| Co-Investigator(Kenkyū-buntansha) |
AWAJI Takeo Tokyo Women's Medical College, School of Medicine, Researchi assistant, 医学部, 助手 (60297546)
SHIRAISHI Koichi Tokyo Women's Medical College, School of Medicine, Researchi assistant, 医学部, 助手 (50256476)
SHIRAKAWA Hideki Tokyo Women's Medical College, School of Medicine, Researchi assistant, 医学部, 助手 (40241070)
ODA Shoji Tokyo Women's Medical College, School of Medicine, Research assistant, 医学部, 助手 (50266714)
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| Research Abstract |
This study was performed as a step to elucidate the molecular mechanism by which the sperm induces an increase in intracellular Ca^<2+>, the pivotal signal for egg activation at fertilization. The following results were obtainde by microinjection techniques, Ca^<2+> image analysis, and confocal laser scanning microscopy.
1)As a phenomenological observation, we established the experimental condition in which sperm-egg fusion is formed only transiently without sperm entry into the sea urchin egg in the presence of a gamete fusion inhibitor, jaspisin. A localized Ca^<2+> rise restricted at the sperm binding site was recorded, separated fron the Ca^<2+> wave which spreads throughout the egg at normal fertilization. Spatiotemporal aspects of the sprem-induced localized Ca^<2+> rise were characterized in detail.
2) Injection of a mouse spermatozoon into an egg (intracytoplsmic sperm injection, ICSI) caused repetitive transient Ca^<2+> rises (Ca^<2+> oscillalions) as seen at fertilization, star
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ting 15 to 30 minutes after sperm injection and persisting for several hours. Each Ca^<2+> rise occurred almost synchronously in the whole egg. The results suggested that a sperm cytosolic factor leaks out from the injected spermatozoon into the egg cytoplasm and can induce Ca^<2+> oscillations, even if sperm-egg binding is bypassed by ICSI.
3) We found that injection of a precursor sperm, round spermatids without flagellum (round spermatid injection, ROSI), can not induce any Ca^<2+> response in the egg but that combined injection of a potent agonist of the inositol 1,4,5-trisphosphate (IP_3) receptor resulted in egg activation (fertilization) associated with male and female pronucleus formation. About 25% of two-cell embryos transplanted into foster mothers developed to normal offspring. The newborn infants grew up to normal adults, which reproduced normal second generations.
4)We partially purified a cytosolic protein from hamster and ascidian spermatozoa that possesses the activity of inducing Ca^<2+> oscillations when injected into hamster, mouse, and ascidian eggs. Microinjection of the sperm factor into a localized region of the egg showed that the peripheral cytoplasm is more sensitive to the sperm factor to generate Ca^<2+> increase (possibly due to Ca^<2+> release from the endoplasmic reticulum through IP_3 receptors), compared with the central region of the egg.
These results suggested that, at fertilization, a sperm cytosolic factor could be introduced into the cortical region of the egg cytoplasm through cytoplasmic continuity formed between the sperm and egg, and induce Ca^<2+> release in the egg. This study provided useful information for advancing further studies in future. Less
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