Ultrastructural aspects of the ovarian follicle and egg envelope of the sea-grass goby Zosterisessor ophiocephalus (Osteichthyes, Gobiidae)

The development of the ovarian follicle, the chorion, the attaching filaments apparatus, and the egg surface are described in the grass goby Zosterisessor ophiocephalus by transmission and scanning electron microscopy. The first appearance of the zona radiata externa is followed by the apposition of a canalicular layer in the outer zona radiata interna, and then of a fibrous tangential network in the inner zona radiata interna, reaching an overall thickness of 5.6 μm in mature eggs. Both zona radiata interna components are likely secreted by the oocyte, whereas the apparently mucopolysaccharidic zona radiata externa is produced early by flattened granulosa cells. During maturation, granulosa cells become cubic in cross section and elongate from the animal pole, marked by the prospective micropyle and oriented along the oocyte meridians. Their cytoplasm is markedly engulfed with enlarged cisternae of the rough endoplasmic reticulum involved, together with mitochondria and well-developed Golgi complexes, in adhesive filaments protein matrix secretion and the making of the basal plate. Filament matrix aggregates in the granulosa cells intercellular spaces, where it is confined by intercellular junctions. The basal plate is hollow in the centre surrounding the micropylar pit and supports a cribriform column of netted filaments eventually fraying off and becoming the egg adhesive apparatus. The comparative functional and systematic relevance of these features among teleosts is discussed and related to environmental and life style adaptations.