24-hour-variation of pineal gland volume, pinealocyte nuclear volume and mitotic activity in male Sprague-Dawley rats.

In two experiments carried out on two alternate days, the 24-h-rhythmicity of pineal gland volume, pinealocyte nuclear size in cortex and medulla and mitotic activity were studied in male Sprague-Dawley rats, to assess to what extent morphological parameters reflect the pronounced day/night differences in pineal melatonin formation. Pineal volume exhibited statistically significant changes in the second experiment only, with a distinct trough at 6 p.m. Karyometry revealed highly variable patterns. In the first experiment, pinealocyte nuclear changes lacked parallelism in cortex and medulla. The cortex exhibited a bimodal curve with peaks at noon of the first day and at 6 a.m. of the second day, and two troughs at 6 a.m. and midnight respectively of the first day. The medulla showed no clear-cut rhythmicity. In the second experiment, cortex and medulla reacted similarly, nuclear size decreasing from 6 a.m. to 6 p.m., remaining low thereafter. Mitotic activity of pinealocytes is low (on average 23 mitotic figures/gland). In both experiments statistically significant differences existed between certain times, pointing in the direction of 24-h-rhythmicity, but whereas the curve exhibited a peak at midnight in the first experiment, mitotic activity in the second experiment showed a trough at midnight. It is concluded that for as yet unexplained reasons morphological parameters do not appear to accurately reflect circadian rhythmicity of pineal melatonin formation.