Transcript
1. (2.5) Despite clear superficial differences, frog and chick gastrulation have similarities. For each of the following structures or regions in frog gastrulation, name what you think most likely to be the homologous chick structure or region, and justify your answer, pointing out similarities and differences.
What is the chick homolog of the frog:
a) (.5)blastopore
The primitive streak. Like the blastopore, it is the opening through which originally cells induced to become mesoderm by the node ingress into the space under the blastodisc.
b) (1) Spemann’s organizer (blastopore lip). Propose an experiment to test your suggestion.
Hensen’s node. Although it moves anteriorly during gastrulation, unlike the organizer, it initiates gastrulation, induces the epithelial-to-mesenchymal transition of blastodisc cells to mesodermal precursors, and plays a key role in patterning the ectoderm and mesoderm along the A-P axis. To test this, you could transplant early Hensen’s node tissue to the anterior end of a host blastodisc and see if twinning resulted. (An even better answer might be: the early organizer corresponds to the blastodisc cells in the posterior marginal zone. The later blastopore lip corresponds to the node. Either could be tested in transplantation experiments.)
c) (1) Nieuwkoop center. Propose an experiment to test your suggestion.
The endodermal cells just beneath the blastodisc cells in the posterior marginal zone. Again, the test would be a transplantation experiment, placing these cells under the anterior blastodisc cells to test for twinning.
2. (2) Assume that you can ectopically express chordin on the ventral side of a Xenopus gastrula stage embryo. Describe two effects you would predict to result from the ectopic chordin expression and explain why.
Secondary neural tube formation: chordin binds BMP4, preventing it from ventralizing. Thus, cells on the ventral side will become dorsal instead of ventral
Ventral mesoderm will be transformed into dorsal mesoderm: ventral mesoderm will be prevented from adopting ventral fates and instead be transformed into dorsal mesoderm (like notochord, for example).
3. (1.5)
a. (.5) As a normal Drosophila embryo is developing, two cells in the region of the future central nervous system, A and B are contacting each other. Cell B has 3 times the number of Delta ligands on its surface than cell A. They have equivalent numbers of Notch receptors. Which of the two cells above will become a neuroblast?
a. A
b. B
c. Neither
d. Both
b. (.5) In a Dl loss of function embryo, which cell will become a neuroblast?
a. A
b. B
c. Neither
d. Both
c. (.5) If B is mutant for Dl but A is not (ie, a mosaic), which cell will become a neuroblast?
a. A
b. B
c. Neither
d. Both
4. (3) In the figure below, the results of an experiment are shown regarding the effects of levels of Sonic hedgehog (Shh) on the differentiation of neurons. Cells from chick neural tube were cultured in vitro in the presence of Shh, then immunostained for proteins characteristic of the different types of neurons shown in the picture.
a. (1) Make a conclusion about the role of Shh from the data shown in the figure.
b. (2) Explain how cells in the neural tube, some of which are exposed to both dorsal and ventral signals, end up taking on a specific fate.
Shh is a signaling molecule that acts as a morphogen. Ventral fate is dependent on concentration of sonic hedgehog. Farther away from source of sonic hedgehog, less ventral the fate.
b. combinatorial control! A cell's fate will be determined by the levels of specific molecules that it is exposed to. In this way, it is very similar to the drosophila embryonic patterning.
5. opinion question
