Hemopoietic differentiation potential of cultured lateral plate mesoderm explanted from Rana pipiens embryos at successive developmental stages.

Ventral blood island mesoderm and dorsal lateral plate mesoderm were removed from Rana pipiens embryos at successive developmental stages (stages 13-19; 50-118 h) and cultured as individual explants in serum-free medium. After 5-7 days, the cultures were harvested, and differential counts were made of Wright-Giemsa-stained cells. Ventral blood island explants gave rise to cells of the myeloid lineage, suggesting that ventral blood island mesoderm was committed to hemopoiesis at the time of explant. Although erythrocytes were present in the cultures, granulocytes and monocyte/macrophages predominated. This differentiation profile occurred without the addition of any exogenous humoral factors. Monocyte/macrophages and immature precursor cells exhibited recurring inverse fluctuations with respect to one another. In all cases examined, cultures of dorsal lateral plate mesoderm showed marginal hemopoietic cell differentiation, suggesting a requirement for exogenous humoral factors and/or cell-cell interactions. When viewed in the context of previous studies from our laboratory, these results demonstrate that, in the amphibian embryo, there are two sources of hemopoietic stem cells separated both in space and time.