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Experimental Embryology
Class 2
Blastula to Gastrula to Neurula
Inductive events
Definitions
Blastula = hollow sphere of cells
A blastula is formed by multiple sub-divisions of cells,
followed by rearrangement of the cells to form a
space in the middle (blastocoel)
Gastrula = gastrulating embryo
Gastrulation is a series of morphogenetic (‘form’ +
‘production’) movements in which cells rearrange
and migrate
Cleavage - Blastula stage
Sea urchin
Xenopus
Making a space in the middle (blastocoel)
How?
–  Changes in cell-cell adhesion
•  higher affinity for neighbors and hyaline membrane (sea
urchin)
–  Tight junction, secretion and change in osmolarity
(frog)
Cadherins anchor epithelial cells to each other
and to the cytoskeleton
Depletion of EP-cadherin mRNA in Xenopus oocyte
Why do you need a space
(blastocoel)?
1.  A space for cells to move into
2.  Keep cells apart to prevent inappropriate interactions
Gastrulation: some examples
for background
In amphibia,
gastrulation begins at the dorsal blastopore lip
G 10.7
G 10.7
Cells move in by involution, first through the dorsal lip, then through the
lateral and ventral lips
Chick
Gastrulation in chick
Epiblast -> 3 germ layers +
extraembyonic tissues
Hypoblast-> extra-embryonic
tissues
G 11.13
G 11.15
Regulative Development
(1)  an isolated blastomere has a potency greater
than its normal embryonic fate, and
(2)  a cell's fate is determined by interactions
between neighboring cells. Such
interactions are called inductions.
What are examples of ‘induction’?
Most embryos depend on segregation of cytoplasmic
determinants AND induction to make cells different
8.11
Xenopus embryo cleavage
Many embryos have asymmetrically distributed proteins
and mRNA
Initial cleavages often separate these components equally
But later cleavages can make cells different
Basic (different) fates = Anterior, Posterior,
Dorsal and Ventral
An example of fate determination by segregation of
maternal components
Normal cleavage
plane bisects the
gray crescent,
which is on only
one side of the
embryo
Forced, aberrant cleavage
One cell gets gray crescent
One cell doesn’t
UV irradiation produces the same “belly” piece
Conclusion: something important is happening at the onecell stage!
Aberrant cleavage:
One cell contains grey
crescent; the other does not
Interlude: What are “dorsal” structures?
Neural ectoderm (the brain and spinal cord) most dorsal
Dorsal mesoderm: notochord, muscle
The other forms of mesoderm are:
intermediate (kidney, gonad)
ventral (heart, blood)
So the question of why altering the
cleavage plane alters
development is really a
question about how the
embryo establishes a dorsal/
ventral axis
10.11
How cortical rotation works from a molecular standpoint
10.25
Gray crescent area has high βcatenin levels, and becomes the
future dorsal lip of the blastopore,
where gastrulation begins
The dorsal blastopore lip can induce gastrulation even
when placed somewhere else..
10.20
The discovery of the ‘organizer’ Spemann & Mangold
Ventral
10.20
Cells from transplant: red
Host cells: yellow
Dorsal
The first inductive event actually helps to establish the
mesodermal cell types
Late
blastula
10.22
What induces mesoderm?
Search for mesoderm inducers:
knew it was diffusible
knew it was from vegetal cells
Candidate: activin
3.19
This compound appeared to act like a “morphogen”
The “French flag” model for patterning by a morphogen
Morphogen
concentration
Distance from morphogen
source
French flag
3.18
Vg1
10.22
There is also induction of neural fate prior to the formation
of the neural tube
But then this experiment was done
Neural “Induction”
…really an inhibition
of an inhibitor
Sanes, Reh and Harris, 2000
Noggin is expressed in the right time and place to be an organizer
molecule
Dorsal
Marginal
Zone
Notochord
10.30
Dorsal
Lip
Noggin “rescues” ventralized embryos by inducing dorsal
mesoderm and dorsal ectoderm (restores function of
organizer)
UV-irradiated
embryos exposed to
increasing levels of
Noggin
10.29
BMP inhibitors from the Organizer:
Noggin
Chordin
Nodal-Related 3
Follistatin
For next week
•  Tuesday: Read Roux and Driesch
•  Answer questions provided (online link)
to help guide reading and understanding
•  Draw a diagram/ “cartoon” of the two
sets of experiments described (one for
Roux, one for Driesch)
•  Thursday: Read Gurdon (online link)