Bone resorption is achieved through the work of Osteoclasts. Osteoclasts are myeloidic cells closely related to macrophages. During differentiation some osteoclasts might undergo fusion to form a giant multi-nucleated cell (polykaryon). Although in recent years new proteins regulating osteoclast fusion have been discovered the cellular characteristics of osteoclast progenitors undergoing fusion is poorly defined. Using time lapse microscopy we show that in a given population of osteoclast progenitors some cells are more prone to fuse, assuming the role of fusion epicenters, which attract other, more passive, cells. Furthermore, while rate of fusion events involving polykaryons increases throughout the differentiation period, fusion events involving only mono-nucleated cells remains constant, suggesting a population of predisposed cells leading the fusion process (founders) and a population of cells being led (followers). In order to determine the distribution of founder cells we cultured osteoclast progenitors on micro-wells and scored for polykarions as cells with three or more nuclei. Our statistical analysis show that the fraction of founder cells in a given population of RAW264.7 differentiating osteoclasts is about 4% of the total progenitor population. Using a system we developed which allows us to track the origin of nuclei of two differently treated populations, we show that founder enriched RAW264.7 and primary osteoclast progenitor population (primed by 48 hours exposure to RANKL) can undergo fusion with cells which are not pretreated with RANKL, demonstrating that fusion is not directly linked to differentiation induced by the known cytokines. These results and observations suggest the existence of a founder-follower mechanism. Understanding this mechanism may provide essential insights regarding the physiological role of fusion in normal, developmental and pathological states of bone.
Disclosure: The authors declared no competing interests.