Masaaki Morisawa

Existence of three mechanisms for blocking polyspermy in oocytes of the mussel Mytilus edulis

We found the existence of a three-step mechanism to block polyspermy in the oocyte of the mussel Mytilus edulis. When the oocytes were inseminated within 30 min after spawning, they underwent monospermic fertilization over a wide range of sperm-oocyte ratios up to 5 x 103. A transient depolarization of the oocyte plasma membrane (fertilization potential) was observed immediately after insemination. Low-sodium seawater induced polyspermy and decreased the amplitude of the fertilization potential, suggesting the existence of a fast block to polyspermy that is dependent on depolarization of the plasma membrane. When the fertilized oocytes were inseminated again at a sperm-oocyte ratio that is great enough to give a high rate of polyspermy in initial insemination, many sperm could not undergo the acrosomal reaction and thus could not penetrate fertilized oocytes. The remaining sperm underwent an acrosomal reaction and the acrosomal process protruded through the vitelline coat, but it did not fuse with the oocyte plasma membrane. These findings suggest the existence of two strategies constituting a late polyspermy block: suppression of acrosomal reaction and block of contact or fusion between the plasma membranes of sperm and oocyte.

Two high molecular mass proteases from sea urchin sperm

Two-types of high molecular mass proteases have been purified from sea urchin sperm using DEAE-Sephacel, hydroxylapatite and Superdex 200 column chromatography. Both proteases showed similar hydrolyzing activities toward synthetic peptides, but they differed in the molecular mass and peptide composition. One was probably identical to a proteasome (multicatalytic proteinase), judging from its molecular mass (650 kDa) and polypeptide composition. The other one was composed of several polypeptides with molecular masses ranging from 24 kDa to 125 kDa and its molecular mass was estimated as 950 kDa by gel filtration. These two proteases, however, were closely related to each other. Immunological studies revealed that the 950-kDa protease comprised at least five subunits of the 650-kDa protease.

Acquisition, initiation and maintenance of sperm motility in the shark, Triakis scyllia

Abstract Shark sperm were immotile in the testis, but motile in the vesicula seminalis, suggesting that motility potential is acquired in the male reproductive tract. Although sperm acquired motility, they were immobile in the undiluted semen, however, motility occurred upon dilution of the semen in electrolyte solutions, whose concentrations are the same as seawater or uterus fluid, suggesting that exposure to these fluids at ejaculation causes the initiation of sperm motility. Duration of sperm motility was longer in glucose-rich uterus fluid than glucose-free media, suggesting the important role of hexose for maintaining sperm motility in the female reproductive tract.

Oocyte Maturation and Furrow Formation in an Unfertilized Egg by Fusion with a Fertilized Egg or Blastomeres in the Ascidian, Ascidia sydneiensis divisa

A technique for fusing an ascidian egg with blastomeres using a chemical fusiogen was established and then used to identify cytoplasmic factors that regulate the process of oocyte maturation in ascidian eggs. Unfertilized eggs fused with fertilized eggs or blastomeres in hypotonic artificial sea water containing 20% polyvinyl alcohol within 10 min. After fusion polar bodies were extruded from the unfertilized portion of the fused eggs. Furrows were formed not only in the fertilized portion but also in the unfertilized portion in the fused eggs. No polar body extrusion and furrow formation occur in either portion of fused unfertilized eggs. These results suggest that fertilized eggs and blastomeres contain a factor that induces oocyte maturation. Polar body extrusion and furrow formation were not suppressed in the fertilized portion of fused eggs, suggesting that unfertilized eggs do not contain a factor that inhibits oocyte maturation.