Review for the third hour exam, part two 19) When a salamander limb regenerates, the muscles near the cut surface "dedifferentiate" (become undistinguishable from cells that had been skeletal cells), and then grow and divide until the cell mass is nearly as big as the amount of tissue removed. Then muscle cells re-differentiate (only) into muscle cells, and skeletal cells redifferentiate (mostly) into skeletal cells. Based on these facts, What can you conclude about the mechanism of pattern formation in limb buds? Is it by rearrangement of cells according to cell type? Hint: yes. Or is it by re-differentiation of cells according to position? > What can you conclude about the usefulness or need for undifferentiated stem cells? 20) Compare these alternatives to what H. V. Wilson hypothesized about the reformation of functional anatomy by dissociated sponge cells. Hint: Wilson assumed that sponges re-formed by re-differentiation of the equivalent of stem cells, or at least by switching from one cell type to another; he hated the idea that being differentiation causes cells to move actively to their correct relative locations, although rearrangement is really what happens, and is also what Wilson is credited with discovering. 21) By radioactive labeling just one cell type at a time (e.g. just the muscle cells or just the chondrocytes (= cartilage cells), researchers discovered that cells don't switch from one cell type to another during regeneration. (New muscles are made of cells that had been muscle cells in the cut off stump, etc.) Assuming this information is true, then argue pro or con whether regeneration of limbs should be considered logically equivalent to the sorting out of dissociated cells. 22) Argue pro or con: If the muscles of a regenerated leg consist entirely of cells that were muscles in the stump, and if all the skeletal cells in the stump become skeletal cells in the regenerated leg, that means that regeneration results from rearrangement of differentiated cells, instead of what most people assume (spatial control of undifferentiated stem cells). 23) What would be some medical uses of a method that could cause cells of one differentiated cell type to convert to cells of a different cell type? 24) Describe the sequence of events that occur when a newt or other salamander regenerates one of its legs. 25) What is a blastema? Where do blastemas develop? Suppose you put a blastema into tissue culture. What might happen? 26) When salamanders regenerate the skeleton and musculature of one of their legs, do any of the previous chondrocytes redifferentiate as muscle cells? Hint: No? What is some experimental evidence that would support or contradict this interpretation? 27) Likewise, do any of the previous muscle cells redifferentiate as skeletal cells? And what evidence would be needed to prove or disprove this? *28) Discuss why, or why not, you would or would not have expected these results, drawing on several particular facts and principles that you have learned in other parts of the course. *29) What are some facts that would have led you to expect these results? *30) In terms of each of the following hypothetical phenomena, please suggest possible reasons why salamanders can regenerate legs but mammals, birds, frogs and reptiles cannot regenerate legs. And for each of these hypothetical explanations, please propose at least one experiment that would be capable of either confirming or disproving the theory. a) Maybe (in mammals, birds and reptiles and frogs), all their myoblasts differentiate into myotubes (=skeletal muscle cells), leaving no undifferentiated myoblasts, but even in adult salamanders enough undifferentiated myoblasts remain (enough to provide the muscles for the regenerating legs)? b) Maybe only salamander muscles can dedifferentiate and separate back into undifferentiated muscle cells? c) Maybe only salamander leg cells continue to be able to crawl and exert traction, sufficiently to rearrange leg cells into their correct anatomical patterns? . d) Maybe only salamander tissues continue to be sensitive to "Positional Information"? e) Maybe only salamander tissues continue to produce "Positional Information"? f) Maybe only salamander cells can switch from one cell type to another? g) Maybe only salamanders do not need an apical ectodermal ridge in order to form a leg, and (also maybe) no vertebrate (except tadpoles) can re-form a new apical ectodermal ridge? Hint: Maybe none of them can regenerate new AERs, but salamanders don't need an AER. (And, yes, I know that regenerating salamanders do form a thickened cap, but not an AER.