1. Chapter 47
Animal Development

2. I. Fertilization
Sea Urchins
Mammals

3. A. Sea Urchins Acrosomal Reaction
a. Release enzymes that “eat through” the jelly coat
b. Acrosomal process tip adheres to receptor molecules on egg’s vitelline layer
c. Plasma membranes fuse
d. Nucleus of sperm enters egg
e. Ion channels open and sodium ions enter (1-3 seconds after sperm binds to vitelline layer) / fast block to polyspermy

5. A. Sea Urchins (cont) Cortical Reaction
a. When sperm and egg fuse a STP produces DAG and IP3 which cause the ER to release Calcium ions
b. Vesicles called cortical granules fuse with the plasma membrane and release contents into perivitelline space
c. Perivitelline layer swells and hardens (fertilization envelope) / slow block to polyspermy / 20 seconds in

7. A. Sea Urchin (cont) Activation of Egg
a. High Calcium ion levels increase metabolism
b. Sperm and egg nuclei merge 20 minutes in
c. Cell division occurs at 90 minutes

9. B. Mammals
Sperm reaches the zona pellucida and ZP3 “receives” the sperm
Acrosomal reaction occurs
The cell depolarizes / fast block
Cortical reaction occurs
Zona pellucida changes as a result / slow block
Microvilli take in sperm
Nuclei of both cells dispense their DNA and divide together

10. Movie

11. II. Formation of Blastula

12. A. Cleavage Rapid cell division occurs
Undergo the S and M parts only / no real G1 or G2 so the embryo does not get bigger
Creates smaller blastomere cells
Zygote has an animal (anterior) and vegetal (yolk) pole
The gray crescent is identifiable (dorsal side)
First 2 splits are meridional and the next are equatorial / 8 cells

13. Blastomere cells

14. B. Blastula Formation
Cleavage continues until there is a solid ball / morula (mulberry)
Blastocoel (fluid-filled cavity) forms
Get a hollow ball or blastula
If there is a lot of yolk just the animal pole divides / meroblastic cleavage
If there is a little yolk, complete division / holoblastic cleavage

15. Morula
Blastocoel
Blastula

16. III. Gastrulation Dramatic rearrangement of cells in the blastula
Cell motility, changes in cell shape, changes in adhesion affect gastrulation
Develop 3 cell layers: ectoderm (skin and nervous system), endoderm (digestive system), and mesoderm (muscle)
Sea Urchin
Frog

17. A. Sea Urchin
Cells detach from the vegetal pole and blastula wall and enter the blastocoel as mesenchyme cells
Rest of the vegetal cells form the vegetal plate and go through invagination
Vegetal plate rearranges more and invaginates more creating the archenteron / filopodia “pull archenteron up
The opening is the blastopore and is the anus
MOVIE!

19. Archenteron
Mesonchyme cells
Filopodia
Blastopore

20. B. Frog
Dorsal lip of the blastopore develops from where the gray crescent was
Cells on the surface “curl” into the lip / involution
Continue to curl up along the roof of the blastocoel
Cells from animal pole spread down over the outer surface
Blastocoel shrinks as the cells keep advancing
Yolk plug forms

21. Movie
Bullet 3

23. IV. Organogenesis Germ layers develop into organs
Folds, splits, and dense clustering of cells
Notochord and neural tube form first by folding in of the mesoderm and neural plate
Amniotes have a fluid sac (amnion) inside an egg or uterus

24. Frog Organogenesis

25. V. Avian (Amniote) Development
Meroblastomic cleavage occurs
Blastodisc forms
Blastomeres sort into the epiblast (embryo develops from this) and the hypoblast / blastula
Epiblast cells move to the midline, detach and move down towards the yolk / produces the primitive streak (like blastopore)
Cells move laterally forming mesoderm / some move down forcing hypoblast out and form endoderm

27. V. Avian Development (cont)
Cells pinch in and form layered tubes
Organogenesis occurs like the frog
Extraembryonic membranes form 4 layers that help egg development:
1. yolk sac – covers the yolk and digests it
2. amnion – encloses embryo in fluid sac / cushions
3. chorion – cushions / respiratory organ
4. allantois – disposal sac for uric acid / respiratory organ

30. VI. Mammalian Development
Holoblastic cleavage occurs slowly ( )
Compaction occurs at 8-cell stage and new proteins form on the surface of the cells (cadherins)
At 7 days 100 cell blastocyst is formed / inner cell mass is present and outer layer is called the trophoblast
This implants in the uterine lining
Trophoblast initiates implantation and extends finger-like projections into the endometrium / will form the placenta

32. V. Mammalian Development (cont)
Inner cell mass flattens and epiblast and hypoblast form
Extraembryonic membranes form
Gastrulation occurs when cells from the epiblast move in through primitive streak
Chorion develops from trophoblast / amnion is a cavity that eventually covers the 3 layers / yolk sac has no yolk, blood cells come from here / allantois is incorporated into umbilical cord
Organogenesis starts with neural tube, notochord, and somites

34. VII. Morphogenesis

35. A. Changes in Cell Shape, Position, and Adhesion
Reorganization of cytoskeleton
Convergent Extension
Cell Adhesion Molecules (CAM) / cadherins / regulate if cells move or not

36. Changes in Shape

37. Convergent Extension

38. B. Cell Fate
Heterogenous distribution of cytoplasmic determinants (not mammals)
Induction – interactions among cells themselves
- Spemann and Mangold “organizer” cells / dorsal lip of the blastopore

40. C. Pattern Formation Development of spatial organization
Relies on positional information from molecular cues
- organizer regions like AER and ZPA