Osteoclasts are multinucleated bone-resorbing cells with a dynamic actin
cytoskeleton. Osteoclasts are derived from circulating mononuclear
precursors. Confocal and stimulated emission depletion (STED)
super-resolution microscopy was used to investigate peripheral
blood-derived human osteoclasts cultured on glass surfaces. STED and
confocal microscopy demonstrated that the actin was curved and branched,
for instance, in the vicinity of membrane ruffles. The overall
architecture of the curved actin network extended from the podosomes to
the top of the cell. The other novel finding was that a micrometer-level
tube containing actin bridged the osteoclasts well above the level of
the culture glass. The actin filaments of the tubes originated from the
network of curved actin often surrounding a group of nuclei.
Furthermore, nuclei were occasionally located inside the tubes. Our
findings demonstrated the accumulation of c-Src, cortactin, cofilin, and
actin around nuclei suggesting their role in nuclear processes such as
the locomotion of nuclei. ARP2/3 labeling was abundant at the substratum
level of osteoclasts and in the branched actin network, where it
localized to the branching points. We speculate that the
actin-containing tubes of osteoclasts may provide a means of
transportation of nuclei, e.g., during the fusion of osteoclasts. These
novel findings can pave the way for future studies aiming at the
elucidation of the differentiation of multinucleated osteoclasts.