1. An important consequence of fertilization: Quiescent (resting) Egg’s metabolism is transformed into a highly active metabolic state - - - > CLEAVAGE

2. Start with first Division ->

3. Cleavage is a stage of VERY fast cell divisions. It starts immediately after fertilization. But, how do we mark its end? Cell cycling slows, indicating end of cleavage stage

4. Typical cell cycle

5. Typical accelerated cell cycle during cleavage stage

6. occurs for most phyla

7. Cell division includes cytokinesis and karyokinesis

8. Table 8.1. Karyokinesis and cytokinesis Major disruptive drug LocationMajor protein composition Mechanical agentProcess Colchicine nocodazole a Central cytoplasmTubulin microtubules Mitotic spindleKaryokinesis CytochalasinBCortical cytoplasmActin microfilaments Contractile ringCytokinesis

9. In MOST cases, cleavage reduces a large egg volume into many smaller cells The relation of nuclear volume to cytoplasmic volume changes during cleavage stage The ratio of: nuclear volume/cytoplasmic volume in each cell, and in the whole organism, increases

12. Two Important Cleavage Geometry Factors -Positioning (& timing) of spindle *maternal factors *here-also sperm influence Here, physical positioning of spindle according to microtubules contributed by sperm cell

13. Two Important Cleavage Geometry Factors -1 Positioning (& timing) of spindle -2 Retardation of traversal of yolk during cytokinesis

14. Retardation of traversal of yolk in cytokinesis

15. Up to Here: Cleavage, generalized. note also: 3. strategies of nutrition 4. when are axes specified? NOW, the specific cases

18. Sea Urchin Cleavage

20. Vade Mecum

21. Blastocoel

22. -Basal lamina chondroitin sulf. collagen laminin fibronectin Peri-vitelline space -Hyaline

24. Spiral holoblastic Cleavage:e.g. Molluscs

27. Factors that regulate spindle placement: Genetic proof for maternal factors.

31. Vade Mecum

33. Cleavage in Zebrafish

35. Chicken – our first amniote Amniotes Lay eggs on land: Reptiles Birds...Mammals END

36. Chicken –AND Zebrafish Discoidal

38. Tunicate: Sea Squirt - Urochordate

41. Cleavage in Zebrafish -Restart 0.5 mm

42. Large to huge amount of yolk Moderate amount of yolk Very little yolk Tetrapods: Even monotremes, but smaller

44. ‘Our’ cleavage (as placental mammals) is very different from birds and reptiles We’ll look at non-placental mammals to bridge the gap, in order to understand ‘our’ cleavage

45. Monotremes – egg laying mammals: Figure 1. Photomicrograph of a uterine egg (4mm diameter) of O. anatinus (Platypus) showing an ellipsoidal blastodisc with eight blastomeres (BL) exhibiting meroblastic cleavage. Scale bar, 2mm. אכידנה Echidna ברוזן Platypus

46. Monotreme vs. Frog 11 mm

47. 1 mm Urchin, human

48. Monotreme vs. Frog, Urchin, Human eggs (and human thumb)

49. Marsupial Phylogenetic relationship of tetrapods, and of Amniotes

50. Chicken – well studied amniote Amniotes Lay eggs on land: Reptiles Birds...Mammals Center of disc – embryo. Edges - extraembryonic

51. Monotremes – egg laying mammals: Figure 1. Photomicrograph of a uterine egg (4mm diameter) of O. anatinus (Platypus) showing an ellipsoidal blastodisc with eight blastomeres (BL) exhibiting meroblastic cleavage. Scale bar, 2mm. Echidna Platypus

52. Diagram of a Monotreme Egg. 1) Shell; 2) Yolk; 3) Yolk Sac; 4) Allantois; 5) Embryo; 6) Amniotic Fluid; 7) Amniotic Membrane; 8) Membrane CHICK

53. NOW: Eutherians – placental mammals:

60. Figure 1 Amniote key adaptation: the amniotic egg Hard-shelled egg or internal gestation. Removed need for moist environment for reproduction.