Biology 441 Spring 2009

Embryology Biology 441 Spring 2009 Albert Harris

Review Questions for Second Embryology Exam, March 2
Stars * indicate very difficult problems, that probably won't be asked.
Draw the arrangements and positions of the four extra-embryonic membranes of birds, reptiles and birds Name each extra-embryonic membrane, and describe its function during development. Draw and/or describe the two extra-embryonic membranes that are part of development of teleost fish.
Which kind of fish is a major "model organism"? List the special advantages of this species for research in embryology and genetics (and specifically for studies of genes that control embryonic development). What are some advantages (and/or disadvantages) of these fish, relative to fruit flies, for the purposes of either embryologists or geneticists? Name two kinds of fish that are not teleosts: name three that are teleosts.
What is meant by meroblastic? What is meant by holoblastic? The amount and distribution of what material in oocytes is primarily responsible for whether cleavage will be meroblastic or holoblastic? *In the evolution of reptiles from amphibians, there must have been intermediate embryos: Suggest possible intermediates in the patterns of cleavage divisions, and the patterns of gastrulation, in the transition from amphibian-type embryos to reptile (~= bird-like) embryos. * Likewise, suggest stages in the evolution of reptile-like (= bird-like) embryos to mammal-like embryos.
In what ways do blastocysts differ from blastulas? What are "deep cells"? What part of the body do enveloping layers develop into? (Trick question!) *Deduce the stages of development of conjoined twin fish embryos. Why are conjoined twin fish always joined at the tail, and side-by-side? If two human embryos share the same amnion, and also the same chorion, then how did twinning occur? (Draw the stage at which separation occurred.) Is it possible for identical human twins not to share the same chorion? Is it possible for identical twins to share the same chorion without also sharing the same amnion? Is the reverse possible? Explain why, or why not, in each case? Describe the three different ways that identical twins develop in mammals. Based on what evidence is this known?
What is meant by embryonic induction? Who discovered the occurrence of embryonic induction? About when was this category of phenomena discovered? By what experimental method? Using early embryos of what kind of animal? Using embryos of what stage of development? Describe at least three or four other examples of embryonic induction (including what differentiated cell types are stimulated to differentiate, and by signals from what other cells?). Explain why you might (or might not) associate induction with regulative development, in contrast with mosaic development? (But I must tell you that this department's own Professor Bob Goldstein first discovered an example of embryonic induction in the Nematode, C. elegans!) To the extent that differentiation of a certain cell type, such as muscle cells in sea squirt embryos, is controlled by localization of special cytoplasmic materials ("the yellow crescent" in sea squirts), then will that tend to make their development more regulative or more mosaic? If in doubt, explain the logic of your reasoning.
What is meant by colinearity, in relation to the relative locations of hox genes on animal chromosomes and the locations where these genes are transcribed within developing embryos? Does this pattern occur only in insects, or only in vertebrates, or only in invertebrates? What sort of effect does a mutation have to have in order to deserve to be called a homeotic mutation? In what sense do conjoined twins have one more plane of reflection symmetry than they should? *Is this a logical necessity, or an interesting fact? * What does it suggest to you about the underlying causal control mechanisms of normal human development that so many conjoined twins are anatomical mirror images of each other? * Sketch the patterns of hox gene expression that you would expect to find within the tissues of a developing pair of conjoined twins (assuming that they are joined as mirror images of each other). * If conjoined twins were NOT mirror images of each other, then figure out what the pattern of hox gene transcription would be.
What is meant by symmetry "breaking"? Does anything actually have to break? Can something break, in symmetry breaking? What kind of symmetry does a frog egg have before it is fertilized? How much of this symmetry remains after the grey crescent is formed? Does this remaining symmetry remain in the adult frog or salamander? What symmetry does an adult starfish have? What symmetry does a pluteus larva have? What about the symmetry of an unfertilized echinoderm egg? ...Of an unfertilized human egg cell? Turing's "reaction diffusion system" of hypothetical chemical reactions would serve to break what kind of symmetry? *What are Liesegang rings? *Would you consider that Liesegang rings are (or are produced by) a reaction diffusion system?
Describe & name the kind of symmetry or combination of symmetries of any letter of the alphabet, or other geometric pattern (for example, Z has two-fold rotational symmetry, A has one plane of reflection symmetry, * has six planes of reflection symmetry), and also the symmetries of embryos at different stages of development.
* The yolk sac is to bird development what the _________ is to mammal development? * The chorion is to bird development as the __________ is to teleost fish development? And explain the analogies in each case.
