In situ hybridization for CCL-9 mRNA during CSF-1 time course, proximal tibia of tl rat. No CCL-9-expressing cells were visible at day 0 (A). CCL-9-positive cells are indicated in panels B-D with arrows. Many small cells were detected at day 2 (B,E), and larger osteoclasts were seen at days 4 (C,F) and 6 (D,G). Higher magnification revealed cytological features of the CCL-9-expressing cells. Arrows in panels F and G indicate some individual nuclei. The cells seen at day 2 were typically smaller, mononuclear, and less differentiated, while at days 4 and 6, osteoclasts generally contained multiple nuclei and were better differentiated. The CCL-9 mRNA is diffusely distributed throughout the cytoplasm in the differentiating mononuclear pre-osteoclasts in panel E, but appears highly concentrated in the ruffled border area in days 4 (F) and 6 (G). *Bone trabeculae. Original magnifications: × 100 (A-D); × 630 (E-G). No counterstain was used.