THE EFFECT OF CYCLOSPORINE-A ON THE PROLIFERATING CHICK-EMBRYO ERYTHROBLASTS

The proportion of proliferating erythroblasts, i.e. proerythroblasts, basophilic erythroblasts and polychromatophilic erythroblasts in blood islands of the chick embryo yolk sac, were counted during embryonic d ays 2-10. From day 2 when high amounts of erythroblasts signalized the onset of embryonic erythropoiesis, the percentage of less mature eryt hroid cells gradually decreased. Intraamniotic injection of cyclospori n A in doses 1.5 or 15.0 mu g per embryo on day 5 led to significant c hanges in the proportion of proliferating erythroblasts in the yolk sa c blood islands. We speculate that these changes were caused initially by the release of the more mature cells into the circulation and late r by a dose-dependent decrease in the number of stem cells. The estima tion of proerythroblast percentage from all proliferating erythroblast s in the yolk sac blood islands may serve as a valuable indication of toxic damage in the late avian embryo.