Active hematopoiesis occurs during teratoma formation. (A) Teratoma section stained with hematoxylin/eosin shown at 10× magnification demonstrating typical teratoma bone marrow–like structures. Trabecular bone, cartilage, and bone marrow are clearly visualized. (B) 60× magnification showing blood elements including neutrophils, lymphocytes, megakaryocytes (MK), and immature blasts (HSPCs) in the bone marrow–like island. (C) LSC showing the presence of human CD45⁺ blood cells (green) and CD34⁺CD45⁺ blood stem/progenitors cells (red and green) indicated by white arrows (right panels, 40× objective, scale bar 20 μm), within the context of an entire teratoma section (left panel, 10× objective, scale bar 500 μm). The profile of intensity in each fluorescent channel along the midline of each cell demonstrates the overlap of the red (CD34) and green (CD45 signals) around DAPI⁺ (blue) stained nuclei (middle panels).