Primitive erythroblasts do not enucleate by splenic pitting or by karyolysis. (A) The estimated number of primitive erythroblasts that undergo enucleation per day between E11.5 and E18.5 of gestation in outbred mice. Enucleation events were estimated using the percentage of enucleated primitive erythroid cells previously determined for each embryonic day, assuming that there are 6.7 × 106 primitive erythroid cells per fetus and that total primitive erythroid cell numbers do not decrease between E12.5 and E17.5 of gestation.12 (B) Immunohistochemistry of transverse section of E15.5 mouse embryo with F4/80 (blue) and anti–ϵγ-globin (red) antibodies reveals the presence of mature macrophage cells and primitive erythroid cells, respectively. At lower power (i), most of the macrophages can be seen in the liver (L) with few in the surrounding body wall and spleen (Sp) or stomach (St). At higher power (ii), infrequent large primitive erythrocytes are seen in vessels in the spleen (arrowhead), but they are not associated with the few splenic macrophages present at this time. Labeled primitive erythroblasts are observed as a subset of blood cells in the aorta (Ao) (iii). Scale bars = 0.1 mm. (C) Analysis of fragmentation of DNA from primitive erythroid cells in E15.5 peripheral blood. Lanes 1 and 2 show no fragmentation of E15.5 peripheral blood DNA in either untreated (lane 1) or after 24 hours of treatment with 0.5 μM staurosporine (lane 2). In contrast, bone marrow DNA shows significant laddering after 6 hours (lane 3) or 24 hours (lane 4) of treatment. Vertical lines have been inserted to indicate repositioned gel lanes.
