LOCALIZATION OF COLLAGENS AND ALKALINE-PHOSPHATASE ACTIVITY DURING MINERALIZATION AND OSSIFICATION OF HUMAN FIRST RIB CARTILAGE

The localization of type X collagen and alkaline phosphatase activity was examined in order to gain a better understanding of tissue remodel ling during development of human first rib cartilage. First rib cartil ages from children and adolescents showed no staining for type X colla gen and alkaline phosphatase activity. After onset of mineralization i n the late second decade, a peripheral ossification process preceded b y mineralized fibrocartilage could be distinguished from a more centra l one preceded by mineralized hyaline cartilage. No immunostaining for type X collagen was found in either type of cartilage. However, stron g staining for alkaline phosphatase activity was detected around chond rocyte-like cells within fibrocartilage adjacent to the peripheral min eralization front, while a weaker staining pattern was observed around chondrocytes of hyaline cartilage near the central mineralization fro nt. In addition, the territorial matrix of some chondrocytes within th e hyaline cartilage revealed staining for type I collagen, suggesting that these cells undergo a dedifferentiation process, which leads to a switch from type II to type I collagen synthesis. The study provides evidence that mineralization of the hyaline cartilage areas in human f irst rib cartilage occurs in the absence of type X collagen synthesis but in the presence of alkaline phosphatase. Thus, mineralization of f irst rib cartilage seems to follow a different pattern from endochondr al ossification in epiphyseal discs.