Answers, problem set B

11) stomach - endoderm

heart - mesoderm

spinal cord - ectoderm

lung - endoderm

ribs - mesoderm

12) a) Sea urchin vegetal plate cells that become primary mesenchyme cells lose adherence to each other and to the hyaline layer, and gain adherence to the basal lamina.

b) A subset of cells undergo these changes.

c) These cells mecome mesodermal tissues

13) The animal hemisphere of a Xenopus oocyte is less dense than the vegetal hemisphere. The animal hemisphere has the nucleus as well as some pigments, whereas the vegetal hemisphere has yolk proteins. The unfertilized oocyte has radial symmetry around the animal-vegetal axis (which is parallel to gravity). Fertilization causes cortical rotation and thereby creates bilateral symmetry.

14) What kinds of tadpoles will be produced by the following treatments of recently fertilized Xenopus eggs:

a) no special treatment? normal tadpoles

b) UV light irradiation? ventralized (no dorsal side)

c) UV irradiation followed by turning the egg on its side? normal tadpoles

d) turning the egg (without UV irradiation)? two-headed tadpoles

15) Animal hemisphere cells give rise to ectoderm, and vegetal cells give rise to endoderm. Mesoderm comes from cells between the future endodermal and ectodermal cells. The brain comes from anterior dorsal ectodermal tissue.

16) Raymond is trying to map the developmental fates of different cells in an amphibian species. To do this, he cuts a 16-cell embryo into clumps of tissue, cultivates the clumps of cells, and observes how they develop.

a) What do you expect the indicated cells will develop into in this experiment?

The animal pole cells will form ectodermal tissue.

The medial cells will also form ectodermal tissue because they haven't been induced yet.

The vegetal pole cells will form endodermal tissue.

b) Is the map that Raymond develops an accurate representation of the future fates of these cells in the normal embryo? Explain your answer.

No. Raymond has developed a specification map, which differs from the true fate map because it doesn't take into account the signaling events that will modify the developmental trajectories of the different cell types.

c) Describe the experimental method you would use to determine a fate map for an embryo with 1000 cells.

To construct the fate map, take multiple embryos at the 1000-cell stage and mark different cells in each embryo with a visible marker (such as a fluorescent dye). Then allow the embryos to develop and watch where the fluorescent dye ends up in each embryo at later stages. The correspondence between the cells that were labelled at the 1000-cell stage and the cells that fluoresce at a later stage indicates the fates of the labelled cells.

17) X doesn't have to be the inducer because it may not be necessary to induce a dorsal axis, or it may normally be present in the wrong place or at the wrong time.

18) While the embryo grew overnight, the animal pole cells that responded on the first day apparently lost competence to respond. Presumably they became determined to adopt some fate, and were no longer receptive to the signal X.

19) a) animal cells become ventral mesoderm, vegetal cells become endoderm

b) animal cells become dorsal mesoderm, vegetal cells become endoderm

c) Either the animal pole cells became determined, or the vegetal cells stopped producing the signal.

20) a) Tilt it in a second dimension (other than that of the normal cortical rotation)

b) Transplant dorsal vegetal cells at the 32- or 64-cell stage (the Nieuwkoop center) to the ventral vegatal region of a recipient embryo or transplant the dorsal blastopore lip of an embryo about to begin gastrulation to a recipient.

21) Multiple signals may combine to produce distinct responses in cells that perceive different combinations of signals. A common signal may induce different responses in different cells if the cells have different regulatory components (receptors, transcription factors, etc.) in place.