Embryology Biology 441 Spring 2010 Albert Harris
Additional Review Questions for Second Examination:
1) If you can take a wall-paper pattern and either move it 2 inches to the right, or to the left, or move it 4, 6, 8 etc inches to either the right or left, and it would look the same, then what kind of symmetry does it have? (Let's assume it looks different if you move it one inch, or 3.5 inches, or any distance but multiples of 2 inches). 2) If this pattern also looks the same after being rotated 60 degrees, 120 degrees, or 180 degrees, then what additional kind of symmetry does the pattern have? 3) If the pattern is all one color, and looks unchanged when moved an inch, or 1/2 inch, or any small distance, and is also apparently unchanged when rotated by any number of degrees, then does it have more or less symmetry than the patterns mentioned above? 4) When an animal develops regularly spaced stripes, in what way has the symmetry of its skin changed? 5) Suppose an animal develops regularly spaced spots: does it then have more, or less, symmetry than a striped animal? (increased or decreased?) 5.5 What combinations of symmetries do Venetian blinds have? 6) Turing's "Reaction Diffusion" mechanism (or "system") is a way to reduce/increase the d----------t symmetry of a spatial pattern?
7) Contrary to what Gilbert's textbook, and many people, believe, Reaction-Diffusion Systems can/can't produce exactly regular patterns? (Hint: I showed you an actual computer program, generating patterns initiated by random fluctuations.)
Notice that this is true despite them being initiated by random fluctuations; 8) Cooke and Zeeman's "Clock and Wave-Front Model" (hypothesis) is similar in what way, and different in what way, from Turing's? *9) If the gradient is made steeper, in Cooke's and Zeeman's hypothesis, why would that result in smaller somites being made? (try visualizing it). 10) If we have two diffusible chemicals, "X" and "Y", or "A" and "B", or "I" and "P", and one of them promotes increasing the concentration of itself and the other, at rates proportional to its own local concentration, while the other chemical promotes the removal or loss of both substances (at a rate proportional to what?), and if the destroying chemical diffuses several times faster than the other chemical, then what will happen? 10A) What kind of symmetry is "broken" by such a mechanism? 10B) The pattern produced depends on magnifying random fluctuations, and is therefore random? ** Why not? 10C) If you had these chemicals distributed in a circular ring, could the break reflection symmetry? 10D)* If these chemicals were confined to a shorter length, or smaller area, or smaller volume, would the number of peaks of concentration become less, stay the same, or become larger? 10E)* If such a mechanism had dilation symmetry, that would mean it could adjust for reduced amounts of material by doing what? Hint, the Clock and Wave-front theory is designed to make the adjustment? 10F) Suppose the variable "A" was not a chemical concentration, but the population density of moving cells, then could it still participate in a pattern generating mechanism? Hint: what could B be? 10G) How would it change the pattern produced by a Turing mechanism if B became able to diffuse faster than previously? 10H*) What if these reactions were going on in a material through which A can diffuse with equal speeds in all directions, but B can diffuse faster in one axis (the dorso-ventral axis) than in the axis perpendicular to this (anterior-posterior)? 10 I*) If you were considering the hypothesis that Zebra Fish skin color patterns are caused by some variation on Turing's mechanism, and your method was to look for mutant fish with abnormal color patterns, then make a list of all (as many as you can think of) the different color patterns that could result from mutations in the genes for A and B. For each, sketch the pattern and state the effect of the mutation on A or B, either their properties, effects, or initial distributions in the embryonic fish. 10 J*) Don't wear out your brain, but can you do the same for the Clock & Wavefront hypothesis? 10 K) Could the Clock & Wave-front hypothesis generate spatial patterns other than somites? (Hint: Why not?) 11) What symmetries, or combinations of symmetries, do each of the letters of the alphabet possess (approximately)? ABCDEFGHIJKLMNOPQRSTUVWXYZ ! # $ % ^ * <> + = ~
Which have rotational symmetry but not reflection symmetry? 12) When the anterior-posterior axis of an amphibian oocyte is caused to form by the location where the sperm enters, then what change in symmetry has been caused. (And likewise for nematode oocytes).
