A histological study of the zona radiata during late oocyte developemntal stages in the Caspian Sea mugilid, Liza aurata (Risso 1810)

RESUMO

Many functions have been attributed do the zona radiata (ZR), a porous layer normally found between the follicular cell layer and the oolemma in fishes. The ZR has frequently been considered as an interface that regulates the movement of essential material from granulosa nursery cells to oocytes, particulary during vitellogenesis. The ZR also anchors oocytes to substractes via processes extending from this layer. The aim of this study was to examine ZR growing characteristics during late stages of oocyte development in the marine teleost Liza aurata (golden mullet). Specimens of L. aurata in the stage IV of ovarian development were caught using beach seines and were injected with an extract from carp hypophysis to induce futher development. The porous nature of the ZR was found to exert an increasing role in sustaining oocytes. No ZR was seen by light microscopy in the previtellogenic (II) stage, but fowling transition to ht early stage III, i.e., early vitellogenesis, the ZR appeared in the form of marginal striations. The ZR developed mainly during the stage IV of ovarian growth, when vitellogenesis was about to end. During development of the oocyte, the ZR is extended as primitive projections towards the inner surface which became more prominent as vitellogenesis progressed, In late vitellogenesis (late stage IV or early V) these projections grew and formed slender finger-like structures arising from follicular cells. The ZR was highly perforates and served as communicating way to the oocyte. This structural organization accounts for the key role of the ZR in egg development.