Cartilage and bone formation in repairing Achilles tendons within diffusion chambers: evidence for tendon-cartilage and cartilage-bone conversion in vivo.

Rodent Achilles tendons were subjected to midpoint tenotomy and allowed to recover for various times in situ before the operated tissue was removed, placed into a Millipore diffusion chamber, and inserted intraperitoneally into syngeneic hosts. Diffusion chambers were then removed at weekly intervals, such that the total time after the operation (i.e., time allowed to recover in situ plus time within the diffusion chamber) was up to 8 weeks, and examined histologically. Ectopic cartilage was produced within the diffusion chamber after a total of 4 weeks but only if the first 2 weeks of recovery were in situ. With increasing time, calcified cartilage, osteoid, and bone were also observed. Overall, the evidence suggests that the cartilage forms via a direct conversion from tendon tissue and that the bone may form as a result of differentiative changes of hypertrophic chondrocytes.