13*) If you fertilize an amphibian or nematode oocyte exactly at the animal pole, and nevertheless it develops a normal anterior-posterior axis (with only one plane of reflection symmetry), then what does this suggest about the underlying mechanism by which a single anterior-posterior axis is chosen from the infinity axes of reflection symmetry that had existed until then? 14) Which symmetries, or combinations of symmetries are possessed by the following? Starfish? Jelly-fish? Propellers? Flowers? Daisies, Sun-flowers, Orchids, Lilies, Trees? Blastocysts? Gastrulas? Paramecia? Diatoms? Honeycombs? Apples? Bananas? Mitotic spindles? Flagella? Microtubules? Actin fibers? Muscle sarcomeres? Spirally-cleaving embryos? Radially cleaving embryos? [don't worry about these 2] Snails? Clams? Limpets? Barnacles? Feathers? Hairs? Claws? Teeth? Lungs? Glands? Lenses of eyes? Vertebrae? Arteries? Muscles and fibers in the walls of arteries? Bamboo? Fern leaves? * Cauliflower? ** Mulberry leaves! Morulae? Knives? Forks? Spoons? Scissors? Other tools? Suggest something else. {Some of these are rather subtle and difficult; but please give each some thought.} 15) What (abnormal and also normal) planes of reflection symmetry are possessed by the bodies of conjoined twins? (so called "Siamese Twins")
16*) Suggest reasons, in terms of embryological mechanisms, why conjoined twins are always (usually?) mirror images of each other? Think about whether embryological control mechanisms would interact, including either chemical gradients or mechanical forces, and what would be the effects of interactions between them. 17*) When human identical twins develop by forming two primitive streaks in the same inner cell mass, then one of these twins will (Usually? Always?) have situs inversus viscerum (aorta on the right side of the heart, etc & everything a mirror image shape and position). Suggest why. 17*) Sessile and slow moving animals often have several planes of refection symmetry (which most biologists naively call "radial symmetry"). Suggest why? For those who have dived on tropical reefs, consider the symmetries of sea fans, relative to the predominant directions of water flow around them. 18) What swimming animals have radial symmetry? And suggest why? 19) Most fish and walking animals have bilateral symmetry; in what sense is this the symmetry of their environment (if we include forward movement as part of their environment)? 20) If a swimming or walking animal spent about as much of its time walking forward as backwards, then what additional symmetry might appear in its anatomy? 21) Burrowing animals (worms and even snakes) tend to evolve back toward "radial symmetry" (= an infinite number of planes of reflection symmetry): is this because they are moving slower? Because they are reversing direction more often? Or because their surroundings consist of front, sideways and rear, with all "sidewases" being equivalent? 22) For a fish or a walking animal, the directions are up, down, rearward, forward and sideways (how many "sidewayses" do they have, not counting flounders? 23) When a jelly-fish swims by pulsation, its directions are forward, rearward, and sideways (how many "sidewayses"?). How is this related to the symmetry of the anatomy of a jelly-fish? Hint: except for the space shuttle, what symmetry to most space rockets have? Is there a reason for this? And if the space shuttle used a parachute to land, would it be an exception to the rule about radial (really "axial") symmetry.
24) Using the cellular automata computer program available on my web site , how would you produce patterns with (a) One plane of reflection symmetry; (b) Two planes, (c) Four planes? (d) Two fold rotational symmetry (without reflection symmetry)?
