Fig. 8. Examination of Mmp9^–/–, Mmp13^–/– and Mmp9^–/–;Mmp13^–/– endochondral bones. (A) Safranin-O staining of 2-week-old tibiae of wild-type, Mmp9^–/–, Mmp13^–/– and Mmp9^–/–;Mmp13^–/– mice. Mmp9^–/– and Mmp13^–/– tibiae have increased zones of hypertrophic cartilage, but a dramatically expanded hypertrophic chondrocyte zone can be observed in Mmp9^–/–;Mmp13^–/– tibia (compare lengths indicated by black bars). Magnifications of secondary ossification sites (insets) of Safranin-O stained 2-week-old tibia of wild type, Mmp9^–/–, Mmp13^–/– and Mmp9^–/–;Mmp13^–/– mice demonstrate delay in initiation of ossification at the secondary site in Mmp9^–/–;Mmp13^–/–. Collagenous bone tissue and colonizing blood cells in wild-type, Mmp9^–/– and Mmp13^–/– secondary sites are indicated by arrows and arrowheads, respectively. (B) Safranin-O staining of 5-week-old and 5-month-old metatarsals from Mmp9^–/–;Mmp13^–/– mice showing that while 5-week-old Mmp9^–/–;Mmp13^–/– metatarsals still display the expanded hypertrophic chondrocyte zone characteristic of this genotype, 5-month-old Mmp9^–/–;Mmp13^–/– metatarsals display a closed growth plate, indicating recovery. (C) Whole skeletal preparations of hind limbs from 12-month-old wild-type and Mmp9^–/–;Mmp13^–/– mice shows a shortening of all elements of the Mmp9^–/–;Mmp13^–/– hind limb when compared with wild type. Scale bars: 300 µm in A; 200 µm in B.