Ultrastructural and Morphometric Study of the Pineal Complex in Indian Major Carp Catla Catla in Response to Continuous Light and Darkness

Abstract

The present communication deals with the ultrastructurai and morphometric study of the pineal complex in a hitherto unstudied Indian major carp Catia catla. In this fresh water teleost, the pineal complex is a discrete structure composed of a distal 'end vesicle' (EV), a long connecting 'pineal stalk' (PS), and a convoluted 'dorsal sac' (DS). Light microscopic study demonstrated that the parenchyma of the EV contains two types of cells, viz., 'light cells', and 'dark cells'; of which only 'light cells' exhibit significant annual cyclic variations in the nuclear diameter. Consequently, attempts have been made to employ transmission electron microscopy for identification and characterization of the photosensory cellular components of the pineal complex in the carps following exposure to continuous illumination (LL; 24L) or continuous darkness (DD : 24D) for 30 days during the different phases of an annual reproductive cycle. The study also indicated the presence of two types of cells which shared the features of photoreceptorand supporting cells in the EV of the pineal complex respectively. Significant changes were noted in the size and shape of various sub-cellular organelles in the photoreceptor-, but not in the supporting cells of the pineal EV following exposure to LL or DD schedules. While an increase in the nuclear diameter was noted in the photoreceptor cells of EV in the carp held under DD in each part of the annual gonadal cycle, significant regressive changes occurred in the photoreceptor cells of the EV in LL group offish but only during the preparatory and the pre-spawning phases of theannual cycle. Commonly, it was found that an increase in the number of secretory granules, synaptic vesicles and synaptic ribbons, and hypertrophy of mitochondria occurred in the pinealocytes of carps held under DD, while regressive changes in the parenchymal cells of EV resulted in the pineal complex following exposure to LL. It was interesting to note that morphometric and uitrastructurai changes in response to altered lighting conditions were demonstrable only in the EV, but not in the PS and DS of the pineal complex. Thus, it appeared conclusive that the pineal complex in Catla catla is directly associated with photoreception, and the EV is the only part of pineal complex that performs the alleged function. However, the etiology of differential responsiveness of the EV photoreceptor cells to LL in different parts of annual reproductive functions remains unknown.