BM architecture. Light microscopy images of murine (panels A,C) and human (panels B,D) BM sections stained with hematoxylin and eosin. Extensive invasions of spongy bones (trabeculae, T) and many fat cells (adipocytes, A), residing between the hematopoietic niches, characterize human BM (B) and distinguish it from the murine BM (A). Murine (C) and human (D) BM contains many small blood vessels (sinusoids, S), which are the site where progenitors home to the BM and where maturing cells and erythrocytes egress to the circulation (B). A monolayer of immature osteoblasts, which lines the bone and the trabeculae, defines the endosteum region (arrowheads, panels C,D), wherein hematopoietic stem cells reside. Images were acquired using an Eclipse E800 microscope (Nikon, Tokyo, Japan) and a Nikon DXM1200 using Nikon ACT-1 software. For panels A and B, objective × 4/15.7 NA was used (original magnification, × 40); for panels C and D, objective × 40/0.14 NA was used (original magnification, × 400).