/. Embryol. exp. Morph. 89, 289-312 (1985) 289 Printed in Great Britain © The Company of Biologists Limited 1985 Mesoderm induction in Xenopus laevis \ a quantitative study using a cell lineage label and tissue-specific antibodies L. DALE, J. C. SMITH* AND J. M. W. SLACK Imperial Cancer Research Fund, Burtonhole Lane, Mill Hill, London NW7 IAD, U.K. SUMMARY We have compared the development of the animal pole (AP) region of early Xenopus embryos in normal development, in isolation, and in combination with explants of tissue from the vegetal pole (VP) region. For the grafts and the combinations the animal pole tissue was lineage labelled with FLDx in order to ascertain the provenance of the structures formed. The normal fate of the AP region was determined by orthotopic grafts at stages 1\ (early blastula), 8 (mid blastula) and 10 (early gastrula). At later stages most of the labelled cells were found in ectodermal tissues such as epidermis, head mesenchyme and neural tube (the last from stages 1\ and 8 only). However, in stage-7^ and stage-8 grafts some of the labelled cells were also found in the myotomes and lateral mesoderm. In isolated explants the AP region of all three stages differentiated only as epidermis assessed both histologically and by immunofluorescence using an antibody to epidermal keratin. The fate of labelled cells in AP-VP combinations was quite different and confirms the reality of mesoderm induction. In combinations made at stages % and 8 the proportion of AP-derived mesoderm is substantially greater than the proportion of labelled mesoderm in the equivalent fate mapping experiments. This shows that the formation of mesoderm in such combinations is the result of an instructive rather than a permissive interaction. The formation of mesodermal tissues in stage-7z combinations was confirmed by using a panel of antibodies which react with particular tissues in normal tailbud-stage embryos: anti-keratan sulphate for the notochord, anti-myosin for the muscle and anti-keratin for epidermis and notochord. Combinations made at stage 10 gave no positive cases and reciprocal heterochronic combinations between stages 1\ and 10 showed that this is the result of a loss of competence by the stage-10 AP tissue. Whereas stage-7| AP tissue combined with stage-10 VP tissue gave many positive cases, the reciprocal experiment gave only a few. We have also tested the regional specificity of the induction. Stage-7? vegetal pole explants were divided into dorsal and ventral regions and then combined, separately, with stage-7^ animal poles. The dorsovegetal tissue induces 'dorsal-type' mesoderm (notochord and large muscle masses) while ventrovegetal tissue induces 'ventral-type' mesoderm (blood, mesothelium and a little muscle). We conclude that mesoderm formation in combinations is an instructive event and propose a double gradient model to explain the complex character of the response. * Present address: Laboratory of Embryogenesis, National Institute for Medical Research, The Ridgeway, Mill Hill, London NW7 1AA, U.K. Key words: Xenopus laevis, mesoderm induction, lineage labelling, regional markers, specification, gradients, keratin, myosin, keratan sulphate.