Embryology Biology 441 Spring 2009 Albert Harris
More review questions for the 2009 final exam.*stars mean difficult questions, of which not too many would be asked**a) Describe and draw the three methods by which Abercrombie proved the existence of contact inhibition of cell locomotion, and measured the amounts of this contact inhibition. For each of the three methods, what would the results have been if he had used some kind of cell in which there was zero contact inhibition? (For example, if a particular kind of cancer cells had zero contact inhibition.) b) Many people believe that there is also such a thing as contact inhibition of cell growth and division. Please re-design each of Abercrombie's three methods so that they would detect whether cell-cell contacts inhibit cell growth and mitoses, and produce quantitative measures of the amount of this inhibition. c) If identical twins are both inside the same amniotic cavity, then at what stage of development did the separation occur? What if identical twins have separate chorions; what does that mean about the stage of development at which splitting occurred? Finally, what is the third way in which identical twinning occurs, and which extraembryonic membranes would be shared, and which not shared, as a result of this third method by which identical twinning sometimes occurs? d) Can identical twinning occur in bird embryos? (There is a photograph posted on one of the web sites for this course). Do there have to be two separate yolks in order for two baby chickens to be formed inside one egg? e)* If a pair of conjoined bird twins are developing on the surface of the same egg yolk, then would you expect both twins always to have the same number of somites (at any given time)? In particular, what theories would be disproved if somite formation was observed NOT to be synchronous in the bodies of the two conjoined twins? Suppose that somite formation is synchronous in conjoined twins, but was not synchronous in the bodies of twins that are not conjoined, but are developing on the surface off the same yolk? (If in doubt, look up "clock and wave-front" in the index of our textbook, although the answer to this question would not depend on that particular theory.) f)* Conjoined human twins are usually, and maybe always, mirror images of each other, and are joined to each other at the same part of each body (hip to hip; head to head, etc.). Propose one or more possible explanations for this, in terms of the normal signaling or other mechanisms that coordinate formation of organs in the body. Conversely, are there any theories of development that would be disproved if pairs of identical twins sometimes were not mirror images of each other? g)* Suggest how "Ontogeny recapitulates phylogeny" may result from the greater probability of mutations producing harmful effects if they occur in those genes that are active in earlier stages of embryonic development. h)*Arms, legs and bird wings all develop from what start out as hemispherical bumps along the sides of embryos. If split in two, they can branch into two legs, etc. ? What change in symmetry occurs in these branching limbs, as compared with the change in symmetry that occurs in the development of a normal leg? Compare this formation of two legs from one limb bud with what Driesch discovered and with what happens when barriers are put between the left and right rudiments of heart development. i) In terms of the mathematical relations between the curvature of a flexible interface (like a sheet of cells), the pressure difference on one side as compared with the other, and the tensions in these interfaces, explain why capillary walls can be so thin, even though the pressure inside capillaries is almost as large as the pressure inside arteries (which have very thick, strong walls, and which inflate and burst when artery walls are not strong enough). j) *There are serious ("thermodynamic") theories that differentiated cells differ in "surface tension". For example, pieces of liver would have lower surface tension than pieces of heart, and pieces of cartilage would have higher surface tension than heart tissue. How would you expect the surface curvatures to differ during a process of side-to side fusion between blobs of different tissues? k) Mutations have been produced that cause embryos not to be able to make the protein type I collagen, and it was discovered that these always die midway through embryonic development, with the cause of death being bursting of many arteries. Why do you think this occurs? How do you interpret this in terms of the normal processes of embryonic development? l)* In a recent research paper, some German scientists measured the tension of the surface of aggregates of fish cells by poking a microneedle into their surface, and measured how much inward force was needed to dent the surface to a certain depth. Krieg, Arboleda-Estudillo, Puech, Käfer, Graner, Müller & Heisenberg, Nature Cell Biology vol 10 number 4 April 2008 pages 429-436. Tensile forces govern germ-layer organization in zebra-fish. Can you deduce the logical basis of their measurements and reasoning? m)* Please argue pro or con whether this experimental method is (or is not) capable of settling the 30-year old debate whether maximization of cell-cell adhesions is the force that causes cell aggregates to round up and resist flattening (what our textbook calls the "thermodynamic" mechanism), or whether active contraction of cells at aggregate surfaces is what drives rounding up and resists flattening. This research paper believes it has settled the issue, in favor of the latter theory; but wouldn't supporters of the thermodynamic theory have predicted the same differences in microneedle force needed to produce a dent? n)* Please argue pro or con whether the needle-denting method of "measuring" tension at a surface treats a tensor variable (mechanical tension) as if it were a scalar variable (i.e. which cannot vary with direction)? o)* Can you invent an improved method that can measure directional variations of tension in the surfaces of cells? Perhaps by mapping the shapes of the dents