Apoptotic chondrocytes from osteoarthrotic human articular cartilage and abnormal calcification of subchondral bone

Osteoarthrosis (OA) is accompanied by altered subchondral bone remodeling. We investigated the role of chondrocytes in the mechanism of abnormal cartilage calcification.

Knee articular cartilage samples from OA and normal tissue were studied. Macroscopic and microscopic observations, alkaline phosphatase staining for light and electron microscopy (bright and dark fields). TUNEL technique, electron diffraction, and x-ray microanalyses were performed.

Chondrocytes from patients displayed a morphology of apoptosis and showed abundant alkaline phosphatase (ALP)-rich matrix vesicles (MV) budding from the plasma membrane with hydroxyapatite microcrystals on their surface. Farther from the cells, hydroxyapatite crystals were detected on the MV surface and increased as they approached the subchondral bone. The concentration of Ca and P and their ratio increased inside the ALP-rich MV in relation to the proximity to subchondral bone. In the OA subchondral bone the ratio Ca/P varied from 3.936 to 0.974. In normal tissue the ratio was very homogeneous (maximum 1.973, minimum 1.781).

In situ, apoptotic chondrocytes correlate with factors known to be involved in the calcification of the extracellular matrix. This suggests that apoptosis is involved in the abnormal calcification of OA cartilage, and consequently in the altered remodeling of the subchondral bone.