CHONDROCYTE MATURATION CYCLE AND ENDOCHONDRAL OSSIFICATION IN HUMAN FETAL CARTILAGE

The initial calcification and the progression of the mineral phase depositing in cartilage and bone matrix was studied in human, fetal cartilage Anlagen at a developmental stage when the calcified cartilage resorption had not yet started. The model allowed a morphological, comparative analysis of the calcification process in both cartilage and osteoid which represented the basic steps of endochondral calcification. The study was carried out with light microscopy and SEM-Edax (combined with hest deproteination and morphometry) on a sequence of thin sections, cut on the same paraffin inclusions. This allowed to analyzed corresponding fields with the different microscopy techniques. In cartilage focal, globular mineral deposits (randomly distributed) represented the initial phase of calcification which progressed with an eccentrical growth of the single deposit. With the increase of calcified globules size and of their density, aggregation led to the extensive mineralization of the inter-territorial matrix. Mineral deposition in osteoid below the periosteal osteoblasts occurred shortly after the collagen fibrils at thin extruded from the osteoblast cell membrane and packed in compact bundles. The fibrils themselves acted to orient and to compact the mineral deposits along a direction of the fibrils. This study documented the progression of the mineral deposition in both cartilage and osteoid and integrated the actual knowledge of the initial Calcium Phosphate nucleation in endochondral ossification.