Cortical membrane-trafficking during the meiotic resumption of Xenopus laevis oocytes.

Changes in the organization of membranous structures in the amphibian oocyte cortex were studied during the process of progesterone-induced meiotic resumption. Progesterone treatment of Xenopus laevis oocytes induced short term and longer term changes in the cortical membranes. In the short term, progesterone induced a burst of endocytosis mediated through coated pits and coated vesicles. Immuno-electron-microscopic localization of progesterone suggested that the progesterone receptor, bound to its ligand, is endocytosed during progesterone-induced endocytosis. Also demonstrated was the existence of a cisternal membrane network, referred to as the primordial cortical endoplasmic reticulum, which surrounds portions of the cortical granules in oocytes. The primordial cortical endoplasmic reticulum is more highly developed in the animal hemisphere than the vegetal hemisphere. Over the long term, during the meiotic resumption, more membrane is recruited into this network to form the cortical endoplasmic reticulum observed by others in the metaphase II egg. This evidence demonstrates that the cortex serves as a site for dynamic changes in membrane organization and that the most extensive changes occur in the animal hemisphere. These data support previous observations that the animal hemisphere is better structured for sperm penetration than is the vegetal hemisphere.

Analysis of proteins in the peripheral and central regions of amphibian oocytes and eggs.

Proteins in oocytes, meiotically mature eggs and zygotes of Xenopus laevis were examined to determine whether proteins in the peripheral region differ from those in the central region of these cells. We show that different regions contain different amounts of newly synthesized proteins and that during meiotic maturation and fertilization the periphery of the animal hemisphere becomes the site where most newly synthesized proteins are found. Examination of two-dimensional gels indicates that most of these proteins are found in all parts of the egg, but certain proteins demonstrate patterns of distribution which are indicative of (1) polarity, (2) developmental stage, and (3) the position within the hemisphere (central or peripheral). These results suggest that the periphery of oocytes, eggs, and zygotes is a site of greater metabolic activity compared with the central region.