Figure 3

Cell proliferation. (A-F) Sections of the tibial articular cartilage of normal Mig-6-flox (A, C, E) and Mig-6-flox;Prx1Cre conditional knockout (Mig-6-cko) knees (B, D, F) immunostained with a Ki67 antibody (brown) and counterstained with hematoxylin. (A, C, E) Scattered proliferating cells (arrows in A) are present in the presumptive articular cartilage of normal Mig-6-flox tibia at postnatal Day 5 (indicated by bars) and in the superficial zones of normal Mig-6-flox tibial articular cartilage at 6 and 12 weeks (arrows in C, E), and rarely in the calcified zone (arrowhead, E). However, in Mig-6-cko knees, abundant proliferating cells are located in these regions (arrows in B, D, F), and the domain of robust proliferation is expanded as early as postnatal Day 5 (compare bars in A, B). In addition, proliferating cells are also detected in the middle zone at six weeks (arrowheads in D). (G) Cell counting shows that proliferating cells present in normal articular cartilage (closed circles) remain at a low and constant level from Day 5 to 12 weeks while proliferating cells present in Mig-6-cko articular cartilage (closed squares) are over three-fold more abundant at postnatal Day 5 (P <0.01) and four-fold more abundant at 6 and 12 weeks of age (P <0.01). Scale Bar = 50 μm (A-F).