ABSTRACT
Most teleosts’ ovaries are paired, saccular structures with a central lumen and they do not have oviducts because of the lack of Müllerian ducts. Consequently, the communication of the ovary to the exterior occurs through the gonoduct. From the ovarian wall, lamellae project into the lumen, which are lined by the germinal epithelium and contain stroma with numerous ovarian follicles. The germinal epithelium contains oogonia among somatic epithelial cells. The lack of oviducts in viviparous teleosts leads to a unique type of intraovarian gestation. Folliculogenesis is completed when the oogonia initiate meiosis and are enclosed by follicle cells and a vascularized theca, becoming an oocyte. Oogenesis involves the morphological and functional processes by which oogonia transform into fertilizable eggs. There are six stages of oogenesis: 1) Oogonial proliferation, 2) The beginning of meiosis forms the chromatin-nucleolus stage, developing lampbrush chromosomes, 3) During previtellogenesis the oocyte has a single nucleolus becoming soon multinucleolar; cortical alveoli and lipid droplets appear in the ooplasm leading to a significant increase in oocyte diameter. 4) Vitellogenesis is characterized by the deposition of yolk in the ooplasm; the follicle reaches its maximum size, becoming a full-grown oocyte. 5) During maturation the germinal vesicle becomes eccentric and migrates to the animal pole, followed 6) by ovulation. Postovulatory follicles contains follicle cells. Atresia is observed in any stage especially at the end of a breeding season. The diversity of ovarian features in teleosts is a main aspect for understanding their essential function, such as: type of spawning pattern; oviparity and viviparity that implicate modifications for intraovarian gestation; hermaphroditism that reveals ovotestis; the change of sex in protogynous and protandric species.
