Increased proliferation in osteoclast precursors derived from Krox20+/− mice. (A) Splenocytes from WT and Krox20+/− newborn mice were cultured for 3 days with 100 ng/mL M-CSF. Krox20 and Krox24 mRNA expression in subconfluent preosteoclasts cultures was assessed by RT-qPCR and corrected for the expression of rpL10A. (B) Spleen-derived preosteoclasts as in panel A were plated in 96-well plates at 2000 cells per well and cultured with 100 ng/mL M-CSF. Medium was changed on days 3 and 5, and proliferation was determined after 7 days using MTT assay. (C) Splenocytes from WT and Krox20+/− newborn mice were plated in 96-well plates at 150 000 cells per well and cultured from the first day with 20 ng/mL M-CSF and 100 ng/mL RANKL. Medium was changed on days 3 and 5, and TRAP staining was performed on day 8. Bars represent relative TRAP+ area. (D-E) Spleen-derived preosteoclasts pretreated with 100 ng/mL M-CSF alone as in panel A were plated in 96-well plates at 5000 cells per well and cultured with 20 ng/mL M-CSF and 100 ng/mL RANKL for 7 days. Medium was changed on days 3 and 5. Bars represent relative TRAP+ area. (E) Representative images of TRAP-positive osteoclasts derived from WT (left) and Krox20+/− (right) mice; bar = 0.5 mm. (F) Krox20 and Krox24 mRNA in primary calvarial osteoblastic cultures from Krox20+/− and control mice was assessed by RT-qPCR and corrected for the expression of rpL10A. (G-H) Primary calvarial osteoblast cultures from WT and Krox20+/− mice were fixed and stained with alizarin red 20 days after confluence. Bars represent percent alizarin red-positive area. (H) Representative images from WT- (left) and Krox20+/−-derived (right) cultures. (I-J) Bone marrow–derived cultures from WT and Krox20+/− mice. Number of alizarin red-positive CFU-Ob colonies (I) and mean colony size (J) were determined in day-20 cultures after fixation and staining. Bars represent mean ± SEM of Krox20+/+ (white) and Krox20+/− (gray) cells. *P < .05 compared with control littermates. Assays were performed in triplicate per animal, and the number of animals is specified in parentheses inside each bar.