OBJECTIVETo study the application of the live cell imaging method to observe the whole process of osteoclast formation induced by monocyte macrophages in the blood system in order to clarify the origin of osteoclasts and their cytodynamics.

METHODSBlood samples (8 ml) were collected from the abdominal aorta of male SD rats weighing 280 g. Mononuclear cells were obtained by density gradient centrifugation and induced by RANKL and M-CSF. The cells were cultured and divided into four groups: inverted phase contrast microscope (IPCM) group, TRAP group, SEM group and live cell imaging (LCI) group. Images of the IPCM group were captured by a digital microscopic imaging system and recorded daily. The TRAP group was identified by enzyme activity staining after a 21-day cultivation period. The SEM group was SEM-observed after a 21-day cultivation period. The LCI group was consecutively and dynamically observed for 35 days.

RESULTSAfter 2-week cultivation, IPCM observations showed the formation of numerous apocytes. These cells displayed round, fusiform, fan-shaped, elliptic or irregular gibbous profiles. TRAP staining showed that most apocytes and monocytes had positive(+)reaction. SEM observations showed many bone absorption lacunae, hollows and channels, in which many osteoclasts with absorption activity were observed. Live cell imaging observations found that multinuclear osteoclasts originating from peripheral blood were generated by fusion of monocytes and apocytes and intercross fusion of monocytes and apocytes,which occurred at the adherent stage of the cells. Cytodynamic observations showed that the cell form of osteoclasts was complex and changeable.

CONCLUSIONRANKL and M-CSF can induce differentiation and formation from monocytes in rat peripheral blood into multinuclear osteoclasts with bone absorption activity. The osteoclasts were formed by various cell fusion processes at the adherent stage. The adherent property of osteoclasts is important for their survival and function. Osteoclasts have phagocytosis and their morphological structure is dynamically changeable, involving not only apocytes but monocytes. The osteoclast property of multinuclear giant cells formed by cell fusion may be a special biological behavior for their adaptation of functional needs and bone absorption efficiency. This experiment has further evidenced the theory of osteoclast origination in the blood system and provided new experimental clues for clarifying the cytodynamic and cytobiological properties of osteoclasts.