Demonstration of circadian rhythm in granular vesicle number in pinealocytes of mice and the effect of light: semi-quantitative electron microscopic study.

Adult (70-100 days) male dd-mice were killed at 7 a.m., 1 p.m., 7 p.m. and 1 a.m. under controlled daily photoperiods (L: 7 a.m.-7 p.m.) and temperature, and at 6, 12 and 18 hours after exposure to continuous lighting which started at 7 p.m. Montage photographs were made from electron micrographs at x8200 based on mesh hole areas, using either 150 or 300 meshes. The number of granular vesicles per unit area was counted (GV Score). The GV Score in distal, mid and proximal portions of the body of the pineal gland showed marked 24 hour change, with high values in the late light period and low values in the dark period. The GV Score in the Golgi region showed a 24 hour change with a smaller amplitude. A peak level was attained at mid-light period and low values during a dark period. Additional lighting inhibited the nocturnal decrease in the GV Score in both the whole cytoplasm and the Golgi region, resulting in a slight increase. It is concluded that the number of granular vesicles in mouse pinealocytes (and in their Golgi regions) follows a 24 hour rhythm, and that the nocturnal decrease in the vesicle number depends upon darkness.