Definition of endosteal, subendosteal, and central zones in BM and demonstration of cell imaging. (Ai-ii) Definition of endosteal, subendosteal, and central zones in BM sections of femur diaphysis representing compact bone. Regions in each transverse bone section were defined as endosteal (≤ 3 cell diameters from endosteal surface; zone 1, depicted in red), subendosteal (4-14 cell diameters from endosteal surface; zone 2, depicted in yellow), or central (> 14 cell diameters from endosteal surface; zone 3). The average relative surface area of each of the 3 regions was calculated, using image analysis software, as 8% ± 0.63%, 31% ± 1.31%, and 61% ± 1.85%, for zones 1, 2, and 3, respectively. (Bi-ii) For trabecular bone sections, 2 zones were evaluated: endosteal in red and the rest. (C) Depiction of homed donor Lin⁻kit⁺ cells to irradiated BM. Transplanted cells are labeled with CFSE (green); red-stained cells (anti–CD31-PE) are vascular cells and megakaryocytes (white arrow). (D) Competitive homing of α4^+/+ (SNARF-1, red) and α4 integrin–deficient (CFSE, green) labeled hematopoietic Lin⁻kit⁺ cells in irradiated BM. The 2 populations were given at a ratio 0.7 (red):1 (green) (see Table 2 for details). The white interrupted line in panels C and D represents the endosteal border.