A comparison of the ultrastructure and opsin immunocytochemistry of the pineal organ and retina of the deep-sea fish Chimaera monstrosa.

The pineal organ and retina of the rat-fish Chimaera monstrosa were compared by electron microscopy and immunocytochemistry using antisera against colour-specific opsins and paying special attention to pineal CSF-contacting neurons and retinal Landolt's clubs. In the retina, a large number of Landolt's clubs and two types of rod-like photoreceptors were found. The outer segments of the numerous electron-dense "tall rods" displayed strong immunoreactivity with the monoclonal OS-2 antibodies--first of all detecting green- and blue-sensitive pigments. These results point out the presence of a chrysopsin-like photopigment. A weak cross-reactivity with the COS-1 and rhodopsin antisera indicates that the photopigment in question has certain amino acid sequence homologies with red and green photopigments. The outer segments of the few electron-lucent "broad rods" reacted with the OS-2 antiserum intensely but weakly with the COS-1 antiserum, a result suggesting the presence of a (blue?) photopigment differing from that of the tall rods. Since in the pineal organ the outer segments of the photoreceptor cells were opsin-immunonegative with all four antisera used, it is suggested that they contain an essentially different (UV-blue?) pigment. The pineal CSF-contacting neurons and retinal Landolt's bipolars were found to be principally similar in cytology. Their ciliated (receptor) dendrite terminals protruding into the photoreceptor space lacked photoreceptor membranes and were opsin-immunonegative. They are supposed to perceive information (on ionic properties?) from the fluid of the pineal lumen and retinal photoreceptor space. On the other hand, by their synaptic connections the CSF-contacting neurons and Landolt's bipolars are considered to be secondary neurons of the light-perceiving pathway of both organs.