Morphological and histological examination at 24 and 52 weeks. Gross appearance, micro-CT scanning, and safranin O staining at (a–e) 24 weeks and (f–j) 52 weeks. Micro-CT scans revealed complete regeneration of the subchondral bone. Histological examination revealed that the regenerated cartilage was maintained with a consistent thickness identical to that of the surrounding original cartilage at both (d) 24 weeks and (i) 52 weeks. Scale bar = 1 mm. (e, j) Tidemarks, which separate hyaline cartilage from the calcified cartilage, were seen (arrows). Scale bar = 100 μm. (k–o) A control rabbit without any implantation at 52 weeks. The defect was mainly filled with fibrous tissue that did not exhibit formation of subchondral bone (m) or sufficient staining with safranin O (n). Although some area was stained with safranin O, tidemark was not formed (o). Downwards arrows indicate the borders of the defect. Representative subjects of three experiments for each time course are shown.