Fine structural changes during sea urchin oogenesis

Abstract

Abstract Fine structural changes during sea urchin oogenesis are described and related to functional changes in the developing ovum. A staging sequence based on the morphology associated with the synthesis of ribosomes, cytomembranes, and yolk and cortical granule proteins is proposed: the oogonium, in which the cytoplasm contains a few scattered mitochondria, lipid inclusions and free‐floating ribosomal aggregates; the first transition stage, in which annuli form in the nuclear envelope and ribosomes become attached to newly synthesized cytomembranes; the vitellogenesis stage, in which the cytoplasm becomes packed with yolk and cortical granules, mitochondria, and lipid droplets; the second transition stage, in which annulate lamellae are delaminated from the nuclear envelope; and the mature ovum, in which the annulate lamellae are distributed throughout the cytoplasm and begin to break down into rough‐surfaced vesicles. Typical Golgi apparatus is first observed at this stage. The plasma membrane forms microvilli and there is a thick jelly layer. The association of ribosomes with cytomembranes and an amorphous material is discussed with respect to their activity in protein synthesis. The relationship of the accessory cells to the developing oocyte is also discussed.