Both germline cysts and the syncytium-like structure participate in oocyte differentiation in zebrafish

Oocyte differentiation occurs within germline cysts in many metazoans, where only a small number of cyst cells differentiate into oocytes, while the majority undergo apoptosis. In zebrafish, the transition of primordial germ cells (PGCs) to germline cysts has been well studied. However, the exact process by which cyst cells develop into oocytes remains unclear. Here, we analyzed the temporal and spatial characteristics of germline cysts and traced the process of oocyte differentiation in the early zebrafish gonad, by transmission electron microscopy and laser confocal microscopy. The experiments showed that germline cysts were first found at the gonadal periphery at 15 days post-fertilization (dpf) in zebrafish. No oocytes with a Balbiani body were observed within these cysts until 21 dpf. Instead, PGC daughter cells within the gonad lumen fused to form a syncytium-like structure between 15 and 16 dpf, where oocytes were formed after mitochondria and nuage aggregated into the Balbiani body since 16 dpf and were released beginning at 19 dpf. On the other hand, strong EdU-positive signals were detected in some cyst cells between 15 and 17 dpf, but not in the germ cells within the syncytium-like structure. By 21 dpf, EdU-positive cyst cells appeared in the syncytium-like structure and finally developed into individual oocytes at 25 dpf. No massive apoptosis was observed in germ cells within the syncytium-like structure. Our findings provide new insights into oocyte differentiation in zebrafish and advance our understanding of early oogenesis in vertebrates.