1. Group Quiz 6 Intro to Development
Today:
Group Quiz 6
Intro to Development
Thursday: Review Paper Draft Due for Peer Review!

2. Backtracking: Advantages and Disadvantages of Sexual Reproduction?

3. Generalizations about Sexual Reproduction
Offspring created by the fusion of haploid gametes to form a diploid zygote.
The female gamete (ovum) is generally large and nonmotile.
The male gamete (spermatozoan) is generally smaller and motile.

4. Generalizations about Sexual Reproduction
Some animals are Hermaphrodites (single individual has both male and female reproductive systems)
Common in sessile or burrowing animals, and in parasites
Advantages? Does this generate genetic diversity?

5. Fertilization Overview
Fertilization: the union of sperm and egg
Can be EXTERNAL or INTERNAL

6. External Fertilization
Photo by Scott Egan,; University of Rhode Island Dept. of Natural Resources; A pair of wood frogs, Rana sylvatica, in amplexus

7. Internal Fertilization
Internal fertilization requires cooperative behavior
and
Sophisticated reproductive systems including organs to deliver sperm and receptacles for its storage and transport to eggs.

8. Mammalian Gamete Production: Females

9. Mammalian Gamete Production: Males

10. Mammalian Gamete Production: Males

11. Preformation vs Epigenesis
Aside: Fertilization
Preformation vs Epigenesis

12. Fertilization
Combines haploid sets of chromosomes to form single diploid zygote
Activation of the egg
Model Organism = Sea Urchin
Why Urchins??

13. Studying Sea Urchin Fertilization
Source: NASA- Photo credit - Joseph Tash

14. Fertilization: The Acrosomal Reaction
External Fertilization in Sea Urchins
Sperm exposed to molecules from the jelly coat surrounding an egg, the acrosome discharges its contents by Exocytosis
= Acrosomal Reaction
The acrosomal process (elongating structure) then penetrates the jelly coat of the egg and adheres to a specific receptor molecule

15. Fertilization: The Acrosomal Reaction
What needs to happen once one sperm nucleus has entered the egg?!

16. Fertilization: Fast Block to Polyspermy
Fusion of the sperm and egg membrane causes ion channels in the egg’s membrane to open
Sodium ions flow into the cell, causing a membrane depolarization (change in membrane potential)
Prevents more than 1 sperm from entering (1-3 seconds!)

17. Membrane potential is restored within 2 minutes! (Uh oh…)

18. Fertilization: Slow Block to Polyspermy
The Cortical Reaction:
Fusion of sperm and egg triggers release of calcium from the ER into the cytosol
Calcium release begins at site of sperm entry and propagates across the fertilized egg

19. Fertilization: Slow Block to Polyspermy
The Cortical Reaction:
High calcium concentration causes the cortical granules to fuse with plasma membrane
Mouse oocyte stained to show cortical granules (small red dots). Genomic DNA in the metaphase plate of the secondary oocyte (top right) appears bluish-white. (From Biology of Reproduction 57: , 1997, Z. Xu, A. Abbott, G. Kopf, R. Schultz and T. Ducibella)

20. Fertilization: Slow Block to Polyspermy

21. Fertilization: Slow Block to Polyspermy
Enzymes from the cortical granules separate the vitelline layer from the plasma membrane
Water is drawn into the perivitelline space by osmosis, swelling it
The swelling pushes the vitelline layer away from plasma membrane where it is hardened by enzymes to form the Fertilization Envelope

22. Fertilization: Slow Block to Polyspermy

23. Fertilization: Activation
Rise in Calcium also induces metabolic changes within the egg
Metabolism increases rapidly!
Nucleus of sperm starts to swell
Sperm nucleus merges with egg nucleus ~ 20 minutes)
First division ~90 minutes

24. Sea Urchin Larva at 13 days
Timing:
Sea Urchin Larva at 13 days

25. Fertilization: Sea Urchins Vs. Mammals
Internal Fertilization (terrestrial!)
Key Initial Differences:
1. Capacitation: molecules in mammalian female reproductive tract alter surface of sperm and increase the motility (~ 6 hrs)
2. Mammalian egg cloaked by follicle cells- capacitated sperm cell must migrate through this layer to reach the zona pellucida

