Chapter 47

Animal Development

I. Fertilization

A. Sea Urchins

B. Mammals

A. Sea Urchins

1. Acrosomal Reactiona. Release enzymes that “eat through” the jelly coatb. Acrosomal process tip adheres to receptor molecules on egg’s vitelline layerc. Plasma membranes fused. Nucleus of sperm enters egge. Ion channels open and sodium ions enter (1-3 seconds after sperm binds to vitelline layer) / fast block to polyspermy

A. Sea Urchins (cont)

2. Cortical Reactiona. When sperm and egg fuse a STP produces DAG and IP3 which cause the ER to release Calcium ionsb. Vesicles called cortical granules fuse with the plasma membrane and release contents into perivitelline spacec. Perivitelline layer swells and hardens (fertilization envelope) / slow block to polyspermy / 20 seconds in

A. Sea Urchin (cont)

3. 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

B. Mammals

1. Sperm reaches the zona pellucida and ZP3 “receives” the sperm

2. Acrosomal reaction occurs

3. The cell depolarizes / fast block

4. Cortical reaction occurs

5. Zona pellucida changes as a result / slow block

6. Microvilli take in sperm

7. Nuclei of both cells dispense their DNA and divide together

Movie

II. Formation of Blastula

A. Cleavage

1. Rapid cell division occurs

2. Undergo the S and M parts only / no real G1 or G2 so the embryo does not get bigger

3. Creates smaller blastomere cells

4. Zygote has an animal (anterior) and vegetal (yolk) pole

5. The gray crescent is identifiable (dorsal side)

6. First 2 splits are meridional and the next are equatorial / 8 cells

Blastomere cells

B. Blastula Formation

1. Cleavage continues until there is a solid ball / morula (mulberry)

2. Blastocoel (fluid-filled cavity) forms

3. Get a hollow ball or blastula

4. If there is a lot of yolk just the animal pole divides / meroblastic cleavage

5. If there is a little yolk, complete division / holoblastic cleavage

Morula

Blastula

Blastocoel

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)

A. Sea Urchin

B. Frog

A. Sea Urchin

1. Cells detach from the vegetal pole and blastula wall and enter the blastocoel as mesenchyme cells

2. Rest of the vegetal cells form the vegetal plate and go through invagination

3. Vegetal plate rearranges more and invaginates more creating the archenteron / filopodia “pull archenteron up

4. The opening is the blastopore and is the anus

MOVIE!

Mesonchyme cells

Archenteron

Filopodia

Blastopore

B. Frog

1. Dorsal lip of the blastopore develops from where the gray crescent was

2. Cells on the surface “curl” into the lip / involution

3. Continue to curl up along the roof of the blastocoel

4. Cells from animal pole spread down over the outer surface

5. Blastocoel shrinks as the cells keep advancing6. Yolk plug forms

MovieBullet 3

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

Frog Organogenesis

V. Avian (Amniote) Development

A. Meroblastomic cleavage occursB. Blastodisc formsC. Blastomeres sort into the epiblast (embryo

develops from this) and the hypoblast / blastula

D. Epiblast cells move to the midline, detach and move down towards the yolk / produces the primitive streak (like blastopore)

E. Cells move laterally forming mesoderm / some move down forcing hypoblast out and form endoderm

V. Avian Development (cont)

F. Cells pinch in and form layered tubes

G. Organogenesis occurs like the frog

H. 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

VI. Mammalian Development

A. Holoblastic cleavage occurs slowly (36-60-72)B. Compaction occurs at 8-cell stage and new

proteins form on the surface of the cells (cadherins)

C. At 7 days 100 cell blastocyst is formed / inner cell mass is present and outer layer is called the trophoblast

D. This implants in the uterine liningE. Trophoblast initiates implantation and extends

finger-like projections into the endometrium / will form the placenta

V. Mammalian Development (cont)

F. Inner cell mass flattens and epiblast and hypoblast form

G. Extraembryonic membranes form H. Gastrulation occurs when cells from the

epiblast move in through primitive streakI. 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

J. Organogenesis starts with neural tube, notochord, and somites

VII. Morphogenesis

A. Changes in Cell Shape, Position, and Adhesion

1. Reorganization of cytoskeleton

2. Convergent Extension

3. Cell Adhesion Molecules (CAM) / cadherins / regulate if cells move or not

Changes in Shape

Convergent Extension

B. Cell Fate

1. Heterogenous distribution of cytoplasmic determinants (not mammals)

2. Induction – interactions among cells themselves

- Spemann and Mangold “organizer” cells / dorsal lip of the blastopore

C. Pattern Formation

1. Development of spatial organization

2. Relies on positional information from molecular cues

- organizer regions like AER and ZPA