[Electron-microscopic observations on pinealocytes in various regions of the pineal gland of young and adult mice].

Qualitative and quantitative electron-microscopic observations were made on pinealocytes in various regions of the superficial pineal gland of young (40 days old) and adult (100 days old) male dd-mice, which were kept under LD 12:12 and killed at various time points over a 24-h period. In young and adult mice, nuclear and cytoplasmic volumes of pinealocytes in the distal and middle regions were larger than those in the proximal region. Nuclear volumes in the distal region and nuclear and cytoplasmic volumes in the proximal region were similar between young and adult animals, whereas cytoplasmic volumes in the distal region and nuclear and cytoplasmic volumes in the middle region were larger in adults than in young animals. In the distal and middle regions, nuclear volumes in young animals and nuclear and cytoplasmic volumes in adults exhibited similar 24-h variations, but there was no significant 24-h rhythm in cytoplasmic volumes in young animals. Nuclear and cytoplasmic volumes in the proximal region were almost constant over a 24-h period in young and adult animals. In adults, nuclear and cytoplasmic volumes at various time points over a 24-h period were similar between central and peripheral areas of the middle region. Thus, regional differences in a proximodistal direction may exist in pinealocytes of young and adult mice, and that larger pinealocytes in the distal and middle regions may be well differentiated, but smaller cells in the proximal region may remain undifferentiated, in adult animals. Chromatin was usually dispersed in pinealocyte nuclei during the light period, whereas it was often condensed during the dark period. Such a rhythm in chromatin occurred in pinealocytes seen everywhere throughout the gland of young and adult mice. Thus, the rhythms in nuclear and cytoplasmic volumes and chromatin may be controlled by different mechanisms.