Ultrastructural study of oogenesis of the Scarus ferrugineus was carried out during the period of oocyte growth, particularly the changes in nuclear morphology, cytoplasmic organelles, and wall formation. Materials produced in the nucleus and transported to the cytoplasm known as nuage were observed in the oogonia and primary oocytes at premeiotic and perinucleolar stages. This material gives rise to germ plasma in the growing oocytes. In the perinucleolar oocyte, the cytoplasm appears loaded with free ribosomes and dense bodies. Short strands of endoplasmic reticulum (ER), perinuclear mitochondrial ring, and a simple layer of flattened granulosa cells were also detected. Protrusion of short buds from oocyte and granulose surfaces to the perioocyte space was clearly detected in the oocyte. From the early vitellogenic stage (yolk vesicle oocyte) the vitelline envelope was observed as a single electron-dense mesh pattern layer, becoming thicker during the vitellogenic stage. The granulosa cells (GCs) become organelle-rich with elongate mitochondria, free ribosomes, dilated tubular RER, and a Golgi system. Both GCs and thecal cells (TCs) show ultrastructural steroidogenic features, suggesting their primary roles in the production and secretion of steroid hormones at this stage of ovarian development. Remarkable ultrastructural changes were shown in vitellogenic oocytes including increase in RER, proliferation of mitochondria, and protrusion of long microvilli from oocyte and granulosa cells into pore canals of the vitelline membrane. The microvilli withdraw by the end of yolk deposition and the granulosa cells dissociate in fully grown oocytes. In atretic oocytes, the vitelline membrane homogenized and fragmented. Also, GCs, and maybe TCs, proliferated, hypertrophied, and phagocytosed the oocyte remnants forming temporary structure. Degeneration of previtellogenic oocytes prior to sex change was described and discussed.
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Vol. 2012 • No. 2