The ultrastructure of the ovarian follicle of medaka,Oryzias latipes

Summary The follicular cells in the oocytes of Oryzias latipes were studied by electron microscopy in order to clarify the fine structure, and the role of the cells during yolk formation and ovulation. The smallest follicles were observed during the early phase of peri-nucleolus stage of the oocyte. The cells have flattened nuclei, and perikarya with undeveloped organelles. But when the oocytes attain diameter of about 250 (yolk vesicle stage), both types of endoplasmic reticula are present. Moreover, the microvilli of the plasma membrane of oocyte as well as the follicles protrude into the pore canals of the zona radiata. In the oocytes of yolk stage the rough-surfaced endoplasmic-reticulum is typically developed and observed around the nuclei. Other organelles (lysosomes, mitochondria and Golgi) increase in number. The relation between the changes of cytoarchitecture in the follicles and yolk formation is discussed.At 17.00 p.m. on the day preceding ovulation the microvilli withdraw somewhat. Ribosomes are attached to the vesicular and cisternal endoplasmic reticula. When the oocytes attain complete maturation (24.00 p.m. at near ovulation), striking changes of the follicles are observed. The microvilli are almost withdrawn. In the degenerating follicles the lamellar structure is formed, and lipids are deposited at the center. At this time the contents of lysosomes have mostly disappeared.