Effects of mobilization and expansion on the BM niche. Representative images of immunohistochemistry staining for F4/80 (brown, left column) and osteocalcin (brown, right column) in serial sagittal cross-sections of the hind limb from C57BL/6 mice treated with (A-B) saline, (C-D) continuous infusion of POL5551 (100 mg/kg per day for 14 days), and (E-F) G-CSF (100 μg/kg per dose, 9 doses every 12 hours) (n = 4). All sections were counterstained with hematoxylin (blue). All images are representative of the endosteal region within the tibial metaphyseal zone. Asterisk in each serial section pair indicates trackable tissue landmarks. (A-D) In saline- and POL5551-treated mice, (B,D, arrowheads) osteocalcin-expressing osteoblasts and (A,C, arrows) F4/80-expressing macrophages can easily be discerned, and they illustrate the F4/80⁺ osteomac canopy structure associated with the osteoblasts. (E-F) In G-CSF–treated mice, few F4/80⁺ macrophages are evident, including dramatic loss of (E) osteomac canopy and (F) osteocalcin⁺ osteoblast-covered bone surface. Original magnification ×60. Scale bars represent 20 μm.