Delayed definitive erythropoiesis in Sp3-deficient embryos. (A) Analysis of the size distribution of circulating erythrocytes at different days of gestation. Yolk sac–derived erythrocytes (primitive) remain nucleated and have a diameter of approximately 10 μm. Fetal liver–derived erythrocytes (definitive) are enucleated and have a diameter of approximately 7 μm. Appearance of definitive erythroytes in the circulation is delayed in Sp3-/- embryos. (B) Cytospins were prepared from blood isolated from the indicated embryos and were stained with a combination of neutral benzidine and histologic dyes.33 Nucleated primitive erythroytes are clearly distinguished from enucleated definitive erythrocytes; both cell types have a normal morphologic appearance in Sp3-/- embryos.
