An organ culture system to model the physiological calcification process was designed using rat embryonic calvaria as a device for analyzing its mechanism. Standardized calvarial explants were dissected from rat embryos aged 18 and 20 days (E18 and E20) and cultured for 1, 3 and 5 days. The calcium content of the cultured explants was quantified by atomic absorption spectrophotometry. Equivalent explants were fixed, embedded in paraffin, sectioned and stained with von Kossa stain combined with hematoxylin-eosin or processed for energy-dispersive X-ray spectroscopy to determine the concentrations of calcium, phosphorus and carbon in the tissue. The total calcium content increased significantly in E18 and E20 cultured calvaria (E18cc and E20cc) over 5 days of culture. All cultured calvaria were von Kossa-positive, whereas the staining was intensified, and sound osteoblasts and osteocytes were observed in the bone matrix only in E18cc during the 5-day culture period. Concentrations of calcium and carbon increased significantly in E18cc over 5 days, whereas E20 showed little increase. Physiological calcification proceeded in E18cc, but not in E20cc. These results indicate that the organ culture system using E18 calvaria is useful for modeling the physiological calcification process in vitro.
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