The objective of this proposal is to further our understanding of the ontogeny of hematopoietic stem cells and their contribution to the developing thymus. This will be accomplished by experimentally manipulating embryonic stem cells followed by injection of these stem cells into intact, cytogenetically distinct larval hosts. The contribution of the injected stem cells to the developing hematopoietic system will be monitored using flow cytometry. The results of these experiments will indicate when lymphoid precursors differentiate during embryogenesis and if there are differential contributions from embryonic stem cell compartments to the developing thymus and other peripheral hematopoietic tissues. The commitment of undifferentiated mesoderm to hematopoiesis appears to be associated with the expression of a surface molecule which may be a member of the Leukocyte Common Antigen family of protein tyrosine phosphatase receptors. It is important to establish the functional role and phylogenetic conservation of this family of signal transduction molecules. Although Xenopus is a phylogenetically primitive vertebrate, many aspects of its hematopoietic and immune systems are similar to more recently evolved species. Thus the proposed investigations of the immunobiology of this organism may provide useful insights into both the ontogeny and evolution of vertebrate defense mechanisms.
