Lmo2 and GATA-3 associated expression in intraembryonic hemogenic sites

It is now widely accepted that hemopoietic cells born intraembryonically ar e the best candidates for the seeding of definitive hemopoietic organs. To further understand the mechanisms involved in the generation of definitive hemopoietic stem cells, we analysed the expression of the hemopoietic-relat ed transcription factors Lmo2 and GATA-3 during the early steps of mouse de velopment (7-12 dpc), with a particular emphasis on intraembryonic hemogeni c sites. We show here that both Lmo2 and GATA-3 are present in the intraemb ryonic regions known to give rise to hemopoietic precursors in vitro and in vivo, suggesting that they act together at key points of hemopoietic devel opment. (1) Lmo2 and GATA-3 are expressed in the caudal mesoderm during the phase of intraembryonic precursors determination. (2) A highly transient c oncomitant expression is observed in the caudal intraembryonic definitive e ndoderm, suggesting that these factors are involved in the specification of intraembryonic hemopoietic precursors. (3) Lmo2 and GATA-3 are expressed w ithin the hemopoietic clusters located in the aortic floor during fetal liv er colonisation. Furthermore, a strong GATA-3 signal allowed us to uncover previously unreported mesodermal aggregates beneath the aorta. A combined i n situ and immunocytological analysis strongly suggests that ventral mesode rmal GATA-3 patches are involved in the process of intraembryonic stem cell generation.