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Abstract

Rabbit osteoclasts and rabbit osteoblast-like stroma cells (OB cells) were plated onto plastic surfaces and the migration patterns of individual osteoclasts and osteoclast-OB interactions were analyzed with time-lapse recording. To induce directed migration, the cultures were exposed to an electrical field of 0.01 or 0.1 V/mm. At 0.1 V/mm, osteoclasts moved directly toward the anode in some cases, clearing OB cells from their path of migration. In other cases, osteoclasts migrated toward the anode for part of the time but then changed direction and moved toward groups of OB cells. Observations were made on osteoclasts interacting with single OB cells or small colonies and on osteoclasts interacting with OB monolayers, at both field strengths; the results were independent of field strength. There were several characteristic behaviors. With single OB cells and small OB colonies, retraction of OB cells upon contact with the osteoclast was the predominant mechanism whereby these cells begin to move out of the path of the osteoclast. A pronounced ruffling or blebbing of the OB cell membrane often followed retraction. When osteoclasts displaced OB cells that were part of a monolayer, extension of an osteoclast lamellipodium underneath the edge of the OB cell layer generally preceded partial retraction of the OB cells involved. It sometimes appeared as if the detached or partially detached OB cells were “pushed” by the osteoclast, which in some cases resulted in OB cells being moved hundreds of μm in a period of a few hours, at rates comparable to the normal speed for osteoclast migration (⋍100 μm/h), much faster than the normal speed for OB cells (⋍10 μm/h). These results imply that osteoclasts have mechanisms to move OB cells out of their path of migration. Further, when these cells were plated onto devitalized bone slices, resorbing osteoclasts apparently were able to clear a path through the OB cells. Laser scanning fluorescent measurements showed that there was no general intracellular [Ca2+] increase in OB cells following contact with osteoclasts, suggesting that an intracellular [Ca2+] signaling cascade does not play a role in the OB displacement. These data support the hypothesis that osteoclasts obtain access to the bone surface by actively displacing osteoblasts or lining cells at sites earmarked for resorption.