> *31) Based on your knowledge of the shape of cross sections through the tips of developing vertebrate legs (including the shape of the apical ectodermal ridge), and also based on what you know about relations between surface curvatures, tensions, and pressures, and the abilities of curvatures and tensions to vary as a function of direction, suggest combinations of changes and differences in tensions and curvatures of limb bud surfaces could explain their shapes. Suggest experiments that could test your hypotheses. 32) If the surfaces of limb buds contract with equal strength in all directions at all locations, and if their inside pressure is equally strong everywhere, then those combinations of mechanical properties would cause limb buds to become what shape? Hint: hemispherical 33) Based on the rule that the tensions in the surfaces of cylindrical tubes are exactly twice as strong in the circumferential direction as compared with the tension in the longitudinal direction, by means of what changes do limb buds change from being hemispheres to becoming round-ended cylinders? For the same reasons, how do hemispherical aggregates of Dictyostelium amoebae change into long cylindrical "slugs" Hint: By doubling their surface tension in the circumferential direction relative to surface tension along the proximo-distal axis (the same thing as medio-lateral axis) 34) What combinations of symmetry do each of the following have? Limb buds before they develop an apical ectodermal ridge Apical ectodermal ridges Hint: a long narrow ridge less symmetrical than either a flat place or a hemisphere) . Would you consider that formation of an apical ectodermal ridge is an example of "symmetry breaking"? 35) Think about some alternative explanations for Apical Ectodermal Ridges: Recapitulation of the evolution of legs from fins A side-effect of directional changes in tension needed for leg elongation A means of breaking symmetry of limb structures A boundary between dorsal and ventral sides of limb buds Please try to invent some others... 36) What are three embryonic organs whose medio-lateral, anterior-posterior and dorso-ventral axes become irreversibly decided during early development? Hint: Limb buds, inner ears, retinas 37) Are all three axes decided simultaneously, or one at a time, or sometimes one at a time, but other times simultaneously? 38) What are two mesodermal organs that start development as two separate organs (right and left), but then their tissues fuse? Hint: heart, uterus 39) Why do limbs branch into three when a limb bud is grafted backwards? 40) How could you produce a 6 legged salamander (or chicken)? (Perhaps as extras in a John Carter of Mars movie?) 41) Draw a sketch of the Apical Ectodermal Ridge in a bird embryo. Contrast the AER structure in embryos of birds, mammals, frogs, salamanders, and fish. 42) Can you figure out any logical reason why the same category of protein (Fibroblast Growth Factor) induces third limbs and also causes the Medio-Lateral axis of limb buds? (I don't know myself why this should be true.) Hint: Substituting for the AER in maintaining limb proximo-distal development is kind of like inducing an additional limb to form. . 43) What are at least five normal functions of programmed cell death (apoptosis)? 44) The protein bcl-2 has what effect on programmed cell death? 45) How was bcl-2 discovered, and how is this method of discovery related to the name? 46) What is the distinction between necrosis versus apoptosis? 47) What are caspases? How are they related to programmed cell death? 48) Suppose that you could invent a treatment that would activate caspase in any cells that had an abnormality that only occurs in cancerous cells? 49) What blocks arteries in atherosclerosis? 50) What are two theories to explain how atherosclerosis occurs? What key fact about atherosclerosis is not predicted by either of these theories? 51) What are Liesegang rings; how are they formed; and what do the look like? 52) Compare and contrast the geometric pattern of Liesegang rings with those produced by other reaction diffusion systems. 53) In the photograph of Liesegang rings on the course web page, can you figure out why rows of dots form in some places, in contrast to continuous lines formed in other places? How could you test whether your hypothesis is correct? Hint: The dark crystals precipitate wherever the concentration of silver ions multiplied by the concentration of bichromate ions gets higher than some threshold amount. {NOTE: should Turing, Meinhart and other hypothesizers of reaction-diffusion systems be paying more attention to concentrations of A multiplied by concentrations of B, rather than just their concentrations? Why or why not?} .