Intravenous injections of colloidal thorium dioxide were made in rabbits and moderate amounts caused no significant changes in the cellular elements of venous blood. Thorium dioxide was held locally with great tenacity in the cells of the reticulo-endothelial system in the bone marrow and showed little tendency to migrate despite drastic stimuli to the marrow in the form of anemia and plethora.
In recently injected rabbits, thorium-laden macrophages abutted each cartilage column in the direction of growth where the cartilage was being removed. In marrow which had formed subsequent to injection the macrophages were thorium-free, allowing recognition of the new tissue by x-ray and histological techniques as clear zones. The growth pattern of marrow could be detected in this way.
The bone marrow increases in length principally in the region of hypertrophic cartilage at the metaphyses and it is evident that the increase is facilitated by the presence of macrophages whose primary function is the resolution of the wasted hypertrophic cartilage cells. Growth in thickness of bone marrow occurs at its circumference. In contrast to the zonal growth of bone marrow, growth of the reticuloendothelial system in liver and spleen is chiefly interstitial.