Development of hematopoietic ontogeny in fetal BM by immunohistochemistry. (A,C,E; ×40, H&E) Shows increasing cellularity, the beginning of hematopoiesis and the appearance of CD45⁺ cells (B,D,F; ×40, anti-CD45 immunoperoxidase). At 33 (A-B) and 39 (C-D) days, very few CD45⁺ cells are apparent. FACS shows evidence of hematopoietic migration by GA37 days, but cell content within the BM does not significantly change until 46 days (E-F), when tissue architecture begins to evolve, demonstrating a slight increase in content of CD45⁺ hematopoietic cells. Scale bars represent 25 μm. Flow cytometry (G) confirms significant expansion of CD45⁺ cells only at the latest time point, with a relatively flat curve throughout the early portion of the analysis.