A morphological study of the circadian cycle of the pineal gland of the rat.

To seek a morphological expression of circadian rhythm, we investigated cytologically pineal glands taken from rats every 2 to 4 h under a lighting regime of 12 h of illumination (6:00 to 18:00) and 12h of darkness. The changes in the number of synaptic ribbons and ribbon fields was observed by electron microscopy. The number of these intracellular elements was greatest at 2:00 and lowest at 14:00, the difference being statistically significant. There were no significant local differences in numbers with respect to the part of the pineal gland examined. The data are similar to those of Vollrath from the pineal gland of a guinea pig, and seem to confirm a circadian function in the pineal gland in mammals.