The role of endoderm in blood cell ontogeny in the newt Pleurodeles waltl.

In order to determine the origin of blood cells we performed embryonic grafts of different portions of mesoderm from various locations between diploid and tetraploid embryos at the tail-bud stage. The tetraploid animals were the hosts. The size differences between tetraploid and diploid cells made identification possible by direct microscopic examination of blood smears. In a previous report we showed the important role of truncal anterior mesoderm in the genesis of blood cells. We now establish that this effect is brought about by the inductive capacity of the hepatic endoderm or by the fact that the environmental conditions are more appropriate for blood stem cell development, whereas in the absence of the hepatic endoderm the blood stem cells fail to appear. Grafting of hepatic anlage containing endoderm and mesoderm gives rise to numerous graft-derived blood cells which last throughout the life span of the hosts. The same result is obtained by grafting truncal posterior ventral or lateral mesoderm onto hepatic endoderm. Heterotopic grafting of truncal anterior mesoderm isolated from its underlying hepatic endoderm leads to the formation of only a few blood cells which last only during larval life. This demonstrates that the whole lateral and ventral truncal mesoderm is able to differentiate into blood cells when associated with hepatic endoderm.