A karyometrical study of circulating erythroblasts of yolk sac origin in the mouse embryo.

Circulating erythroblasts of embryonic mice were karyometrically examined by light microscopy. In the erythroid cells in embryonic blood vessels, mitoses were encountered from 9 to 12 days of gestation. At 9 days, the circulating blood cells consisted of proerythroblasts and less mature cells. The nuclear diameter ranged from 4.8 to 9.8 micron, the majority ranging between 6 and 8 micron. At 11 days, hemopoietic cells with a nuclear diameter larger than 6 micron disappeared from embryonic circulation, and more than 90% had a nuclear diameter of less than 5 micron. Between 12 and 16 days of gestation, the smallest orthochromatic erythroblasts measuring 3.4 micron nuclear diameter showed the highest peaks. The progress of primitive erythropoiesis in embryonic circulation is discussed in comparison with that of definitive erythropoiesis.