Association of an extracellular protein (chondrocalcin) with the calcification of cartilage in endochondral bone formation.

Abstract

Bovine fetal epiphyseal and growth plate cartilages were examined by immunofluorescence microscopy and immunoelectron microscopy using monospecific antibodies to a newly discovered cartilage-matrix Ca-binding protein that is now called chondrocalcin. Chondrocalcin was evenly distributed at relatively low concentration in resting fetal epiphyseal cartilage. In growth plate cartilage, it was absent from the extracellular matrix in the zone of proliferating chondrocytes but was present in intracelluar vacuoles in proliferating, maturing and upper hypertrophic chondrocytes. The protein then disappeared from the lower hypertrophic chondrocytes and appeared in the adjoining extracellular matrix, where it was selectively concentrated in the longitudinal septa in precisely the same location where amorphous mineral was deposited in large amounts as demonstrated by von Kossa staining and EM. Mineral then spread out from these nucleation sites to occupy much of the surrounding matrix. Matrix vesicles were identified in this calcifying matrix but they bore no observable morphological relationship to these major sites of calcification where chondrocalcin was concentrated. Since chondrocalcin is a calcium-binding protein and has a strong affinity for hydroxyapatite, chondrocalcin thus may play a fundamental role in the creation of nucleation sites for the calcification of cartilage matrix in endochondral bone formation.