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Chapter 47
Animal Development
I. Fertilization
A. Sea Urchins
B. Mammals
A. Sea Urchins
1. 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
A. Sea Urchins (cont)
2. 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
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
Blastocoel
Blastula
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!
Archenteron
Mesonchyme cells
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 advancing
6. Yolk plug forms
Movie
Bullet 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 occurs
B. Blastodisc forms
C. 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 lining
E. 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 streak
I. 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