Atresia and Apoptosis in Preovulatory Follicles in the Ovary of Danio rerio (Zebrafish)

Although apoptosis, the programmed cell death, is the main mechanism in follicular atresia and especially postovulatory regression in mammals, its role in fish ovary is still controversial. In order to discuss the possible functions of atresia as a context of apoptosis, the preovulatory follicles of the freshwater teleost, Danio rerio (zebrafish) were investigated by light microscopy. The main characteristics of atresia of vitellogenic oocytes of zebrafish were determined as a significant hypertrophy and hyperplasia of the granulosa cells. The theca cells also became slightly hyperplasic while the zona radiata was degenerated and the basal membrane located between granulosa and theca was also disintegrated. The aggregation of compacted chromatin in a crescent pattern, and cell fragmentation that indicates to formation of apoptotic bodies were evaluated as the morphological evidences for apoptosis in granulosa cells of the oocytes of zebrafish. Towards the end of atresia, the vitellus is fully degenerated and absorbed, some apoptotic bodies were also observed between the translucent clusters of theca cells. Apoptotic process was still continued when an atrium had been formed.