STAGES OF OOCYTE DEVELOPMENT IN THE ZEBRAFISH, BRACHYDANIO-RERIO

Oocyte development has been divided into five stages in the zebrafish Brachydanio rerio, based on morphological criteria and on physiologica l and biochemical events. In stage I (primary growth stage), oocytes r eside in nests with other oocytes (Stage IA) and then within a definit ive follicle (Stage IB), where they greatly increase in size. In stage II (cortical alveolus stage), oocytes are distinguished by the appear ance of variably sized cortical alveoli and the vitelline envelope bec omes prominent. In stage III (vitellogenesis), yolk proteins appear in oocytes and yolk bodies with crystalline yolk accrue during this majo r growth stage. Oocytes develop the capacity to respond in vitro to th e steroid 17alpha,20beta-dihydroxy-4-pregnen-3-one (DHP) by undergoing oocyte maturation. In stage IV (oocyte maturation), oocytes increase slightly in size, become translucent, and their yolk becomes non-cryst alline as they undergo final meiotic maturation in vivo (and in respon se to DHP in vitro). In stage V (mature egg), eggs (approx. 0.75 mm) a re ovulated into the ovarian lumen and are capable of fertilization. T his staging series lays the foundation for future studies on the cellu lar processes occurring during oocyte development in zebrafish and sho uld be useful for experimentation that requires an understanding of st age-specific events. (C) 1993 Wiley-Liss, Inc.