ROLE OF CARTILAGE CANALS IN THE FORMATION OF SECONDARY CENTERS OF OSSIFICATION

  • 1 January 1979
    • journal article
    • research article
    • Vol. 129  (OCT) , 493-506
Abstract
Development of cartilage canals was studied in the rat and rabbit femur and humerus. Investigations were centered around the primitive connective tissue cells, which are always present between the blood vessels and are more numerous in newly formed canals. Serial araldite sections were examined by light microscopy to trace the blind end of extending canals, where large multinucleate cells were discovered. These cells gave a positive acid phosphatase reaction, and EM examination revealed that their cytoplasm was identical to that of osteoclasts. These cells were chondroclasts, their function being to remove cartilage matrix and thus extending canals. In the rat a massive proliferation of the connective tissue cells in the canals was observed as they reached the hypertrophied cartilage at the designated secondary center of ossification. Autoradiography showed .apprx. 30% of these cells to be labeled with 3H-thymidine 45 min after injection. EM examination revaled that while many of these cells were primitive and could be regarded as osteoprogenitors, some differentiated to become osteoblasts and others, at the canal periphery, became osteoclasts. When hypertrophied cartilage cells degenerated, demarcation between the canal and the surrounding matrix disappeared and cells within the canals populated the cartilage spaces, establishing the secondary center. Hypertrophied cartilage, or a substance secreted by it, may be responsible for the proliferation and differentiation of the connective tissue cells within the canal.