Down-regulation of membrana granulosa cell gap junctions is correlated with irreversible commitment to resume meiosis in golden Syrian hamster oocytes

One of the currently popular hypotheses for the regulation of meiotic resumption in mammalian oocytes proposes that the preovulatory surge of luteinizing hormone causes down-regulation of follicular gap junctions, which in turn disrupts transfer of a meiotic arrester from the somatic cells into the oocyte. The present study has investigated this hypothesis by examining the integrity of membrana granulosa cell gap junctions during the period of irreversible commitment to maturation of golden Syrian hamster oocytes in vivo. Our results have revealed a significant progressive decrease in the fractional area of cell surface occupied by gap junction membrane with increasing percentage of oocytes irreversibly committed to mature (1.946% and 0.921% fractional gap junction area at 0% and 100% oocytes irreversibly committed to mature, respectively, P less than 0.05). This net loss of membrana granulosa cell gap junctions from the cell surface was accompanied by a significant decrease in density of gap junction particles, whether they were arranged in rectilinear or non-rectilinear packing patterns. Furthermore, the number of gap junction particles per unit area of surface membrane scanned also underwent a significant progressive decrease with increasing percentage of oocytes irreversibly committed to mature. These data with the hamster are consistent with the hypothesis that down-regulation of membrana granulosa cell gap junctions may be of central importance in the regulation of gonadotropic stimulation of meiotic resumption in mammalian oocytes.     

Calcium signals in neutrophils can be divided into three distinct phases

Rabbit neutrophils were loaded with the fluorescence probe indo-1 and cytosolic free calcium levels were monitored during chemotactic peptide (fMet-Leu-Phe) stimulation. The fMet-Leu-Phe-induced calcium signal consisted of three consecutive phases: (1) an initial peak that was independent of extracellular calcium, (2) a secondary shoulder that required extracellular calcium but was totally blocked by hyperosmolality and (3) a final plateau of elevated calcium that was dependent on extracellular calcium but insensitive to hyperosmolality.     

Extracellular coats on the surface of Strongylocentrotus purpuratus eggs: stereo electron microscopy of quick-frozen and deep-etched specimens

Summary Sea urchin (Strongylocentrotus purpuratus) eggs were fixed, quick-frozen, deep-etched, and rotary-replicated, and the three-dimensional structure of the external surface of the egg visualized using stereo electron microscopy. The cell surface is coated with three layers of filaments: the sheetlike vitelline layer adhering closely to the plasma membrane, a second layer of oblique fibrils extending from microvillar tips to the vitelline layer below, and a third, outermost layer of horizontal filaments coursing in bundles over the microvillar tips. After fertilization, the newly elevated vitelline envelope is transformed into a three-layered structure, the central layer being a tightly knit network of fine filaments decorated on each side with a loose network of thicker fibrils. Subsequently, the envelope becomes coated with paracrystalline protein released from the cortical granules, and microvillar casts are reshaped into angular, jagged peaks having two to five sides. The final structure of the fertilization envelope consists of a thick central layer of compact fibrillar material that is coated on each side with thin plates of paracrystalline protein.