6.5 days post-coitum (d.p.c.; Wilkinson et al., 1990) and is required for axial mesoderm formation, particularly notochord (Herrmann, 1991). Abnormal development of the neural tube occurs in regions where notochord formation is disrupted in Brachyury T homozygous mutants consistent with an essential role for notochord in neural induction in the mouse (Chesley, 1935). The homeobox gene goosecoid (Blumberg et al., 1991) is expressed specifically in the organizer region of Xenopus embryos at the dorsal lip of the blastopore and migrates with the sheet of invaginating dorsal mesodermal cells. Additionally, injection of goosecoid mRNA into ventral mesoderm is sufficient to induce a complete secondary body axis suggesting that this gene has a central role establishment of Spemann’s organizer activity (Cho et al., 1991). Goosecoid mRNA is also expressed in the mesoderm of the mouse and is first detected at the site where primitive streak formation first occurs (Blum et al., 1992). As the primitive streak develops, goosecoid expression occurs specifically in the anterior region of the streak over a short period (approximately 6.4 to 6.9 d.p.c.). Further, this region of the mouse embryo has also been shown to have organizer-like activity when transplanted into Xenopus embryos. Based on these observations, it has been suggested that expression of goosecoid in the mouse gastrula reflects the localization of the murine equivalent to Spemann’s organizer of amphibians (Blum et al., 1992). If this is the case, anterior 37 Development 120, 37-47 (1994) Printed in Great Britain  The Company of Biologists Limited 1994