TRAP-positive osteoclasts in tl rats in response to CSF-1 injections. (A) TRAP staining of proximal tibial metaphysis reveals few osteoclasts by day 2 (red, arrows), and well-established populations at days 4 and 6 (images were obtained with a Zeiss Axioskop 2 Plus microscope with a 20 ×/0.5 NA brightfield objective). None were seen at day 0 (not shown). (B) Cell counts were performed in a window of defined size in 2 areas, area A near the chondro-osseous junction, and area B, in the lower metaphysis, in replicate slides of multiple animals (n = 24-36; bars show mean + 1 SD).