LOCALIZATION OF TYPE-I, TYPE-II AND TYPE-X COLLAGEN AND OSTEOCALCIN IN INTRAMEMBRANOUS, ENDOCHONDRAL AND CHONDROID BONE OF RATS

Chondroid bone is a unique calcified tissue intermediate between bone and cartilage. To clarify its characteristics, we examined the distrib utions of the ECMs associated with chondrogenic differentiation and ma trix calcification in the chondroid bone of the rat glenoid fossa, and compared them to those in two typical bone tissues, alveolar bone of the maxilla (intramembranous bone) and the growth plate of long bone ( endochrondral bone), using immunofluorescence techniques. Morphologica lly, the glenoid fossa consisted of the fibrous, progenitor and cartil aginous cell layers and the cartilaginous cell layer was further divid ed into the superficial non-hypertrophic layers (secondary cartilage) and the deep hypertrophic cell layers (chondroid bone). The co-distrib ution of type I and type II collagens was observed in secondary cartil age and chondroid bone, whereas type X collagen was restricted to the pericellular matrix of hypertrophied cells (chondroid bone). Osteocalc in, which was absent from the calcified cartilage of endochondral bone formation, was also present in the ECM of the chondroid bone, but not in cells. These results demonstrate that chondroid bone of rats, whic h is adjacent to secondary-type cartilage in the glenoid fossa, has ph enotypic expressions associated with both hypertrophied chondrocytes a nd osteocytes.