Mobilization of macrophages during normal fracture healing. (A) RT-PCR assay of the expression levels of molecules related to macrophage recruitment or endocytosis. CCL2, CCL7 and CCL8 are chemokine (CC motif) ligand 2, 7 and 8, respectively. CCR2 is CC chemokine receptor type 2. MSR1 is macrophage scavenger receptor 1. (B,C) In normal unfractured tibiae, macrophages are detected in subcutaneous tissues. Macrophages are stained brown after immunohistochemistry using anti-F4/80 antibody. (D) At 3 days after fracture, abundant macrophages are present in granulation tissues (E), between injured muscles (F), and in the region of periosteal reaction (G). (H) At 7 days after fracture, macrophages are observed in fibrous callus (I) and between newly formed trabecular bone (J), but not in cartilage (K). (L) At day 14, macrophages are present at the periphery of fracture callus (M). A small number of macrophages were also detected at the front of endochondral ossification (N, arrow) and in the new bone (O, arrow). (P) At day 21, macrophages are observed at the periphery of callus (Q). (R)Macrophages were also detected lining trabecular bone in the callus (arrows). The dashed lines in B, D, H, L and P show fracture ends. The dashed line in G shows cortical bone. Bars, 1 mm (B,D,H,L,P); 50 μm (C,E–G,I–K,M–O,Q,R).