Diurnal rhythms in the collagen-synthetic activities of cartilage cells and osteoblasts in the rat mandibular condyle.
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Diurnal variations in the collagen-synthetic activities of cartilage cells and osteoblasts in the mandibular condyle were determined in growing rats by means of autoradiography with 3H-proline. Male Wistar rats that had been adapted to a 12/12 hr light/dark illumination program (with light from 07: 00 to 19: 00) were divided into six groups, and injected intraperitoneally with <SUP>3</SUP>H proline at 00: 00, 04: 00, 08: 00, 12: 00, 16: 00, or 20: 00 respectively. One hour later, they were sacrificed and the mandibles were dissected and fixed in a fixative.<BR>After decalcifying and embedding the specimens in paraffin, autoradiograms of frontal sectionsof condyle (7-μm thick) were prepared. Condylar cartilage was histologically classified into five layers: fibrous, proliferative, transitional, mature, and hypertrophic. The number of autoradiographic grains per unit area was counted over each cartillage cell layer, and over trabecular bone tissue near the cartilage-bone boundary.<BR>The results showed that total incorporation of <SUP>3</SUP>H-proline into each cartilage cell layer followed a distinct diurnal rhythm with a peak at 13: 00 and a nadir at 1: 00. To further assess the diurnal variation in the secretory activities of hypertrophic cartilage cells and osteoblasts, the number of grains over the cells and those over extracellular matrix were counted. The matrix/cell ratio (M/C ratio) showed essentially the same diurnal phasing pattern as in <SUP>3</SUP>H-proline incorporation in both types of cells.<BR>In summary, the present study clearly demonstrated that the collagen-synthetic activities and secretory activities of cartilage cells and osteoblasts in rat mandibular condyle fluctuate according toa diurnal rhythm, with a peak in the middle of the light period (resting phase of the animal) and a nadir in the middle of the dark period (active Phase of the animal).
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