Studies of the development of frog hemopoietic tissue in vitro. I. Spleen culture assay of an erythropoietic factor in anemic frog blood

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Abstract
A new in vitro technique has been described for demonstrating the presence of an erythropoietic factor in the circulating blood of frogs. The assay system consisted of MC33 medium, erythropoietically active spleen cells from Rana pipiens, and plasma or serum from frogs made anemic via phenylhydrazine or bleeding. The spleen cells, which remain erythropoietically active for up to nine days, were found to incorporate 59Fe, [3H] thymidine, [3H]uridine, and [3H]leucine at a greater rate in the presence of plasma or serum from anemic versus normal frogs. The hormones triiodothyronine, prolactin, and erythropoietin were not effective in eliciting an hemopoietic response. The data presented suggest that the spleen from that adult frog is a major site of erythroid differentiation and maturation.