Chapter 47
I) Stages of Early Embryonic
Development
1) preformation vs.
epigenesis
A) Fertilization
1) The acrosomal
reaction
a) enzymes
released by
acrosome
b) acrosomal
process penetrates
the jelly coat
c) proteins on tip of
process bind to
receptors on vitelline
layer, just outside of
plasma membrane
c1) lock and key:
only sperm from that
species will fit
d) membrane of sperm
fuses with
membrane of egg
d1) ion channels
open in egg,
allowing Na into egg
d2) this polarizes
egg, blocking entry
of any more sperm =
fast block to
polyspermy
2) The Cortical
Reaction
a) egg releases
Ca2+ from ER at
site of sperm entry
b) this spreads
throughout cytosol,
causing release of
more Ca2+
c) Ca causes cortical
granules to fuse with
membrane and release
contents into perivitelline
space
d) vitelline layer separates
from membrane and
hardens, creating a
fertilization envelope =
slow block to polyspermy
http://www.youtube.com/watch?v=9zv3TRl
ctTY
http://worms.zoology.wisc.edu/urchins/pha
secal.html
3) Activation of the egg
a) transport of H+ out
raises pH, allowing an
increase in
metabolism and
protein synthesis
a1) some protein
synthesis occurs even
without nucleus, so
mRNA was stockpiled
in egg
a2) activation can be
artificial and without
nucleus, showing
that molecules
involved in activation
were also present in
cytoplasm
4) Fertilization in
mammals
a) similar to
invertebrates
b) zona pellucida
surrounds the egg,
which hardens after
the cortical reaction
to be the slow block
to polyspermy
B) Cleavage
1) rapid division of
embryo with no
cellular growth
between divisions
a) small cells called
blastomeres
2) the following occurs
except for mammals
a) High amounts of
yolk are stored in
vegetal pole
b) animal pole has
little yolk, and
divides faster
c) in amphibians, animal
pole is dark gray.
c1) cytoplasm rotates
toward point of sperm
entry, leaving a grey
crescent
**sperm can only enter
in the animal pole
along the
animal/vegetal border
3) first two rounds of
cleavage occur
along animal-vegetal
pole axis
4) next round divides
perpendicular to
this, mainly in
animal pole because
vegetal pole is too
thick.
a) holoblastic
cleavage: complete
division of cells(sea
urchin and frog)
b) meroblastic
cleavage:
b1) incomplete
division from large
concentration of yolk
* birds: yolk is an
egg cell, with small
disc of yolk free
cytoplasm at top of
animal pole.
5) Morula: solid ball of
cells
6) blastula: has a fluid
filled cavity called
blastocoel
C) Gastrulation
1) production of 3
(germ) cell layers
a) ectoderm: skin,
hair, nervous system
b) endoderm:
digestive tract
c) mesoderm:
muscles, bones,
blood
2) gastrulation of sea
urchin
a) invagination:
mesenchyme cells
at vegetal pole
migrate toward
animal pole and pull
vegetal plate up
b) pouch formed is
called archenteron
(primitive gut)
c) blastopore:
opening of
archenteron
3) gastrulation in frog
a) invagination of
cells forms a dorsal
lip at the
blastopore (at gray
crescent)
b) cells migrate around
embryo toward blastopore,
until a full circle is formed,
creating a yolk plug.
http://www.youtube.co
m/watch?v=ojqXV06
2CNI
animation
D) Organogenesis
1) Notochord
a) dorsal mesoderm
condenses
2) Neural tube
a) dorsal ectoderm
folds downward and
pinches off, creating a
tube
video
3) Somites
a) clusters of
mesoderm that
surround the neural
tube and turn into
vertebrae/muscles
E) Amniote embryos
1) amniotic sac: fluid
filled sac
surrounding the
embryo
2) birds
a) early cleavage
creates a blastodisc,
a cap of cells on the
yolk that create the
embryo
a1) epiblast
a2) hypoblast
a3) blastocoel
b) cells of upper layer
roll down and into
blastocoel
b1) forms a
primitive streak on
top of blastodisc
(shows anteriorposterior alignment)
b2) embryo only
comes from epiblast
b3) germ layers
outside of embryo
become extra embryonic
membranes
* yolk sac: nutrients
* amnion: fluid filled
sac around embryo
* allantois: uric acid
storage/ gas
exchange
* chorion: shock
support/ gas
exchange
3) Mammals
a) blastocyst: circle of
cells with an inner
cell mass
b) inner cell mass:
cells that will
become embryo and
extra-embryonic
membranes
c) trophoblast: outer
cells that form fetal
half of placenta
d) follows similar
gastrulation as bird
d1) epiblast and
hypoblast form
d2) yolk sac forms
and creates blood
cells that migrate
into embryo
d3) allantois turns
into umbilical chord
II) Cellular and Molecular Basis of
Differentiation
A) changes in cell
shape, position, and
adhesion
1) microtubules
shorten or elongate
to change the shape
of the cell
2) Convergent
extension
a) cells wedge in
between each other,
making the region
narrower, but longer
3) Extracellular matrix
a) glycoproteins
secreted by cells to
control their, and
other cells’,
movements
4) Cell Adhesion
Molecules(CAMs)
a) glycoproteins on
the surface of a cell
that bind to other
CAMs
b) cadherins: CAM
that requires Ca to
function properly
B) Fate Mapping
1) cells and their
progeny can be
traced using dyes or
other markers
C) Cytoplasmic Determinants
1) Polarity and body
plan
a) bilaterally
symmetrical
organisms have:
a1) anteriorposterior axis
a2) dorsalventral axis
a3) left and right
sides
b) established by
positioning of
materials in the egg
2) Restriction of
cellular potency
a) cleavage can
divide materials
evenly or unevenly
a1) inclusion of
grey crescent allows
full development
D) Inductive signals drive
differentiation
1) The “organizer” of
Spemann and
Mangold
a) found that the
dorsal lip of
blastopore controls
neurulation
a1) called this
region the “primary
organizer”