Genetic control of hematopoietic development in Xenopus and zebrafish

Blood development has been highly conserved during evolution. Hematopoietic cells in amphibian and fish embryos, as in mammalian embryos, emerge and progressively differentiate in several locations. Hematopoiesis, including of the immune system, is similar in the amphibian, Xenopus, to mammals and the embryos are ideal for tissue transplantation and lineage labelling experiments, which have enabled the elucidation of the distinct origins of embryonic and adult hematopoietic cells, as well as their migration pathways and organ colonisation behaviours. The zebrafish hematopoietic system is less well understood, but these embryos have recently emerged as a powerful system for both genetic analysis and imaging. In this review, we summarise our current knowledge of the cellular and genetic basis of ontogeny of the hematopoietic system in Xenopus and zebrafish embryos.