The locomotion of body cells is more like that of which category of amoebae? Which specific differentiated cell types are capable of crawling locomotion? Describe the forces that propel crawling tissue cells. Will body cells move preferentially onto (and into) more materials to which they are more adhesive? What is meant by "Haptotaxis"? Is this preferential adhesive movement onto more adhesive materials because cells are physically pulled by the physical process of forming more or larger adhesions? (Hint: No). By what method were the locations and directions of this force discovered?
Can cell traction forces move non-living materials around in the body? [Hint: Yes] What are two examples of structural materials being moved around inside living animals? What happens to fluorescently-labeled collagen injected into developing embryos? Does the collagen need to be from the same or different species than the embryo that it is injected into? [Hint: No]
* Many cancers have consistent abnormalities of shape and behavior, which are visible in tissue culture. Cancer cells' locomotion and shapes do not change as much as normal cells do in response to being cultured on relatively adhesive as on relatively non-adhesive plastics or other materials: what explanations can you hypothesize?
* Try to invent hypotheses to explain the following facts: Most non-cancerous cells become shaped differently when crawling on less adhesive surfaces, as compared with how they behave on glass or other sticky surfaces. On less adhesive surfaces, the normal cells mimic shapes and movements of cancer cells. ** Cancer can be induced by several kinds of solid materials, including some forms of asbestos, if these solids become imbedded in your tissues. ** Hypothesize the mechanism. * Which is more true: a) That cells pull themselves by adhesions to external materials, or b) That cells are pulled by formation of adhesions (that expansion of adhesions exerts a pulling force)? (Hint (a) is more true)
The "Differential Adhesion Hypothesis" explains sorting out of differentiated cells from one another by assuming which of the two alternatives in the preceding question? Does the author of our textbook seem to accept the Differential Adhesion Hypothesis as having been proven true? (Hint, he sure does!) Compare the opinions of Hans Holtfreter and J. P. Trinkaus about this ("DAH") theory. Compare their opinions about complicated theories, in general, as applied to embryology. In terms of the "DAH" what would it mean about differences between normal and cancer cells if they could sort out from each other, starting with random mixtures, and the cancer cells tended to wind up selectively on the outside surface of the masses of cells? True or false: The anatomy of every kind of multicellular animal consists of some particular geometric arrangement of differentiated cells? (Hint, yes, although it's a little more complicated than just arrangement of cells; collagen and other extracellular materials also have to get arranged, and epithelial cells have to get the correct orientations.
What is a reaction-diffusion system? What is an example of a combination of chemical reactions and diffusion rates that will generate a spatial pattern ("break" translational symmetry). [Imagine there were two chemicals, X and Y, and Y diffused faster than X, and X caused increases in its own concentration... and what other rules would need to be obeyed? Who was Turing, and what key idea did he contribute to embryology? What are some possible structural patterns (ribs, stripes, etc.) that might be caused by some reaction-diffusion system? ÊAre reaction-diffusion systems a hypothetical way of generating embryonic pre-patterns? (Hint: Yes) ÊÊSuppose that the differentiation of cells to form ribs was caused to occur at each location in the developing body where the concentration of a certain signal molecule was higher than some threshold: would the pattern of variation of concentration of this signal substance be a pre-pattern? (Yes). Please explain why that system of controlling where ribs form would NOT be an example of "positional information". Argue pro or con whether Liesegang rings are a result of a reaction-diffusion system. * Can you invent a bioassay for identifying chemicals that participate in a reaction-diffusion system? Briefly explain Wolpert's concept of the worm with the anatomical pattern of the French flag. Also, how is this supposed to help explain embryonic regulation? Does this theory fit the actual observations of concentration patterns of the "bicoid" protein in early fly embryos?
What are some examples of fusion of membranes that are necessary parts of normal fertilization? What is an acrosome? What kind of cell has an acrosome? What kind of cell has 20,000 +/- cortical vesicles? What is a vitelline membrane? How is it related to the zona pellucida? How does the fast block to polyspermy work? What ions leak from inside the oocyte to outside it? What ions leak into the oocyte? How does the speed of this process compare to that of a nerve impulse? Describe two different kinds of slow blocks to polyspermy, and state what classes of animals have which kind.
Describe how sperm cells meet oocytes in sea urchins, sea squirts, fish, amphibians, reptiles, birds and mammals. From a distance of several feet away, how can you tell if frog eggs have been fertilized? What cytoplasmic rearrangements take place in oocytes after fertilization? What determines the anterior - posterior axis of the embryo in nematodes? in frogs?
What hormones control ovulation and sperm production in mammals? Where are they secreted? How do birth control pills inhibit ovulation? Where does fertilization take place in humans? How does the cleavage pattern in mammalian embryos differ from that in fish, frogs, or birds? What is compaction? How is it related to implantation? What happens when implantation occurs in the fallopian tube?
Remember that there are also review questions at the end of some of the pages of lecture notes.