Recruitment of osteoclasts in the mandibular condyle of growing osteopetrotic (op/op) mice after a single injection of macrophage colony-stimulating factor.

The purpose was to elucidate histological changes in the mandibular condyle and ramus in growing osteopetrotic (op/op) mice after a single injection of macrophage colony-stimulating factor (M-CSF). M-CSF (5 microg) was injected into 6-, 11-, 26-, 56- and 86-day-old op/op mice, and the mice were killed 4 days after the injection. In normal mice, the condyle was substantially wider than the ramus beneath it, and enlargement and ossification of the condyle occurred after weaning. These changes were not found in the uninjected and injected op/op mice, the condyles of which were occupied by hypertrophic cartilage cells, and the hypertrophic cell layer was thicker and more irregular in the arrangement of epiphyseal cell columns. In spite of the lack of bone resorption in uninjected and injected op/ op mice, ossification of the mandibular ramus occurred, but later than that of normal mouse. The number of tartrate-resistant acid phosphatase-positive cells in the injected op/op and normal mice approached a maximum at 30 days and then gradually decreased up to 90 days of age, although the numbers were substantially different for all ages. The uninjected op/op mice had no visible osteoclasts until 15 days and their number then increased significantly from 60 to 90 days of age. These results were considered due to the difference in biological responses of bony structures to M-CSF injection in the op/op mice. The influences of mechanical stimuli from masticatory functions, which are deficient in op/op mice, might also be responsible for the differences in bony architecture between the op/op and normal mice.