Development of hematopoietic ontogeny in FL by immunohistochemistry. (A,C,E; ×20, H&E) Shows the structural maturation and expansion of CD45⁺ cells (B,D,F; ×20, anti-CD45 immunoperoxidase). At 33 days (A-B), the tissue appears immature, and relatively few CD45⁺ cells are apparent. By 39 days (C-D), CD45⁺ cells begin to appear, and by 46 days (E-F), obvious vascular architecture and a more mature histologic appearance correspond with a significantly increased concentration of CD45 hematopoietic cells and CD34⁺ progenitors. Scale bars represent 50 μm. Again, flow cytometry confirms the presence of CD45⁺ hematopoietic progenitors throughout the period of analysis, increasing exponentially with cellular division (G).