Mitotic activity and its 24-hour rhythm in the rat pineal gland

Previous studies have yielded equivocal results concerning the 24-hour rhythmicity of mitotic activity in the rat pineal. The aim of the present study was to re-investigate this problem by carrying out three separate 24-hour experiments on alternate days. The results obtained confirm previous findings showing that in the pineal gland of adults mitotic activity is low. On average 22.3 mitotic figures of pinealocytes are seen per pineal gland, corresponding to a mitotic index of 0.2-0.6/1,000 pinealocytes. Mitotic activity is distinctly higher at daytime than at night. The timing of the peaks and troughs differs slightly from experiment to experiment. The majority of observations now indicate that in the rat pineal gland mitotic activity is higher at day time than at night.