9. Fertilization a process at the beginning of ourselves

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

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

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

13. 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

14. 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.

15. 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.

16. 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.BackBack Therefore

17. 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 Ca 2+, which spreads through the cell in a wave. The rise in Ca 2+ activate the egg and initiate the cortical reaction.wave

18. Second block to polyspermy cortical granules located just under the plasma membrane of the egg release their contents by exocytosis. A,BAB 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. EndEnd

19. A mature Xenopus egg, ready for fertilization. For detailFor detail

20. The zona pellucida The zona pellucida. For detail For detail

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

22. Drawing of the midpiece of a mammalian sperm

23. Depolarization & Cortical reaction are the mechanics of Block to Polyspermy continue continue

32. Acrosome Reaction Sperm penetrate the zona pellucida by acrosome reaction. Process of acrosome reaction.Process of acrosome reaction Back

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