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FERTILIZATION

FERTILIZATION

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FERTILIZATION

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  1. FERTILIZATION Gametes: fuse to create a new organism sex: combining the genes of both parents reproduction: creation of a new organism Steps .Contact and recognition between gametes . Regulation of sperm entry . Fusion of genetic material . Activation of egg

  2. 7.2 The modification of a germ cell to form a mammalian sperm

  3. 7.2 The modification of a germ cell to form a mammalian sperm (Part 3)

  4. 7.3 The motile apparatus of the sperm Axoneme

  5. 7.4 Structure of the sea urchin egg at fertilization

  6. 7.5 Stages of egg maturation at the time of sperm entry in different animal species

  7. 7.6 The sea urchin egg cell surface • Enclosing the cytoplasm is the egg cell membrane which must be capable of fusing with the sperm cell membrane and must regulate the flow of certain ions during fertilization. • Outside of the cell membrane, there is a fibrous mat involved in sperm-egg recognition. In nvertebrates: vitelline envelope.

  8. 7.7 Hamster eggs immediately before fertilization • In mammals, the vitelline envelope is a separate and thick extracellular matrix called zona pelucida. • The mammalian egg is also surrounded by a layer of cells called the cumulus. • The inner most layer of cumulus cells is called corona radiata. • Beneath cell membrane is the cortex (globular actin molecules and cortical granules).

  9. 7.8 Summary of events leading to fusion of egg & sperm plasma membranes in the mouse

  10. 7.9 Sperm chemotaxis in the sea urchin Arbacia punctulata (Part 1) Resact is a 14 aa peptide that diffuses readily in seawater. 1 sec 20 sec 40 sec 90 sec

  11. 7.10 Model for chemotactic peptides in sea urchin sperm

  12. 7.11 The acrosome reaction in sea urchin sperm • The acrosome reaction has 2 components: • 1) the fusion of the acrosomal vesicle with the sperm cell membrane: the contact causes the exocytosis of the sperm’s acrosomal vesicle and the release of proteolytic enzymes. • 2) the extension of the acrosomal process: the actin molecules assemble to produce microfilaments extending the acrosomal process onwards.

  13. 7.12 Species-specific binding of acrosomal process to egg surface in sea urchins

  14. 7.13 and 7.14 Localization of bindin on the acrosomal process and Bindin receptors on the egg

  15. 7.15 Scanning electron micrographs of the entry of sperm into sea urchin eggs

  16. 7.17 Membrane potential of sea urchin eggs before and after fertilization Fast block to polyspermy- change in the resting potential of the egg cell membrane. Egg cell membrane increase the Na permeability. Sperm cannot fuse with membranes having a positive resting potential.

  17. 7.18 Formation of the fertilization envelope and removal of excess sperm • Cortical granule serine protease: dissolves the protein posts that connect the vitelline envelope proteins to the cell membrane, and if clips off the bindin receptors and any sperm attached to them. • Fertilization envelope is formed (components of the cortical granules + vitelline envelope). • Mucopolysaccharides: elevate the fertilization envelope. • Peroxidase enzyme: hardens the fertilization envelope • Hyalin: forms a coating around the egg providing support during cleavage Slow block of polyspermy- the cortical granules remove sperm

  18. 7.19 Cortical granule exocytosis (Part 1)

  19. 7.19 Cortical granule exocytosis (Part 2)

  20. 7.20 Wave of Ca2+ release across a sea urchin egg during fertilization • Upon fertilization, the concentration of free Ca2+ in the egg cytoplasm increases greatly. • The release of calcium comes from within the egg itself. • The release of Ca2+ starts at one end of the cell and proceeds actively to the other end. • Calcium ions are directly responsible for propagating the cortical granule reaction. • The calcium ions are stored in the endoplasmatic reticulum .

  21. 7.22 Postulated pathway of egg activation in the sea urchin (Part 1)

  22. 7.22 Postulated pathway of egg activation in the sea urchin (Part 2)

  23. 7.30 Sperm-zona binding Sperm - SED1- ZP complex Galt activates G-protein that open Ca channels and initiate the acrosome reaction.

  24. 7.33 Entry of sperm into a golden hamster egg (Part 1) • Sperm fusing the egg. • Sperm head passing through the zona pelucida • Hamster sperm fusing parallel to the egg plasma membrane.

  25. 7.33 Entry of sperm into a golden hamster egg (Part 2)

  26. 7.34 Pronuclear movements during human fertilization Microtubules- green DNA- blue White arrows: sperm tail