Kutum (Rutilus frisii kutum, Kamensky 1901) is an economically important Cyprinid species endemic to the Caspian Sea. This species is anadromous, meaning it enters rivers for spawning. In this study, the oocyte development in the developing oocyte and fertilized egg of kutum was investigated with emphasis on the zona radiata (ZR) structure by light and scanning electron microscopy. Histological features of developmental stages of oocytes were described in detail using light microscopy. The results showed that ZR was not observed in the previtellogenic phase. The ZR was observed as a simple structure between the follicular layer and oocyte membrane during the cortical alveolus stage. The study of the ZR ultrastructure showed some pore canals and microvilli at the vitellogenic stage expanded structurally as the vitellogenesis process proceeded. The ZR maximum thickness (14.9 ± 1.6 μm) and maximum length of extended microvilli (10.8 ± 1.4 μm) was achieved at the end of the vitellogenic stage. During ovarian maturation, reduction in ZR thickness and microvilli length was found to be gradual. The transformation of ZR structure, i.e., clogging of the pore canals and retraction of the microvilli occurred functionally during fertilization and later on the ZR renamed as chorion (diameter 9.6 ± 0.4 μm). The chorion on the surface of fertilized eggs possessed microvilli (8.0 ± 1.1 μm in length) and some slender processes. In general, considering the changes of ZR thickness and microvilli length, it seems that they play an important role in easier transfer of yolk materials into the oocyte. In addition, after fertilization, the chorion (transformed ZR) helps adhesion of eggs to the bottom with regard to the environment of spawning.
