Transplanted HSPCs home rapidly to the BM where analysis of the spatial distribution of individual HSPCs reveals their location and allows an accurate assessment of homing efficiency. (A) At 15 minutes after transplantation, HSPCs are already in the BM and are classified as being within BVs (top image), crossing BV endothelium (middle image), or being within the extravascular space (bottom image). Because of perfusing the fixative through the BVs, these structures are mostly cleared of blood. The thin-walled endothelium is visible at higher magnification (not shown). In the middle micrograph, red blood cells remaining in the BV are distinguished by their reddish hue, which is a result of the dual filter used to visualize the images. *BV. Bone (b) is outlined by a dashed line. Scale bar represents 25 μm. Micrographs captured using a 60× (UPlanApo NA 1.2) objective on an Olympus BX51 microscope fitted with a SPOT RT-SE6 camera. (B) The percentage of HSPCs in each of these areas during the first 15 hours after transplantation is shown: 15 minutes, n = 7 (424 HSPCs); 30 minutes, n = 6 (287 HSPCs); 45 minutes, n = 8 (605 HSPCs); 5 hours, n = 3 (129 HSPCs); and 15 hours, n = 5 (635 HSPCs). (C) Homing efficiencies determined by flow cytometry were corrected for the percentage of HSPCs in vessels. Symbols represent individual recipients. (B-C) Experi-mental repeats are ≥ 2. Data are mean ± SEM.