Fig. 7

Necrotic cartilage canal. Cyan: Two-photon-excited fluorescence (TPEF), Glowing red: Second harmonic generation (SHG). a Cross-section of a cartilage canal containing degenerated blood vessels but with absence of chondrocytes in its lumen. The canal is therefore considered to be necrotic and not chondrifying. b At the site where the canal crosses the osteochondral junction, the canal appears to be disturbed by the ossification front, and a cone of cartilage persists around the canal into the ossification front. c-d Higher magnification of the regions indicated by dashed rectangles (only TPEF shown). Near the distal terminus, the canal contains remnants of blood vessels and fragments giving high TPEF signal, interpreted as debris or necrotic perivascular cells. More proximally to the arterial source, the canal is thicker and contains more complex tissue. However, the blood vessels and the perivascular tissue appear degenerated, and aggregating cells are present in the lumen of a vessel remnant (asterisk). The images were generated by a 25×–focusing objective. The bars correspond to 500 μm in a and b and are equal to 50 μm in c and d