Study of the Zona Radiata Structure in Oocytes of the Persian Sturgeon (Acipenser persicus) before and after Fertilization

Egg envelope or otherwise called zona radiata (ZR) is an acellular area that originates next to oolemma externally. It grows and thickens gradually from oolemma and follicular epithelium both. Microscopic studies on the tissue sections hava shown that during developmental stages of oocytes, ZR could not be observed at stage I (Primary growth stage) in Acipenser persicus. It appeared in stage II (Cortical alveolar stage) and during stage III (vitellogenesis), ZR thickness was greatest and had highest complexity. The striated appearance of ZR at this stage, were identified to be pore-canals involved in yolk material transportation. In stage IV (Maturation stage), a decline was observed in ZR thickness and complexity, a process which continued after fertilization. In mature egg, an uneven layer of chorion over ZRe (ZR, external) and a jelatinous coat extrachorion were developed. Immediately after fertilization, ZRi (ZR, internal) turned to a homogenous layer (fertilization envelope), probably because of cortical reaction. 30 minutes after fertilization, two layers recognized in ZRi and chorion and extrachorion were disappeared.