Objective To investigate the formation and special cell biological behavior of osteoclasts.Methods The live-cell imaging technology was adopted to consecutively and dynamically observe the whole process of multinuclear osteoclast formation induced by RANKL and M-CSF from rat peripheral blood monocyte.Meanwhile,the inverted phase contrast microscopy,TRAP staining,and scanning electron microscopy were also applied to identify the osteoclast.Results After 2-week cultivation,a great number of apocytes were found by the inverted phase contrast microscopy,and most apocytes and monocytes had positive reaction after TRAP staining.Moreover,many bone resorption lacunae in which osteoclasts were perhrming bone resorption function could be found in the bone slice under the scanning electron microscope.Live-cell imaging observation showed that the multinuclear osteoclasts were generated through self-fusion of monocytes,fusion of monocytes and apocytes,as well as fusion between apocytes.All fusion processes occurred under the condition of cell adherence.Time-lapse Microcinematography observation showed diverse shapes of osteoclasts and the cell division of multinuclear osteoclasts.Conclusion Rat peripheral blood monocyte can develop into osteoclast under induction of RANKL and M-CSF.Osteoclast can form gigantic apocyte via various types of cell fusion to increase its nucleus number and cell volume,vary its shape,and increase the area of plasma membrane.On the other hand,osteoclast can decrease its cell volume and nucleus number via cell division to adapt the needs of local morphology,biomechanics and bone resorption dynamics.It suggests that this non-mitosis cell division is a special cell biological behavior of osteoclast,which may be the basis of exerting its function and improving bone resorption efficiency.