(e) Four-fold rotational symmetry (without reflection symmetry) 25) What symmetry does the counting of neighbors have? (in this program) 26*) Can you imagine and suggest some other symmetry in the counting? What if the right and left neighbors counted twice as much as the above and below neighboring squares? What if the right hand square counted twice as much as the left hand square? Could these differences affect the symmetries of the patterns? (hint: sure, But can you visualize what forms the alterations would take?) 25) Can you create a sequence of patterns with time displacement symmetry? (That keeps cycling through a repeated series of identical spatial patterns) [For an easy way to do this try the "opposite" buttons] 26) One a pattern has a given symmetry, do later patterns ever lack this symmetry? Do later patterns ever have MORE symmetry? Can you explain why? What did Pierre Curie have to say on the subject? 27**) Invent something analogous to a Turing mechanism (for cellular automata) that can decrease ("break") symmetry in some regular way, not just becoming random. 28) What is the relation between the lack of reflection symmetry in the body and the lack of reflection symmetry in flagella? 29) When a force, or balance of forces have spherical symmetry, then what shape will they tend to remold a cell into? 30) If you see a mass of cells changing from other shape into a sphere, then what do you tentatively conclude about the forces responsible? 31) If a gene could make an internal force less symmetrical, then could that cause cells and tissues to change from one symmetry to another? 32) Becoming less symmetrical means losing (?) or gaining (?) elements of symmetry? Either way, what alternative kinds of mechanisms can be used to choose which specific planes or axes, or other symmetry elements, will be gained or lost? (Sperm entry? Gravity? Random flucuations? Anything else? What if you just poked it at some time of special sensitivity? 33*) To prove the occurrence of an actual Turing mechanism, would you need to isolate and identify the actual chemicals? Or can you invent experimental criteria, such as how the resulting patterns are altered by water currents or by more viscous fluids, or barriers to permeability, or something else. 34*) Invent a new kind of symmetry, based on distortions in shape produced by curved mirrors. Could some shapes look the same? Or could reflection, plus some other change (like inflation), in combination, leave some shapes looking the same.
35*) What the symmetries are approximated by the path of a meandering river?
(Take a look on Google Earth, or the next time you fly somewhere.)
36) Does Driesch's entelechy have dilation symmetry!?
37) Try comparing the symmetries of brick locations in brick walls or sidewalks:
how many different combination of symmetries can you find? 38) When Dictyostelium amoebae aggregate to form a hemisphere, symmetry has increased? In what way? And by what cause? 39) When this sphere distorts itself into a long slug, has symmetry changed? How? Why? 40) When such a slug is cut in pieces, and the fragments become smaller slugs that have the same length to width as the original slug had had, then what sort of symmetry are we going to say this is an example of? * What symmetries in the causal forces are possible explanations?
41) What are at least two different reasons why an aggregation of cells can behave as if it has a contractile surface? 42) If you measure how much flattening of cell aggregations is produced by a given centrifugal force, then what cell property are you measuring? If A is true? If B is true? If some third force causes cell aggregates to round up? Then what would you measure? 43*) According to my reinterpretation, Steinberg's measurements of "reversible works of cell-cell adhesions are real (perfectly accurate) measurements of what other properties of each cell type? [in dynes (force) per centimeter] 44*) Can you invent some practical experiments by which one could prove, one way or the other, whether the rounding up of cell aggregates is driven by maximization of cell-cell adhesions, or whether rounding up is driven by stronger contraction of cells parallel to the surface of the aggregates? 45**) Can you invent a third explanatory hypothesis, different from either Steinberg's or Harris's, that can explain the rounding up and sorting out of multicellular aggregates. 46) If Steinberg is right about cell sorting, then the driving forces of gastrulation and neurulation are what? 47) If Harris is right about cell sorting, then what are the driving forces of gastrulation and neurulation? Are these embryologically important questions, or semantic details?
48*) Since Holtfreter spent his life trying to understand what forces drive and control gastrulation, neurulation and other active cell movements, can you suggest possible reasons why he was so contemptuous of Steinberg and me, both? 49*) Do you see any logical connections between this disagreement and the studies of cell wrinkling on thin sheets of rubber? 50*) Similarly, what logical connection can you suggest between my research on cell movements on grid patterns of metal-coated glass and plastic, relative to the debate about theories of cell sorting? [Hint: are cells pulled by maximization of adhesion, or do cells pull themselves by active traction. And which does Steinberg assume, concealed under the abstractions of thermodynamics?]
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