The pineal organ as a folded retina: Immunocytochemical localization of opsins

The most simple pineal complex (the pineal and parapineal organs of lamprey s), consists of saccular evaginations of the diencephalic roof, and has a r etina-like structure containing photoreceptor cells and secondary neurons. In more differentiated vertebrates, the successive folding of the pineal wa ll multiplies the cells and reduces the lumen of the organ, but the pattern of the histological organization remains similar to that of lampreys; ther efore, we consider the histological structure of the pineal organ of higher vertebrates as a 'folded retina'. The cell membrane of several pineal phot oreceptor outer-segments of vertebrates immunoreact with anti-retinal opsin antibodies supporting the view of retina-like organization of the pineal. Some other pineal outer segments do not react with retinal anti-opsin antib odies, a result suggesting the presence of special pineal photopigments in different types of pinealocytes that obviously developed during evolution. The chicken pinopsin, detected in the last years, may represent one of thes e unknown photopigments. Using antibodies against chicken pinopsin, we comp ared the immunoreactivity of different photoreceptors of the pineal organs from cyclostomes to birds at the light and electron microscopic levels. We found pinopsin immunoreaction on all pinealocytes of birds and on the rhodo psin-negative large reptilian pinealocytes. As the pinopsin has an absorpti on maximum at 470 nm, these avian and reptilian immunoreactive pinealocytes can be regarded as green-blue light-sensitive photoreceptors. Only a weak immunoreaction was observed on the frog and fish pinealocytes and no reacti on was seen in cyclostomes and in the frontal organ of reptiles. Some photo receptors of the retina of various species also reacted the pinopsin antibo dies, therefore, pinopsin must have certain sequential similarity to indivi dual retinal opsins of some vertebrates. ((C) Elsevier, Paris).