Fine Structure of Micropylar Cell in the Ovarian Follicle of the Teleost, Plecoglossus altivelis

The micropylar cell in the ovarian follicle of the teleost, Plecoglossus altivelis has been studied with light and electron microscopy in order to clarify the mechanism of maintenance of its highly modified structure, and to determine the role of the cell during egg maturation. As oogenesis proceeds a monolayers of squamous follicle cells at the animal pole transform into stratified cuboidal cells in a depression in the attaching layer of the chorion. The micropylar cell assumes the contours of a mushroom and differentiates to form the cell body and the micropylar process. In the cytoplasm of cell body 10 nm filament form bundles about 1μm in diameter which encircle the upper part of nucleus. Branched bundles extend to the periphery and into the micropylar process. Well developed endoplasmic reticulum with cisternae which seem to be empty make the cell appear like a sponge. The micropylar process, which inserts into the cortex of oocyte to a depth of 1μm, contains a diplosome oriented nearly longitudinally with respect to the axis of the process. Many amorphous dotted satellites are associated with the diplosome. Microtubules extended to the periphery of the process and to the cell body. Functionally, the micropylar cell has three kinds of secretory systems. Two of them, which appear also in the follicular epithelium, are forthe production of the follicular liquid and the attaching layer. The third kind is particular to the micropylar cell and involves secretion from the tip of the micropylar process directly into the vitellus.