Since the work of DODT1), the photosensitivity of pineal organ of fish has been detected electrophysiologically in some fishes2-5). Furthermore, some structural evidences for the photosensory function are presented by the electron microscopic studies on piscine pineal organs.6-10) However, only a few species of fishes were used in these studies.On the other hand, some observations had been made on the pineal window of deep sea fishes and migratory ones such as tunas and scombrids, taking phototaxis and color change of fish into account11-14). But no detailed histological informations are available on the pineal organ in various species of fishes.The present report deals with histological investigations on the pineal organ of 15 species of teleosts, as the first step in the studies on the pineal organ of fishes. Materials and MethodsThe species used in the present examination are shown in Table 1. Using living fishes, firstly macroscopic observations were made on the pineal window which is found in the dorsal head skin between the larteral eyes. Then fishes were decapitated and entire heads were fixed in 10% neutral formalin, ZENKER-formol, BoulN's or ELFTMAN'S fluids for histological investigation. After fixing for 24-48 hours, the specimens were decal cified by means of electric decalcification, dehydrated in alcohol, and embedded in paraffin.

Ultrastructural changes of the pineal organ were investigated in the blind cave fish, Astyanax mexicanus, kept under continous artificial light (5000 lux), in continuous darkness, and under natural light conditions. The pineal end-vesicle of the fish kept under natural photoperiod consisted of photoreceptor cells and supporting cells mixed with a few ganglion cells. The photoreceptor cells possessed well-developed outer segments with regularly arranged lamellar membranes. The supporting cells contained a number of lipid droplets and large globular cisternae filled with fine granules. In the fish kept under continuous light or in darkness, the pineal end-vesicle displayed a dilated lumen, and the outer segments of the receptors showed signs of degeneration. Furthermore, alterations of cell organelles were observed in the photoreceptor and supporting cells.

No abstract

The pineal organ of the chicken was investigated electron microscopically during embryonic and post-hatiching development with special regard to photosensory and secretory features. Throughout the developmental period both pinealocytes and supporting cells, of which the pineal parenchyma is composed, were rich in ribosomes, granular endoplasmic reticulum and mitochondria, but lacked agranular endoplasmic reticulum. The outer segments of pinealocytes barely showed formation of lamellar structures (disks) at the 17th and 21st day of incubation. Before and after hatching the follicular lumen was ofter filled with amorphous material presumed to be derived from outer or inner segments. By 15 days after hatching thw whorl-like structures were occasionally connected to bulbous outer segments, and their relation appeared similar to that of the adult. Mitochondria disappeared from the inner segments after 21 days of incubation. Dense core vesicles (about 80-120 nm in diameter), regarded as secretory granules, appeared first at the 10th day of incubation in the supranuclear region of the pinealocyte. With the extending of basal processes the dense core vesicles gradually migrated into these processes, attained maximum number one month after hatching and increased further in the adult; they are located both around the Golgi apparatus and in the basal process. These results provide evidence that secretory activity is maintained from embryonic stages to adulthood.

Light microscopic immunolocalization of taurine, a sulfur-containing free amino acid, was investigated in the developing retina of a lefteye flounder, Paralichthys olivaceus, which exhibits metamorphic changes with rod cell addition for 3-5 weeks after hatching. This immunocytochemical study of the developing retina revealed: 1) From 3 to 13 days after hatching, intense immunostaining was shifted from the surroundings of neural cells to the neural somata and processes in the inner retina. 2) Intense immunoreactivity appeared also in the outer and inner segments and basal processes (pedicles) of cone cells within 6 days or 13 days after hatching. 3) Lack of immunoreactivity was found in the outer segment of rod cells from their appearance during metamorphosis. These findings are discussed with the possible functional roles of taurine in the fish retina: 1) involvement in cell differentiation and/or development; 2) protection of the outer segments against light stimuli; and 3) regulation of neural transmission.