Ultrastructural Features of Follicle Cell-Oocyte Interface during Different Stages of Follicles in the Caecilian Ichthyophis tricolor

Abstract

This study is the first to describe the major stages of follicles of a caecilian from ultrastructural perspectives. The female reproductive system of Ichthyophis tricolor is described. Assembly of the folhcles involves the origin of primary oocytes from oogonia and the pre-granulosa cells from ovarian stroma, each from a separate nest, and the arrival of pre-granulosa cells around the meiotic oocyte to become granulosa cells. Appearance of a perivitelline space between the oocyte and the granulosa cells marks the beginning of pre-vitellogenic follicles. Macrovilli arise from the granulosa cells and ramiiy in the perivitelline space. In the vitellogenic follicles the macrovilli are further more prominent, and microvilli arising from the oolemma intercalate between the macrovilli. Yolk platelets appear in the cortical ooplasm, which are initially small and scattered but later the smaller ones fuse, and lipid and glycogen are added on resulting in large yolk platelets. The material of the vitelline envelope appears in the perivitelline space, which later condenses into the vitelline envelope. Some of the follicles undergo atresia when the granulosa cells become atrophied, and amoeboid cells from the theca enter the ooplasm and phagocytose the yolk.