Purpose: Diurnal variation is an essential feature of bone physiology. However, characteristics of this phenomenon were not fully understood in the bone field. The purpose of this study was to investigate diurnal variations in the matrix-synthetic and secretory activities of cartilage cells and osteoblasts in various bone-forming sites in growing rats by radioautography with 3H-proline and 3H-galactose. Materials and methods: After acclimatization to a 12/12h light/dark cycle for 2 weeks, male Wistar rats were divided into six groups and injected with the tracers at 00:00, 04:00, 08:00, 12:00, 16:00 or 20:00, respectively. One hour after the injection, the animals were sacrificed, and femora, mandibles and skull were removed and processed for radioautography. Femoral epiphysis, spheno-occipital synchondrosis and sagittal suture were examined for the 3H-proline-injected animals. Femoral epiphysis and mandibular condyle were examined for the 3H-galactose-injected animals. Results: In all area examined, both total number of silver grains and matrix/cell ratio for both cell types varied significantly over a 24-h period, showing essentially the same pattern of diurnal fluctuation with a peak at 13:00 and a nadir at 01:00. These diurnal rhythms revealed considerable excursions ranging from 138.5% to 240.6%. Conclusion: Our results clearly demonstrated that in growing rats, the matrix-synthetic and secretory activities of cartilage cells and osteoblasts in various bone-forming sites followed almost the same diurnal rhythm with a peak in the middle of the light period and a nadir in the middle of the dark period.
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