Three types of photoreceptors in the pineal and frontal organs of frogs: ultrastructure and opsin immunoreactivity.

The pineal complex in frogs (Rana esculenta, R. temporaria, R. tigrina, R. arvalis) was studied by conventional electron microscopy and postembedding rhodopsin immunoelectron microscopy. Three types of photoreceptor cells were found in both the pineal and frontal organs. In the pineal organ, most of the photoreceptors exhibited rhodopsin-immunoreactive outer segments and large inner segments with a large ellipsoid of densely packed mitochondria ("rod-like" photoreceptors). A small number of photoreceptors was rhodopsin-immunonegative ("cone-like" photoreceptors). In both Rana esculenta and R. temporaria, the latter were either supplied with an oil droplet and an ellipsoid in their inner segment, or they were electron-lucent with a small inner segment without an ellipsoid. In contrast, the frontal organ displayed many immunonegative "cone-like" outer segments and few rhodopsin-immunoreactive "rod-like" photoreceptors. In both organs, the basal processes of the photoreceptor cells were found to form ribbon-containing axonal pedicles which synapsed with the dendrites of secondary neurons. The latter rarely received any further afferences by conventional synapses. The frog pineal organ is considered a predominantly "rod-type" and the frontal organ a "cone-type" photosensory organ. The presence of three kinds of pineal/frontal photoreceptors is discussed in connection with the occurrence of different photopigments (rhodopsin/porphyropsin, iodopsin, ultraviolet and/or blue pigments) enabling the animal to discriminate by the pineal complex environmental light in various ranges of the spectrum.