A functional and morphological study of cells adjacent to ectopic bone implants in rats

Subcutaneous implantation of bone chips into normal and osteopetrotic (ia) rats results in the formation of multinucleate giant cells (MNGCs) adjacent to the bone surface. In this study the resorptive and morphological characteristics of the cells surrounding these implants were assessed to determine if the bone-resorbing defects seen in ia animals would be mimicked, thus giving validity to the use of this system as a model for the study of osteoclastic lineage and function. Direct measurement of in vivo bone resorption was achieved through the use of (45)Ca-labelled bone-chip pairs that were primarily osteoid-exposed and freeze-thawed (FT), primarily mineral-exposed and bleached (B), or primarily mineral-exposed and collagenase-treated (CT). Comparison of the (45)Ca content of implanted chips to that of controls indicated the total (45)Ca release during a two-week implantation period. There was no significant difference in the amount of label released between normal and ia animals. Both normal and ia rats showed 23% greater total (45)Ca release from mineral- versus osteoid-exposed matrix. Cellular events occurring on the bony substrate were evaluated by light and electron microscopy. At 3 days, bone chips were surrounded primarily by mononuclear cells. By 14 days, MNGCs were present at the bone surface in both ia and normal animals. In mineral-exposed implants, 40-50% of the bone surface was covered by MNGCs as compared to 20% of the osteoid-exposed surface. These MNGCs possessed occasional clear zones, but did not exhibit ruffled borders; therefore, they could not be classified as osteoclasts. Thus, the defects seen in ia mutants were not reproduced in this implant system. The (45)Ca release that occurred was probably due to the action of mononuclear phagocytes and macrophage polykaryons rather than to true osteoclastic bone resorption.