Blood cell induction in Xenopus animal cap explants: Effects of fibroblastgrowth factor, bone morphogenetic proteins, and activin

Cultures of Xenopus blastula animal caps were used to explore the haematopo ietic effects of three candidate inducers of mesoderm: basic fibroblast gro wth factor (bFGF), bone morphogenetic proteins (BMPs) and activin A. In res ponse to either bFGF or activin A, explants expanded into egg-shaped struct ures, and beneath an outer layer of epidermis, a ventral mesodermal lining surrounded a fluid-filled cavity containing "blood-like cells". Immunocytoc hemistry identified some of these cells as early leukocytes, but erythrocyt es were rare. BMP-2 or BMP-4 induced primitive erythrocytes as well as leuk ocytes, and a high concentration was required for these cells to differenti ate in only a small proportion of explants. BMP-2 but not BMP-4 induced ven tral mesoderm concomitantly. High concentrations of activin A dorsalized ex plants, which contained infrequent leukocytes, and an optimal combination o f activin A and bFGF caused differentiation of muscle with few blood cells. By contrast, BMP-2 or BMP-4! plus activin A synergistically increased the numbers of both leukocytes and erythrocytes. Explants treated with BMPs plu s activin contained a well organized cell mass in which yolk-rich cells mix ed with blood cells and pigment-ed cells did not. BMP-2 plus bFGF also indu ced numerous leukocytes and fewer erythrocytes, but BMP-4 antagonized the l eukopoietic effect of bFGF The data suggest that the signalling pathways th ese three factors use to induce leukopoiesis overlap and that erythropoiesi s may be activated when inducers are present in combination.