Development and fine structure of the yolk nucleus of previtellogenic oocytes in the medaka Oryzias latipes

The development and fine structure of yolk nuclei in the cytoplasm of previ tellogenic oocytes were examined by electron microscopy during several stag es of oogenesis in the medaka, Oryzias latipes. Shortly after oogenesis sta rts, oocytes 20-30 mum in diameter have much electron-dense (basophilic) cy toplasm, within which a continuous or discontinuous, irregular ring-shaped lower electron-dense area of flocculent appearance (LF) begins to emerge ar ound the nucleus. The yolk nucleus is first recognized within an LF area as a few fragments of dense granular thread measuring 20-25 nm in width. The threads consist of two rows of very dense granules resembling ribosomes or ribonucleoprotein (RNP)-like particles in size and electron density. These thread-like fragments gradually increase in number and length until they as semble into a compact, spherical mass of complicated networks. Analysis of serial sections suggests that the yolk nucleus is a complicated mass of num erous, small deformed vacuoles composed of a single lamella with double lay ers of ribosomes or RNP-like granules, rather than a mass of granular threa ds. When oocytes develop to greater than 100 mum in diameter, the yolk nucl eus begins to fragment before dispersing throughout the surrounding cytopla sm, concomitantly with the disappearance of LF areas. At this stage of ooge nesis, a restricted region of the granulosa cell layer adjacent to the yolk nucleus becomes somewhat columnar in morphology, fixing the vegetal pole r egion of the oocyte.