Histologic examination of representative spine joints from wild-type versus ank/ank mice. (A) Vertebra from an 8-week-old wild type mouse. V, EC, AF, and IVD denote vertebra, endplate cartilage, annulus fibrosus, and intervertebral disc, respectively. (B) Vertebra from an 8-week-old ank/ank mouse. Thick arrow shows calcium apatite crystals at the edge of the vertebral bodies. Thin arrow shows hypertrophic chondrocyte-like cells replacing the annulus fibrosus and extending into the intervertebral disc. (C) Vertebra from a 12-wk-old ank/ank mouse. Thin arrow shows a large amount of crystal deposits. Thick arrow shows early syndesmophyte formation at the edge of the vertebral bodies. (D) Vertebra from an 18-week-old ank/ank mouse. Thick arrow shows ankylosis with fibrous connective tissue adjacent to some incomplete bony bridging. Thin arrow shows the intervertebral disc at this stage, and it was replaced by proliferative fibroblasts and large chondrocytic cells.