SYNAPTIC RIBBONS OF THE GUINEA-PIG PINEAL-GLAND IN STERILE, PREGNANT AND FERTILE BUT NON-PREGNANT FEMALES AND IN REPRODUCTIVELY ACTIVE MALES

Pinealocyte ultrastructure was studied in 4 sterile, 4 pregnant and 5 fertile but non-pregnant females and in 3 reproductively active male pigmented Duncan Hartley guinea pigs. Synaptic ribbons were dense, rod-like structures with a linear arrangement of clear vesicles periodically spaced on both sides of the rodlet. Although these structures were observed in the pinealocytes of all of the animals studied they were scarce and difficult to locate in tissue from the fertile, non-pregnant females and from the reproductively active males. They were numerous in the pineal glands of the pregnant and sterile females. Typically they lay perpendicular to the cell membrane of the pinealocyte polar process and in close proximity to a polar process of a neighboring cell. The increased incidence of synaptic ribbons in pinealocytes which appeared to be in a heightened state of activity strongly suggested a function for this structure. Synaptic ribbons were present in sensory systems such as rods and cones of the retina, hair cells of the organ of Corti and hair cells of the vestibular apparatus. This fact, plus the photoreceptor function of pinealocytes in lower vertebrates, lends credence to the possibility that this structure may serve a sensory or receptor function in the guinea pig gland.