| Summary: | Long term repopulating HSC activity has been found to increase within AGM region explant cultures indicating that elements of the supporting microenvironment for definitive HSC expansion can be captured <i>in vitro</i>. In this study an explant culture system was developed to examine the inductive properties of the AGM region on differentiating ES cells. A highly significant increase in the number of primate haematopoietic progenitors, as measured by <i>in vitro</i> colony assays, was observed after co-culture of ES cells with the AGM region from a 10.5 day embryo. However, engraftment <i>in vivo</i> of ES-derived progenitors after transplantation was not achieved. The effect of three stromal cell lines derived from the AGM region and foetal liver on the differentiation of ES cells in co-culture was also examined. Although the cell lines were similar to each other in terms of the expression of surface markers, they exhibited diverse effects on the differentiation of ES cells. Preliminary data also suggest that the AGM region-derived factor(s) responsible for the increase in haematopoietic differentiation of ES cells is dependent on direct cell-cell contact. The AGM region culture system was also used as a novel investigative tool in the analysis of a putative haematopoietic phenotype in a mouse mutant deficient in the murine homologue of the ubiquitin conjugating enzyme, Ubc7, generated previously by a gene-trapping technique. The numbers of haematopoietic progenitors in AGM regions from wildtype, heterozygous and homozygous embryos after culture were compared, but in this case no measurable haematopoietic defect was observed.
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