Objective To observe the morphological changes of cellular condensations from marrow stromal cells in calcium nodules, and to provide experimental data for further studying bone regeneration. Methods Rabbit marrow stromal cells were isolated through whole marrow culture method and amplified in flasks, using the osteoblast inducing conditional media served as the culture media. Morphological changes of cellular condensations in minerized nodules were investigated by tetracycline labelling, alkaline phosphatase staining（ALP）, alizarin red S staining（ARS）, scanning electron microscope（SEM）and transmission electron microscope（TEM）. Results Calcium nodules with cellular condensations revealed bright golden yellow fluorescence under fluorescence microscope when labelled with tetracycline. Histochemical study with ARS and ALP staining showed positive calcium reaction and positive alkaline phosphatase reaction compared with pheripheral spreading cells, respectively. Under SEM, condensed cells with spheral shape secreted abundant granules heaped up in the cell body. Rich matrix vesicles could be seen under TEM. Conc