The effects of constant light and constant darkness on daily changes in the morphology of the pineal organ in the goldfish,Carassius auratus

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Abstract
Summary The fine structure of photoreceptor cells in the pineal organ of the goldfish was found to vary quantitatively over a 24-hour period. Stereological analysis revealed significant daily changes in the volume of the cell and inner segment, nuclear volume and nucleolar diameter, volume of endo-plasmic reticulum and Golgi bodies, area of both rough and smooth endo-plasmic reticulum, and number of vesicles associated with each Golgi body. Peak values of these variables occurred either during the dark phase or latter part of the light phase. These findings agree closely with those reported in higher vertebrates, and suggest that metabolic activities, and possible secretory functions, of the pineal organ of fishes are synchronized to the light: dark cycle. Daily changes in these variables generally persisted in fish exposed to constant darkness for seven days, with the peaks in these rhythms coinciding closely with those observed in fish exposed to a light: dark cycle. In contrast, the rhythms in all variables were abolished in fish kept in continual light for seven days. Photoreceptor cells from fish exposed to continuous light had larger nucleoli and greater amounts of rough endoplasmic reticulum indicating a further effect of light on pineal metabolism in lower vertebrates.

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