RESUMO
OBJECTIVES: We examined mice with gene deletion of Receptor activator of nuclear factor-κB (Rank) ligand (Rankl) to histologically clarify whether they contained progenitor cells committed to osteoclastic differentiation up to the stage requiring RANK/RANKL signaling. METHODS: The tibiae and femora of ten-week-old male wild-type, c-fos-/-, and Rankl-/- mice were used for immunohistochemistry and transmission electron microscopy (TEM). RESULTS: In Rankl-/- mice, we observed osteoclast-like giant cells, albeit in low numbers, with single or two nuclei, engulfing the mineralized extracellular matrix. TEM revealed that these giant cells contained large numbers of mitochondria, vesicles/vacuoles, and clear zone-like structures but no ruffled borders. They often engulfed fragmented bony/cartilaginous components of the extracellular matrix that had been degraded. Additionally, osteoclast-like giant cells exhibited immunoreactivity for vacuolar H+-ATPase, galectin-3, and siglec-15 but not for tartrate-resistant acid phosphatase, cathepsin K, or MMP-9, all of which are classical hallmarks of osteoclasts. Furthermore, osteoclast-like giant cells were ephrinB2-positive as they were near EphB4-positive osteoblasts that are also positive for alkaline phosphatase and Runx2 in Rankl-/- mice. Unlike Rankl-/- mice, c-fos-/- mice lacking osteoclast progenitors and mature osteoclasts had no ephrinB2-positive osteoclast-like cells or alkaline phosphatase-positive/Runx2-reactive osteoblasts. This suggests that similar to authentic osteoclasts, osteoclast-like giant cells might have the potential to activate osteoblasts in Rankl-/- mice. CONCLUSIONS: It seems plausible that osteoclast-like giant cells may have acquired some osteoclastic traits and the ability to resorb mineralized matrices even when the absence of RANK/RANKL signaling halted the osteoclastic differentiation cascade.