Avaliação da remodelação ovariana pós-desova da tilápia-do-nilo (Oreochromis niloticus) em condições de cultivo

Abstract

Tilapias are the second most important group of fish cultivated in aquaculture worldwide and the Nile tilapia Oreochromis niloticus (Linnaeus, 1758) is the main species of tilapia currently farmed. Studies about the ovarian regeneration after spawning in species of commercial importance, such as O. niloticus, provides important reproductive parameters for the cultive of the species. Fish ovaries after spawning are also excellent experimental models for studying mechanisms of programmed cell death, especially due to tissue remodeling involving follicular growth and regression for initiation of new reproductive cycle. Thus, the present study aimed to evaluate the dynamics of ovarian remodeling of Nile tilapia after spawning in captivity. To achieve this goal, ovarian samples were collected weekly for histological, ultrastructural, in situ TUNEL, immunohistochemistry (caspase 3) and morphometric analysis. Initially, the quantitative and morphological dynamics of follicular growth after spawning was evaluated, and then, the role of apoptosis and autophagy was studied during follicular growth and regression after spawning in O. niloticus ovaries. Recently spawned ovaries contained follicles at all stages of development, with a predominance of early primary growth (~42%) and full-grown follicles (~20%). Post ovulatory follicles represented approximately 5% of the ovarian follicles after spawning, and less than 1% after 7 days. Atretic follicles accounted for approximately 2% of the follicles during the study period. Primary growth follicles maintained a stable stock during ovarian remodeling, indicating their availability for continuous recruitment. Twenty-one days after spawning, the full-grown follicles predominated in the ovaries, representing approximately 35% of the follicles. All follicles, except post-ovulatory and atretic, grew significantly along the ovarian remodeling. The ultrastructure of growing follicles revealed an intense synthesis activity of the follicular cells, which actively contributed to formation of the zona radiata and oocyte development after spawning. The results showed a rapid ovarian recovery and follicular growth of O. niloticus, in 21 days at 29.5°C, to allow the next spawning. The second stage of the present work revealed, during follicular growth after spawning, a low occurrence of apoptosis in the follicular cells and theca of the follicles. Shortly after spawning, post-ovulatory follicles exhibited hypertrophied follicular cells with intense synthesis activity, high rates of follicular apoptosis and autophagy to cell clearance. Post ovulatory follicles, 7 days post-pawning, showed follicular cells with abundant autophagic machinery and lower apoptosis. Early atretic follicles showed follicular cells containing numerous synthesis organelles, heterophagous activity for phagocytosis of the yolk and low apoptosis. In advanced atresia, the yolk continues to be phagocytosed by the follicular cells, which exhibited decreased synthesis activity accompanied by increased apoptosis and autophagy. In late atretic follicles, the follicular cells presented a markedly electron-lucid cytoplasm and increased apoptosis. These results indicate that autophagy and apoptosis act cooperatively for efficient regression and elimination of post-ovulatory and atretic follicles in ovaries of Nile tilapia and are essential mechanisms in ovarian remodeling post-spawning. Additionally, apoptosis plays an important role in the homeostasis of growing follicles after spawning.