The Structure of the Pineal Complex in a Common Indian Teleost, Catla catla: Evidence for Pineal‐Induced Inhibition of Testicular Function Within an Annual Reproductive Cycle

Abstract

The structure of the pineal complex and the annual reproductive cycle in a major Indian carp, Catla catla, were investigated in the present study. Additionally, given the well‐known inhibitory effects of the pineal on reproductive function in mammals, attempts were made to investigate whether or not the pineal exerts an inhibitory influence on reproductive function in this piscine species as well. Sexually adult animals were utilized in all experiments. The cytomorphology of the pineal complex and a number of parameters for testicular function—such as testicular cytology, serum testosterone levels, and testicular activities of two steroidogenic enzymes, 17β‐hydroxysteroid dehydrogenase (17β‐HSD) and Δ5‐3β‐hydroxysteroid dehydrogenase (Δ5‐3β‐HSD) were examined over a period of two years. Our studies showed that the pineal complex in this species consists of three separate but distinctly connected components: (a) an end vesicle (EV); (b) a long pineal stalk (PS); and (c) a dorsal sac (DS). Of these, the epithelial lining of the EV consists of cells that have rounded vesicular nuclei and long apical cytoplasmic processes that reach the lumen, features suggestive of pineal photoreceptor cells. The cells of the PS have some similarity with those of the EV, while DS cells appear columnar and ciliated. With regard to gonadal activity, testicular germ cell profiles revealed that this species has four distinct phases during the annual reproductive cycle: (a) preparatory (January–April); (b) pre‐spawning (May–June); (c) spawning (July); and (d) post‐spawning (August–December). During the spawning phase (July), seminiferous tubular diameter, percentage of late spermatids within seminiferous tubules, and serum testosterone levels showed the highest values compared to those obtained in most of the other phases of the reproductive cycle. Also in July, along with peak serum testosterone levels, the activities of 17β‐HSD and Δ5‐3β‐HSD were at their highest levels. In a correlation between the pineal cytology and testicular functional status, it was noted that both the nuclear diameter and the apical cytoplasmic projections of the EV photoreceptor cells showed a significant reduction, thus suggesting a reduced synthetic activity, during the month of July, the spawning phase of the reproductive cycle. In contrast, the same features of the EV cells during the other phases of the reproductive cycle showed an increased cellular and metabolic activity—a time when the gonads were less active and in a quiescent stage. These data suggest an inhibitory role of the pineal on gonadal function and thus provide additional credence to the concept that, as in higher mammals, there exists an inverse relationship between the pineal activity and gonadal function in teleost fishes as well.