Intraembryonic origin of hepatic hematopoiesis in Xenopus laevis

The liver is a major site of hematopoietic stem cell differentiation during vertebrate development. Hepatic hematopoiesis is dependent on colonization of the organ by extrinsically derived stem cells which, in mammals, are thought to originate only in the yolk sac. However, in birds and amphibians two distinct embryonic stem cell sources have been identified. The yolk sac or extraembryonic compartment is associated with the developing vitelline veins, and the para-aortic or intraembryonic compartment is associated with the dorsal aortae and postcardinal veins. The homologous compartments in the Xenopus embryo are the ventral blood island (extraembryonic) and dorsal lateral plate (intraembryonic) mesoderms, which contribute to primitive larval erythrocyte and definitive late larval and adult erythroid populations, respectively. The role of these embryonic stem cell sources in hepatic hematopoiesis has not been determined. We have examined the development of hepatic hematopoiesis in Xenopus 2N/3N stem cell chimeras using two-color FACS analysis. DNA content was determined using Hoechst 33342, and subpopulations of hematopoietic cells were identified with specific mAbs. Here we show that hepatic erythrocytes, leukocytes, and B lymphocytes in the liver of Xenopus larvae were derived from stem cells that originated from the intraembryonic mesoderm.