Cytologic changes during primitive erythroid maturation. (A) Giemsa-stained cytospin preparations of blood from wild-type embryos at E9.5 to E14.5. Scale bar, 20 μm. Circulating blood cells from E9.5 (B) and E10.5 (C) embryos, showing loss of nucleoli (red arrows in B) within the intervening 24-hour period. Scale bar, 10 μm. (D) Enucleated definitive (*), larger nucleated primitive (black arrow) and enucleated primitive (white arrow) erythroid cells in circulation at E14.5. Scale bar, 10 μm. (E) Diameters of circulating E9.5 to E14.5 embryonic blood cells (□) and their nuclei (⊡), measured on cytospin preparations using the Axiovision program. (F) Ratio of mean cross-sectional area of nuclei and cytoplasms of circulating embryonic blood cells. A dramatic decrease in nuclear diameter and cross-sectional area was observed, coincident with nuclear condensation (compare with panel A). Data in panels E and F are expressed as mean ± SD.