Diagrams of the protocol of BM sample preparation and confocal microscopy. For mouse BM, sternums were bisected sagittally into 2 to 3 segments. For human BM, core biopsies were bisected into segments (2-4 mm). Segments were further transected, fixed (except for EGFP-expressing tissues), and fluorescently stained as whole-mounts. Fluorescently labeled BM tissues were subjected to confocal microscopy, and series of images were used to reconstruct the 3D architecture of BM (related images in supplemental Video 1).