Formation of zona radiata and ultrastructural analysis of egg envelope during oogenesis of Chinese perch Siniperca chuatsi.

During oocyte maturation, egg envelope undergoes a series of changes in structure and function. We investigated the follicular envelope, zona radiata and plasmalemma of Chinese perch, Siniperca chuatsi at five different developmental stages by light, scanning and transmission electron microscopy. An octopus-like structure in the zona radiata was observed for the first time. The increasing thickness of zona radiata during these stages suggests the microvilli's functional transition from transferring vital materials for the protection of the oocyte. That the egg envelope fully matures later than the oocyte is well supported by the disappearance of the follicular envelope. We have discovered several characteristic structures in the semi-floating egg and propose that the octopus-like structure is essential for oocyte migration and fertilization.