Problem Set #2
Due Feb 2, 2012
Grader: Na
Office Hours: BI 11, Mon& Fri 5-6pm
1
a). In your own words, describe 4 cell movements during gastrulation. (use 1
sentence for each movement). (2pt)
self-explanatory
b). Name one difference between chick and mouse gastrulation. (Hint: the
direction of node movement) (1pt)
chick gastrulation: node moves from anterior to posterior
mouse gastrulation: node moves from posterior to anterior
2.
Will the organizer form in an embryo with no cortical rotation? (1pt) What
about ventral and dorsal tissues? (1pt)
No. Without cortical rotation, beta-cat will not be stabilized on the dorsal
side. As a result, the nieuwkoop center will not form. The nieuwkoop center
induces the formation of the organizer. Therefore the organizer will not form
without cortical rotation. Without the organizer, there will be no dorsal
tissues and the embryo will be ventralized.
3.
a) Name an early signal for mesoderm induction. Where is it expressed in a
frog embryo? (0.5pt)
VegT---ventral side of the embryo
or
Beta-cat-dorsal side of embryo
b). In a mutant screening project, you found a frog embryo with no
mesoderm formation. The protein expression of beta-catenin is comparable
to that of wild type embryos. What then is causing the mutant phenotype?
Suggest one possibility (1pt) and design experiments to validate your
hypothesis. (0.5pt)
Perhaps vegT is missing or mis-expressed in this embryo. To test this
hypothesis, perform in situ hybridization and immunohistochemistry to look
at vegT expression. You expect to find no vegT mRNA in the ventral side of
this mutant embryo. If this is the case, you can then inject vegT mRNA in to
the ventral side of this mutant embryo and expect see partial or complete
recovery of mesoderm formation.
Problem Set #2
Due Feb 2, 2012
Grader: Na
Office Hours: BI 11, Mon& Fri 5-6pm
4. A previous post-doc working on your new SURF project isolated sonic
hedgehog (shh) in zebrafish and your PI has told you that her preliminary
results suggest shh acts like a morphogen. However, the post-doc and your PI
had a falling out and the notes were destroyed.
a. What role does the notochord and the non-neural ectoderm play in
specification of cell types in the spinal cord? (0.25 points)
The notochord and the non-neural ectoderm set up the dorsal-ventral (DV)
specification in the neural tube. [The roof of the neural tube is exposed to
signaling molecules like BMP4 and BMP7 secreted from the epidermis
(non-neural ectoderm), and the floor of the neural tube is exposed to Shh
secreted from the notochord.]
b. Discuss the concept of a morphogen and how it can work in cell type
specification in the spinal cord. (1 point)
Morphogens are proteins made in specific sites in the embryo, diffuse over
long distance, and form concentration gradient where the highest
concentration is at the point of synthesis and gets lowers as the morphogen
diffuses away from its source and degrades over time.
e. g. In the neural tube, shh is secreted from the notochord, ventral to the
neural tube, and hence its concentration is the highest in the ventral side.
As it diffuses upwards, the concentration gets diluted along the DV axis
(high in ventral, low in dorsal). On the other end, BMP4 and BMP7 are
secreted from the non-neural ectoderm overlying the neural tube. These
proteins diffuse downward along the DV axis (high in dorsal, low in
ventral). Different types of neurons of the neural tube are formed by their
exposure to these gradients of paracine factors. (e.g. high shh induces
formation of motor neuron in the ventral, while high BMPs induces
formation of sensory neurons in the dorsal) (Chapter 12, p384.)
c. Where is shh expressed in the trunk of the embryo? (0.25 points)
Shh is expressed in the notochord and the floor plate, with the highest
concentration in the ventral side of the neural tube.
d. How would you experimentally test whether shh is a morphogen. (1.5
points)
Design an experiment to show that the concentration of shh is important
for formation of different types of neurons in the neural tube.
e.g. take the undifferentiated neural tube cells, treat it with different
concentrations of shh. Should varied concentrations of shh caused
formation of different neuronal types, it suggests that shh functions in a
gradient manner and is a morphogen