Gastrulation begins 180° from site of sperm entry
1. Blastopore Formation
sperm
entry
(That looks
familiar!)
Fig. 7.6
How do cells
choose fates before
gastrulation?
The dorsal-ventral
axis in the frog
Animal pole
A frog’s
mother sets
up the
initial axis
of polarity
Vegetal pole
An outline of frog gastrulation
Future Ventral Side (belly)
Sperm always
enters at the
animal hemisphere
Future Dorsal side (back)
Sperm entry is the critical asymmetric cue
in setting up the dorsal-ventral axis
of the frog Xenopus
See Figure 7.1
Sperm entry triggers a rotation of the outer cortex
relative to the inner cytoplasm, fixing the site of
gastrulation initiation
Sperm entry
point
Animal
cytoplasm
Vegetal
cortex
Yolky
cytoplasm
Gray crescent = junction of
vegetal cortex with animal
cytoplasm
Animal
cortex
30
gastrulation
initiates
here.
cortex
rotation
Rotation occurs during the first division,
and requires the sperm centriole
Experimental alteration of cortical rotation changes the
blastopore location
Sperm entry
1. Rotate egg so that sperm
entry point is now on top.
Sperm entry
point
point
2. Prevent normal
cortical rotation
Sperm entry
point
gray
crescent
3. Gravity rearranges cytoplasm
so that the junction of
vegetal pole cortex with animal
pole cytoplasm is next to sperm
entry point instead of opposite
it.
4. Gastrulation
now occurs next to
sperm entry point
Information from neighbors:
Mother cell
Cell division
Cell type A
Cell type B
Cell division places
daughter cells in different
environments, which can
lead to different cell fate
choices
Induction: information from
neighbors influences cell fate
responder
inducer
Competence: ability to respond to a certain
inductive signal
responder
Cell not competent to respond
inducer
Succesive inductions: can generate many
cell types from just a few interactions
inducer
Cell not competent to respond
responder
Types of signals
Inducer
Responder
Release into the
Fallopian tube
Signals can
act globally
throughout the
body
Signals can also act
in a graded fashion
Induction:
An initial difference can be
amplified into many cell types
How is cell fate determined
-- the dorsal-ventral axis in newts
See p. 255
Spemann & Mangold: the organizer
can influence neighboring cells to
form a secondary body axis:
Figure 7.17
Induction of the
organizer
can influence
neighboring cells
to form a
secondary
body axis:
If one transplants a second inducer
of the organizer the embryo forms two body axes
Figure 7.19
Induction of
mesoderm in
the frog embryo
Initial asymmetry of Xenopus egg leads to some
cell fate differences
Animal pole
Vegetal pole
Mesoderm arises at endoderm/
ectoderm junction
Neither vegetal or animal
pole alone can make mesoderm.
Experiment:
Figure 7.18
Vegetal pole cells can induce animal pole cells to
make mesoderm
Fig. 7.18
Conclusion: a signal in the vegetal part is needed to induce
mesoderm in the animal cap.
Induction of dorsal mesoderm by dorsalmost vegetal cells
The 3-step model of mesoderm induction
#3
#1
#2
Figure 7.18
The 3-step model of mesoderm induction
#3
#2
#1
Model for mesoderm induction and organizer formation
Figure 7.23
Neurulation separates the ectoderm into
neural and epidermal cell fates
Neurulation is driven by cell shape
and adhesion changes similar to those
that drive gastrulation
Alberts Figure 20.26
Dorsal-Most Cells
Neural tube
formation
Figure 9.4
Differential adhesion helps the neural
tube and skin to separate from each
other
ectoderm
E-cadherin
N-cadherin
See Fig 9.6
Hatta and Takeichi (1986) Nature
Experiment: Differential adhesion helps the
neural tube and surface ectoderm separate from
each other
ectoderm
Inject N-cadherin mRNA so
that surface ectoderm
expresses N-cadherin
Fig 9.6
Neural crest cells
migrate from the CNS
to form the peripheral
nervous system
Defects in neural tube closure are
among the most common birth defects
Figure 9.5
Numbers indicate regions of
neural tube closure.
It is estimated that 50% of human neural tube
defects would be prevented if all women of
childbearing age take supplemental folic acid
(vitamin B12)
400 mg/day
Expression of a folate receptor in mice
neural tubes prior to fusion
Figure 9.7