Clinical Connections

New bone formation is a key feature of ankylosing spondylitis (AS). Previous histologic studies of joint material from AS patients suggested that a fibroblast-rich granulation tissue erodes the subchondral bone plate, which separates the bone marrow from the cartilage. This process is driven by osteoclasts. In an immunohistologic study of facet joints from AS patients, Bleil et al analyze whether this granulation tissue also has bone-forming capabilities. They show cells lining the granulation tissue, which express RUNX-2, CD56, and most importantly, type I collagen, identifying them as osteoblasts. No evidence of chondrocyte hypertrophy is seen. At contact zones between granulation tissue and cartilage, bony regions are observed, suggesting that successive bone generation at these sites promotes the bony bridging of the facet joints that is typical of AS. KEY POINTS