Phenotypic analysis of cell subsets in the circulation at different days of gestation. (A) The graph shows the frequency of nRBCs vs enucleated RBCs (enRBCs) between E13 and E18 (determined as percentage of total RBCs). (B) At E15, nRBCs and large mononuclear cells with features typical of progenitors are readily found on cytospin preparations stained with modified Giemsa. (C) The differential white blood cell count in fetal blood between E13 and E18 is shown. Cells included within the category “other” include primordial stem cells, monocytes, basophils, and eosinophils. (D-E) In addition, fetal LysEGFP+ and LysEGFP− blood leukocytes were investigated for their expression of the neutrophil marker Gr1 at (D) E14 and (E) E18.