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Fertilization a process at the beginning of ourselves

Fertilization a process at the beginning of ourselves. Structure of Sperm and Egg. Sperm A human sperm. Egg Overview The zona pellucida. Scanning electron micrograph of a human sperm contacting a hamster egg. For detail.

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Fertilization a process at the beginning of ourselves

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  1. Fertilizationa process at the beginning of ourselves

  2. Structure of Sperm and Egg • Sperm A human sperm. • Egg Overview The zona pellucida.

  3. Scanning electron micrograph of a human sperm contacting a hamster egg. For detail

  4. Scanning electron micrograph of a clam egg with sperm bound to its surface. For detail

  5. Block to polyspermy • A depolarization of the egg plasma membrane a rapid, primary block to polyspermy • Egg cortical reaction a slow, secondary block to polyspermy

  6. primary block to polyspermy • Na+ influx triggered by fusing with sperm make the membrane potential of egg shift from –60mV to +20mV within a few second. • Low concentration of Na+ raise the incidence of polyspermy. Artificially depolarized an unfertilized egg prevent sperm from fusing with egg.

  7. primary block to polyspermy • It seems likely that the depolarization alter the conformation of a crucial protein in the egg plasma menbrane so that the sperm membrane can not fuse with the egg membrane. • The egg membrane potencial returns to normal within a few minutes after fertilization.

  8. Therefore • a second mechanism must provide a long-term barrier to polyspermy. • In most egg, including those of mammals, this barrier is provided by Cortical Reaction.Back

  9. Second block to polyspermy • When the sperm fuses with the egg plasma membrane, it activates the inositol phospholipid cell-signaling pathway in the egg. This, in turn, causes a local increase in cytosolic Ca2+, which spreads through the cell in a wave. The rise in Ca2+ activate the egg and initiate the cortical reaction.

  10. Second block to polyspermy • cortical granules located just under the plasma membrane of the egg release their contents by exocytosis. A,B • The enzymes released by the cortical reaction change the structure of the zona pellucida, which becomes hardened so that sperm no longer bind to it, thereby providing a slow, secondary block to polyspermy. End

  11. A mature Xenopus egg, ready for fertilization.For detail

  12. The zona pellucida The zona pellucida. For detail

  13. A human sperm. It is shown in longitudinal section

  14. Drawing of the midpiece of a mammalian sperm

  15. Depolarization&Cortical reactionare the mechanics ofBlock to Polyspermycontinue

  16. AcrosomeReaction • Sperm penetrate the zona pellucida by acrosome reaction. • Process of acrosome reaction. Back

  17. Process of Fertilization • Acrosome reaction • Deplarization • Cortical Reaction • 2nd oocyte complete Meiosis Ⅱ • Pronucleus • Replication of DNA in pronucleus • First Mitosis

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