produced by the microneedles? Are these dents conical? Or does their width differ with direction? p) Would you expect the tension in the wall of a capillary (or other blood vessel) to vary with direction? (specifically, tension along the length of the capillary, as compared with tension in the circumferential direction, around the capillary?) q) Can you regard this as a symmetry question? If pressures and tensions have spherical symmetry (the same strength in all directions), then what is the only shape that they can produce? r) In an artery wall, (at least in a healthy artery) the long axes of the smooth muscle cells are strongly oriented around the circumferential direction, and so are the axes of the collagen fibers. In terms of their functions, why is this directional orientation important? s)* Suppose that some of these smooth muscle cells began to contract in random directions: what effect would you expect this to have on the effective strength of the artery wall, and on the orientations of other cells and fibers in the artery wall? (hint: try to imagine a vicious cycle, in the sense of positive feedback with bad results). t) Imagine an anti-cancer drug that is not poisonous in the form that is injected into the patient, but that reacts with lactic acid to produce a chemical that is strongly poisonous to individual cells. Explain why this might be useful. u) Imagine another anti-cancer drug that can be converted to a poison if phosphate groups become covalently attached to it? Explain why it might be useful? v) Imagine yet another anti-cancer drug that becomes poisonous when it encounters unusually high concentrations of cytoplasmic actin that is NOT assembled into actin fibers. w) Imagine a 20 amino-acid peptide, that only cancer cells could cleave into two 10 amino-acid fragments; and also imagine that people could be vaccinated against one or other of the 10-amino-acid fragments, so that T-lymphocytes would attack and initiate programmed cell death in all cells that contained the 10-amino-acid fragments, but would not attack cells containing the original 20-amino-acid peptide. x) If a certain kind of leukemia is caused by synthesis of an abnormal enzyme, that is never produced by normal cells, and that causes cells to grow and divide without control, then rank the following treatments: good, better, best?
A drug that is converted into a poison by this abnormal enzyme, but is not affected by normal enzymes? A drug that blocks the synthesis of this abnormal enzyme? A drug that selectively kills faster-growing cells? A drug that slows down growth rates in normal cells, but has no such slowing effect on leukemia cells? The combination of the preceding two drugs?
z) The original journal article that proposed the modern theory of how the immune system works called it the "Natural Selection" theory of antibody production. Briefly explain the similarities between Darwin's theory of evolution of millions of different species over many millions of years, as compared with the mechanism within each person's body that produces billions of B-lymphocytes and T-lymphocytes during your embryonic development, and subsequent life (and avoids making antibodies whose binding sites fit antigens that are normal parts of your own body).
A) If you have type A blood, or type AB blood, then you would quickly die if any of your B-lymphocytes secreted antibodies whose binding sites fit the "A blood group antigen". However, people with type O blood and type B blood have lots of B-lymphocytes that secrete antibodies that bind to the A antigen.
Which is the explanation:
2) During the embryonic development of each person with type A blood, the VDJ recombination method did create B-lymphocytes which made antibodies that fit the A antigen, but in people who inherit that antigen, some unknown mechanism either killed, inactivated or re-programmed those and all other anti-self lymphocytes? 3) By means of genetic linkage, people with the genes for type A blood never have the genes that code for antibodies whose binding sites fit the A antigen? C) Because humans and other mammals have two X chromosomes in females, and in each cell randomly (and permanently) inactivate transcription in one or the other of these chromosomes, how was it possible to take advantage of this fact to prove that cancers and also atheromas (atherosclerotic regions of arteries) are "clonal" in origin, in the sense that they develop by one cell undergoing an inheritable change that makes them grow and spread abnormally. Such clonal origin is in contrast with events in which one cell becomes abnormal, and that induces nearby cells to develop the same abnormality, like catching a disease, instead of inheriting a disease. D) * The word "clone" is used in many different ways in modern biology, sometimes as a noun and sometimes as a verb, and often to mean quite different processes. *Make a list of as many different uses of this word as you can, and the differences between them. **Is the word clone ever used as an adverb or a participle? (Don't worry about this last question!)
E) * With only 2 or 3 exceptions, all autoimmune diseases occur at higher rates in women as compared with men (often MUCH higher frequencies: for example, there is a nine to one ratio in Lupus). Nobody knows why this is, but the usual explanation is that it has something to do with estrogen, if you call that an explanation! (Which I don't! Even if true, it wouldn't be the kind of fact I consider to be explanatory.) F) Why did embryologists not expect that eye development would be stimulated in mammals by transcription factors that have very nearly the same amino-acid sequences as the transcription factors that stimulate eye development in flies and other insects? (So similar, in fact, that the mammal protein can induce fly embryos to form eyes at very abnormal locations in their anatomy!) G) * A Russian embryology textbook that I am reading considers sorting out of dissociated cells to be "non-Drieschian" embryonic regulation. Although we don't usually think about things that way in the US, it seems to me to be a good point. Can you explain it to me?
