The heart, blood and vasculature arise from parts of the mesoderm in vertebrate animals. Recent sttudies in frog have shown that the mesoderm itself is induced from ectodermal tissues by components secreted by the endoderm and that these components include members of the FGF and TGF- beta superfamilies of growth factors. Although experimental embryological studies have suggested that the mesoderm may divided into a ventro- lateral and dorsal component, the exact complexity of the mesoderm has not been established at the time of gastrulation, nor are the processes by which the mesoderm differentiates further understood. In this proposal we plan to develop markers for the early mesoderm tissues in the frog to identify in detail the paths that lead from ventral mesoderm to the blood and vascular lineages and from dorsal mesoderm to the heart lineage. Using these markers we plan to identify further requirements for the formation of these tissues. We also plan to examine the role of PDGF in mesoderm induction and the utility of the frog PDGF receptor for distinguishing dorsal from ventral mesoderm. In addition the question of competence in the response of the ectoderm cells to FGF will be examined by cloning the FGF receptor in rog eggs and examining its pattern of expression. Finally given a number of experiments that suggest that later signals from the endoderm are required to generate the blood, vasculature, and heart from already induced mesoderm, we plan to develop specific bio-assays for soluble components in the induction process along the lines that have been successful for the mesoderm inducing activities and assays known cytokines for these activities. Through these experiments we hope to better define the role of growth factors in the early development of tissues and to develop a molecular understanding of how complexity develop in the mesoderm.
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