Appearance of primitive “pyrenocytes” in the fetal bloodstream. (A) Immunohistochemistry fetal blood, stained with anti–ϵγ-globin antibodies (red) and DAPI (blue) to label primitive erythroid cells and nuclei, respectively. The upper panel is a darkfield combined fluorescent image of E14.5 blood. The numerous smaller definitive erythrocytes are not evident because they are both ϵγ-globin– and DAPI-negative. The lower panel is E15.5 blood with fluorescent images layered over a Hoffman contrast image to show the abundant unstained definitive erythrocytes compared with the larger ϵγ-globin–positive primitive erythroblasts. Primitive pyrenocytes with small rim of ϵγ -globin–positive cytoplasm are indicated by arrows. Scale bar = 10 μm. (B) Morphometric analysis of E14.5 blood images indicates that pyrenocytes are significantly (P < .001) smaller than surrounding primitive orthochromatic erythroblasts (EryP). However, there is no significant difference in the size of their nuclei. Error bars are SEM.