Day-night changes in the ultrastructure of pinealocytes in the Syrian hamster: a quantitative study

The ultrastructure of pinealocytes was studied in Syrian hamsters maintained under 12:12 light: dark cycle, both during the daytime (1300h) and at night (0100h). At night the pinealocytes showed ultrastructural features indicating an increased metabolic and synthetic activity of these cells. It was expressed by higher relative volumes of mitochondria, lysosomes, lipid droplets, granular endoplasmic reticulum and vacuoles containing flocculent material at night in comparison with those during the daytime. By contrast, the number of the dense-core vesicles, both in the cell body and in the endings of pinealocyte processes was higher during the daytime than at night. Quantitative morphological results correlate with the well known nocturnal enhancement in biochemical activity of the pineal gland, especially the circadian rhythm of the secretion of the pineal hormone-melatonin.