Fertilization and activation

1. - eggs are fertilized externally. - eggs are surrounded by a jelly coat.

2. Fertilization in Sea Urchins

3. Prevention of polyspermy – only one sperm can enter -

4. Fast block-Depolarization of membrane

5. Slow block-Cortical reaction resulting in fertilization membrane

6. 1)Cortical reaction - follows the fusion of thousands of enzyme-rich cortical granules with the egg membrane.

7. 4)-Cortical granules release contents between the membrane and vitelline envelope. -Creates an osmotic gradient

8. 2)-One cortical granule enzyme causes the vitelline envelope to harden. * Now called the fertilization membrane. *Block to polyspermy is now complete. -Similar process occurs in mammals.

9. 3)-The increased Ca2+ concentration in the egg after the cortical reaction results in an increase in the rates of cellular respiration and protein synthesis. -The egg is activated.

10. 5) -In a fertilized egg of a sea urchin --Many events occur in the activated egg

11. ex: Sea urchins - models for the study of the early development of deuterostomes .

12. Fusion of Pronuclei

13. After sperm and egg membranes fuse, the sperm loses its flagellum

14. Fusion of male and female pronuclei forms a diploid zygote nucleus.

15. Contact between sperm and egg

16. -Broadcast spawners : often release a chemotactic factor that attracts sperm to eggs. -Species specific -Sperm enter the jelly layer.

17. Egg-recognition proteins on the acrosomal process bind to species-specific sperm receptors on the vitelline envelope

18. Fertilization

19. Molecules and events at the egg surface play a crucial role in each step of fertilization

20. Sperm penetrate the protective layer around the egg

21. Receptors on the egg surface bind to molecules on the sperm surface

22. Changes at the egg surface prevent polyspermy, the entry of multiple sperm nuclei into the egg

23. Enlarged sperm nucleus is the male pronucleus and migrates inward to contact the female pronucleus