Light and electron microscope structural study of the zona radiata in the oocyte of zebrafish (Danio rerio)

Abstract

Teleosts account for more than half of the vertebrate species. A key feature of the evolutionary success of teleosts is their reproductive systems, which are efficient and functional under all aquatic environmental conditions. The zona radiata (ZR) is a non- cellular envelope over oolemma of teleost fish oocytes. The morphology of the ZR varies in different ovarian developmental stages and also among various fish species, which reflects adaptation to ecological conditions. In this research the structure of the ZR in the oocytes of zebrafish in different stages of ovarian development and oocyte growth were studied. The fish ovary was fixed in Bouin's solution. Histological sections were studied by light and scanning electron microscope after subjecting them to tissue preparation stages. In immature and primary growth stages (I and II), the ZR was not observed. In stage III, the ZR was observed as a thin layer around the oocyte. It increased in thickness at stage IV (vitellogenesis) but reduced in stage V (maturation). In early stage III, the ZR striations started to appear and in stage IV (vitellogenesis) fence-like striations were completely formed. SEM micrographs showed stria to be deep with crenelated margin. In late stage IV fence-like striations disappeared gradually and in matured egg (V) the stria turned into pit-like remnants. In zebrafish no adhesive filaments were observed and the ZR surface was smooth.