A quantitative ultrastructural study of the pinealocyte of the chipmunk (Tamias striatus) during the daytime and at night

Day‐night differences in the areas of granular endoplasmic reticulum, Golgi apparatus, dense‐core vesicles, and lysosomelike bodies were analyzed morphometrically in the pinealocytes of the chipmunk. The area of lysosomelike bodies was greater at night and lower during the daytime. In contrast, the areas of granular endoplasmic reticulum and dense‐core vesicles were greater during daytime and lower at night. Our observations suggest the existence of 24‐hour rhythms in the areas of these structures, perhaps indicative of a pineal rhythm in secretion, whether indolic or peptidergic.