ELECTRON MICROSCOPIC STUDIES ON THE UPTAKE AND STORAGE OF THORIUM DIOXIDE MOLECULES IN DIFFERENT CELL TYPES OF FRACTURE CALLUS

The pattern of uptake and storage of exogenous, electron dense macromolecules—thorium dioxide particles—was studied in an attempt to classify and elucidate the functional significance of the different cell types observed during formation and remodeling of fracture callus in the rat. Two major groups of cells were distinguished on the basis of ability for endo‐cytosis and storage: (a) Those mainly specialized for phagocytosis (monocytes, histiocytes, macrophages and osteoclasts) and, in addition, immature fibroblasts; and ( b ) fibroblasts, chondroblasts, chondrocytes, osteoblasts and osteocytes (known to participate in the elaboration of extracellular collagenous material). The latter group of cells may—in addition to their bone forming capability—also have limited degradative functions since they are able to perform heterophagy. Occurrence—in monocytes, histiocytes, macrophages, and immature fibroblasts—of collagen‐like fibers in thorium dioxide‐containing lysosome‐like bodies suggests a role of these cells in the remodeling of the callus and the resorption of the matrix. In all the different cell types of the callus, marker molecules were stored for long periods of time indicating lacking or low ability for exocytosis of lysosomal content, and stability and longevity of the lysosomes. The findings suggested that uncoated Golgi‐associated vesicles in the different osteogenic and phagocytic cells of the callus were derived from the plasma membrane (and represented endocytotic vesicles) while coated “Golgi vesicles” might represent primary lysosomes.