1. Step one: Contact between the sperm and the eggshell

2. Step two: Contact between the sperm and the egg’s plasma membrane

3. Fertilization: a multistep process (ECM)

4. Sperm-eggshell contact triggers the acrosome reaction Example 1-- the sea urchin enzymes Species specificity (equivalent to zona pellucida)

5. attracts/activates sperm Figure 7.8

6. Sperm-eggshell contact triggers the acrosome reaction Example 2: Mammals Sperm have ZP3 receptor

7. Figure 7.8

8. BLOCKING POLYSPERMY 1.Fast/transient: prevent sperm fusion 2.Slow/permanent: removal of other bound sperm

9. Fast block 1-3 seconds lasts 1 min Slow block 20-60 seconds permanent

10. How do we know this? We can measure these events directly! Figure 7.17Membrane potential of sea urchin eggs before and After fertilization. resting -70 (sperm can fuse with egg) (sperm can’t fuse with egg) +20 mV

11. Cortical granules: poised for release 15,000 cortical granules in a sea urchin egg contain enzymes that clip the egg’s bindin receptor and any attached sperm alter the vitelline envelope (zona pellucida in mammals) Blocking polyspermy: SLOW BLOCK Figure 7.6

12. Sperm-eggshell contact triggers the acrosome reaction Example 2: Mammals Sperm have ZP3 receptor

13. A wave of increased calcium can be visualized moving across the egg from the point of sperm entry Figure 7.20

14. Fast block 1-3 seconds lasts 1 min Slow block 20-60 seconds permanent

17. Fig. 8.15 Fig. 8.20 Cleavage and Gastrulation Gastrulation

18. Many embryos fail to successfully complete early cleavage divisions or gastrulation CLEAVAGE GASTRULATION

19. EARLY CLEAVAGE  rapid increase in the number of cells after fertilization  egg contents distributed to cells, often asymmetrically blastomeres

20. Early BlastomeresNormal Cells The cell cycle is different during cleavage Much shorter cell cycles during early development-- no growth phases and cells rely on stored material Figure 5.1 mitosis DNA synthesis

21. Mitosis-promoting factor (MPF) Mitosis is dependent on presence of Cyclin B Figure 5.1 Activation of MPF controls start of cleavage

22. actintubulin mitotic spindle contractile ring Fig. 5.2 CLEAVAGE and the cytoskeleton

23. II. MEROBLASTIC (INCOMPLETE CLEAVAGE) A. Telolecithal (Dense yolk throughout most of cell) Fig. 5.3 Different organisms have distinct cleavage patterns

24. What controls the pattern of cleavage? 1.Amount and distribution of yolk in the egg 2.Factors in the egg that influence the angle and timing of mitotic spindle formation

25. Sea urchin egg yolk-rich Fig. 5.6

26. II. MEROBLASTIC (INCOMPLETE CLEAVAGE) A. Telolecithal (Dense yolk throughout most of cell) Fig. 5.3 Different organisms have distinct cleavage patterns

27. Discoidal cleavage in fish and birds  creating a cellular region above the dense yolk Figure 7.40 zebrafish

28. What controls the pattern of cleavage? 1.Amount and distribution of yolk in the egg 2.Factors in the egg that influence the angle and timing of mitotic spindle formation

29. Radial cleavage in sea urchins: simple planes of cleavage a) Placement of mitotic spindles

30. Synchronous vs. asynchronous cleavage 1  2  4  8  even cell stages odd numbered cell stages are frequent b) Timing of mitotic spindle formation

31. c) Symmetric vs. asymmetric cell divisions Fig. 5.6

32. Mammalian cleavage is unique

33. Rotational cleavage in mammals Figure 8.16 meridional equatorial (radial cleavage)

34. Synchronous vs. asynchronous cleavage 1  2  4  8  even cell stages odd numbered cell stages are frequent

35. Compaction during mammalian development: A sudden change in cell adhesion 8-cell embryos: uncompacted compacted

36. Compaction requires the cell adhesion protein E-cadherin Control embryos Embryos treated with antibodies blocking E-cadherin

37. Development of a Human Embryo From Fertilization to Implantation Fig. 8.15 8-cell stage compaction

38. Early mammalian development follows (in function) the pattern seen in other animals morula

39. hatch from zona pellucida Fig. 8.15 Fig. 8.20 Development of a Human Embryo From Fertilization to Implantation

40. End of cleavage = mid-blastula transition controlled by nuclear/cytoplasmic (N/C) ratio

41. The Mid-Blastula Transition Growth phases added to Cell Cycle Embryo starts transcribing its own RNA Cells start moving around in preparation for gastrulation

42. Mid-blastula transition and the N/C ratio Lets say you have an organism which normally undergoes 4 cell divisions before the MBT pre-cleavage: N/C ratio = 1/16 = 0.0625 (close to 0) after 4 cell divisions (1 > 2 > 4 > 8 > 16): 16/16 = 1