H) Discuss possible ways to explain why development in some species is so much more mosaic than in more "regulative" species, especially mammals, in terms of the following differences:
> Whether cell differentiation is reversible, or for how long it can be reversed? > Whether cell cleavages always occur in the same geometric patterns? > Whether there is an exact cell lineage? > Whether monozygotic ("identical") twinning ever occurs, or can be caused to occur? > The degree to which cell differentiation is controlled by cell-to-cell signals ("induction")? > The extent to which differentiation is controlled by different cytoplasmic materials, unequally distributed between cells during mitotic cleavages? > The speed of embryonic development? (time between fertilization and independent feeding) > The size of the eventual animal being produced? > Whether geometric variations in anatomy interfere with normal physiological functioning? > The extent to which the eventual locations of differentiated cells is simply the result of where these cells originally differentiated, versus being the result of locomotion of cells from their original location to other positions in the body? > Please invent as many more possibilities as you can.
J) * Explain or argue why you think mosaic development versus regulative development does or not result from some fundamental difference in mechanisms of development. (What differences? How fundamental?) K) Could mosaic development have evolved as a way of avoiding cancer? L) Could regulative development have evolved as a way of improving wound healing and/or regeneration. M) Argue pro or con whether regulative development should be considered as a special case of wound healing, that differs in that it occurs very early in development? N) If Dolly the sheep, or an animal like her, had been produced by transplanting the nucleus of a B-lymphocyte into an enucleated egg cell (instead of using the nucleus of a tissue culture cell derived from a mammary tumor, which is what those people in Scotland actually used), then why would the resulting animal only been able to make antibodies whose binding site had one particular shape. (All the animal's antibodies would have the same specificity! Note: this has actually been done with mice.) O) If an embryologist were to remove a few thousand lymphocytes from early mouse or chicken embryos, keep these in tissue culture (or maybe frozen) for an hour or so, and then inject these cells back into the same animal (or an animal genetically identical to it), and repeat this experiment for many different 1 or 2 hour periods of development, then explain what it would probably mean if the animals consistently developed autoimmune diseases when lymphocytes had been kept out of the body during certain, consistent ages of embryonic development? [Hint: suppose there is a certain age during which anti-self lymphocytes are weeded out.] P) * Can you invent some other embryological experiments capable of determining the time and mechanism by which anti-self lymphocytes are weeded out? Q) Experiments have shown that when dizygotic twin cows develop partly fused placentas, then both these non-identical twins will accept skin and organ grafts from the other one. No immune rejection occurs. Grafts are accepted equally well between identical twins. Explain these events in terms of the mechanism of immune self-tolerance. R)* I have never heard or read that any (or all, or some) auto-immune diseases occur higher or lower frequencies in people who have non-identical twins. What would it suggest about the causal mechanisms of such diseases if they were to occur at unusual frequencies in people who are non-identical twins? S) What if your chance of getting an autoimmune disease were increased by previous treatment with "nitrogen mustard" cancer chemotherapy? What might that tell you about the cause(s) of autoimmune diseases? T) Ditto, the previous question, if the effect of prior chemotherapy were to DECREASE the frequency (& probability) that people will later get some autoimmune disease? U) Early development of vertebrate brains is mechanically analogous to the inflation of a balloon: In this analogy, what plays the mechanical role of the air inflating the balloon? What plays the role of the rubber part of a balloon? What variation of Young's modulus causes the roof of the brain to inflate in a series of sphere-like lobes? (If you look at a picture of the brain of an early chicken or other embryo, it may help you visualize this situation. And if this question gets asked on the final exam, you should be able to include a sketch of the early brain, during its period of hydraulic inflation.) V) If you could invent medicines whose effects were to increase and to decrease the mechanical tension in the cells and collagen fibers of the cornea, then explain how you could use these medicines to treat farsightedness and nearsightedness. How could you use such medicines to replace "Lasik Surgery"? W) * The terms "orphan diseases" and "orphan drugs" refer to situations in which not enough people suffer or die from a given disease to make it profitable enough to develop, test or manufacture drugs to test the disease. * What policy changes would tend to help such situations? Making drug patents last longer? Eliminating drug patents? Creating a government program that subsidizes drug development, without preventing patenting of drugs that result from this program? Giving very large government prizes for cures? What other possibilities can you invent, that would motivate people and companies to search for cures that would only save a few thousand lives? X) Why is the curvature of the surface of the left ventricle of the heart very much larger than the curvature of the surface of the right ventricle of the heart? And why does the inter-ventricular septum bulge toward the right, and become concave on the side toward the left? Y) Deduce what it means that this asymmetry of curvatures of the right and left ventricles develops only gradually after birth. Before birth, the curvatures and thicknesses of the ventricle walls are symmetrical: Why? Why do the atria never become so asymmetrical? Z) ** If banks or insurance companies could die of cancer or lupus, then would enough money get appropriated to cure these diseases?
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