MLL-AF9 leukemia from fetal liver differs from that originating in postnatal marrow. (A) The latency of leukemia in recipient mice that received 100 hematopoietic MLL-AF9 stem cells (HSCs) from either embryonic day (E) 14.5 fetal liver or adult bone marrow. (B-D) Immunohistochemisty for myeloperoxidase (MPO), CD45R/ B220, and morphology of hematoxylin and eosin–stained cells on the sections from liver of leukemic mice showing leukemia that developed in recipients of fetal liver or bone marrow. All vertical panels are at the same magnification; bars represent 50 μm. Images were taken with a Spot Insight Wide-field 4 MP CCD Scientific Color Digital camera (Diagnostic Instrument) mounted on a Nikon Eclipse 80i microscope (Nikon Plan Apo 20×/0.75 or Plan Fluor 60×/0.85); image size was adjusted in Photoshop CS3 Version 10.0.1 (Adobe Systems). (B) Recipient of HSCs from adult marrow. Leukemia cells are MPO-positive, CD45R/B220-negative and show nuclear lobulation and indentation consistent with granulocytic differentiation. (C) Recipient of HSCs from fetal liver. Leukemia cells are MPO-negative, B220-positive and are large with large round, blastlike nuclei consistent without evidence of lineage-specific differentiation. (D) Recipient of unsorted cells from fetal liver. Leukemia cells are both MPO- and B220-positive and show no evidence of lineage-specific differentiation.