26. Fertilization: Sea Urchins Vs. Mammals

27. Fertilization: Sea Urchins Vs. Mammals
The Zona Pellucida = filamentous network of glycoproteins (extracellular matrix)
One of the glycoproteins, ZP3 functions as a sperm receptor
Binding induces the acrosome to release its contents (as with sea urchins)
Enzymes from acrosome help sperm penetrate the zona pellucida

28. Fertilization: Sea Urchins Vs. Mammals
As with sea urchin, binding of egg triggers depolarization of egg membrane (fast block to polyspermy)
A Cortical Reaction functions as the slow block (granules in cortex release contents; enzymes catalyze changes in the zona pellucida)

29. Fertilization: Sea Urchins Vs. Mammals

30. Unfertilized Star Fish Eggs
Fertilization Reviewed
Unfertilized Star Fish Eggs

31. Fertilized egg (zygote) - the nucleus is no longer visible; the fertilization envelope is present
Let’s Watch:

32. Fertilization is followed by 3 stages:
Stages of Development
Fertilization is followed by 3 stages:
1. Cleavage
2. Gastrulation
3. Organogenesis

33. CLEAVAGE Succession of rapid cell divisions
Partitions the cytoplasm into smaller cells, BLASTOMERES, each with its own nucleus
Each region of cytoplasm contains different cytoplasmic components

34. 2-cell and 4-cell stage of Sea Urchin Cleavage (Blastomeres)
What happened to the size of each blastomere??
2-cell and 4-cell stage of Sea Urchin Cleavage (Blastomeres)

35. Polarity of Zygote and Egg
Most animals (except mammals) produce asymmetrical eggs and zygotes
Distribution of yolk, mRNA and proteins is not uniform
Sets the stage for subsequent developmental events!

36. (Becomes the dorsal side)
Yolk is concentrated at the vegetal pole.
The opposite pole is the animal pole
(Becomes the dorsal side)

37. Polarity of Frog Eggs
In the final stage, a frog oocyte is pigmented dark brown in one hemisphere (animal pole).
The other hemisphere (vegetal pole) shows the yellow color of the egg yolk.

38. The third division is horizontal, producing eight cells.
Cleavage
The first two cleavage divisions are vertical producing four cells extending from animal to vegetal pole.
The third division is horizontal, producing eight cells.
Continued divisions (16-64 cells) produce a solid ball of cells, the morula.

39. A fluid-filled cavity, the blastocoel forms within the morula, creating a hollow ball of cells, the blastula.
Above right. Human- day 3-4
Note the difference the presence of yolk makes!

40. Sea Urchin Morulas - 8-16 cell stage
Sea Urchin Blastula - 32-cell stage, blastocoel

41. Sea Urchin: Late Blastula - shows thickened layer of cells at one end, the vegetal pole

42. Morula (16-64 cells)  Blastula (>128 cells)
A fluid-filled cavity, the blastocoel, forms within the morula creating a hollow ball
Urchin
Source: courtesy of Dr. J. Hardin, Univ. of Wisconsin

43. ectoderm, endoderm and mesoderm
Blastula  Gastrula
Changes in cell motility, shape and adhesion result in the spatial rearrangement of an embryo
Results in three familiar embryonic tissue layers:
ectoderm, endoderm and mesoderm

44. 2. Gastrulation
Source: raven.zoology.washington.edu/ embryos

45. Sea Urchin: Early Gastrula - blastocoel, blastopore, primitive gut (archenteron)

46. Sea Urchin: Late Gastrula - endoderm, ectoderm, mesoderm

47. Blastula  Gastrula in a Frog

48. Frog Development Great model organisms: HUGE eggs! (> 1mm)
External fertilization and development
Vertebrates!
Fertilization  Neuralation in 18 hours under good conditions

49. anatomy.med.unsw.edu

50. Zooming in on the Frog Gastrula

52. How do we know??

54. Fertilization is followed by 3 stages:
Stages of Development
Fertilization is followed by 3 stages:
1. Cleavage
2. Gastrulation
3. Organogenesis

55. In chordates, the neural tube and notocord form first
3. Organogenesis
Morphogenetic changes – folds, splits and clusterings (condensation) begin the process of organ building
In chordates, the neural tube and notocord form first

56. Sea Urchin: the bipinnaria larva (bilateral symmetry) develops into a brachiolaria larva; this larva undergoes metamorphosis to become the adult starfish, which has radial symmetry.

57. Earliest Organogenesis in Frog (Chordate) Embryos

58. Let’s Watch –

59. Formation of the Neural Tube in a Chordate
Species: Mouse Day Gestation: 8