Embryology   Biology 441   Vertebrate Embryology, Spring 2016   Albert Harris


Review questions for the last part of the course - part one

1) 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.
    What can you conclude about the mechanism of pattern formation in limb buds? ?
    Is it by rearrangement of cells according to cell type? ?
    Or is it by re-differentiation of cells according to position?
    What can you conclude about the usefulness or need for undifferentiated stem cells?

2) 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 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.

3) 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).

4) Argue pro or con: Will undifferentiated stem cells be able to regenerate complicated anatomical structures? (instead of just particular cell types, that either have a very simple geometry, like skin and intestine, or have no geometry, like blood)

5) 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?

6) Describe the sequence of events that occur when a newt or other salamander regenerates one of its legs.

7) What is a blastema?

8) 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 of the experimental evidence for or against this fact?

9) Likewise, do any of the previous muscle cells redifferentiate as skeletal cells? And what evidence would be needed to prove or disprove this?

*10) 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.

*11) What are some facts that would have led you to expect these results?

*12) In terms of each of the following hypothetical phenomena, please explain 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"?

    ) 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).

*13) 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.

14) 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

15) Based on the rule that the tensions in the surfaces of cylindrical tubes are 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?
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)
For the same reasons, how do hemispherical aggregates of Dictyostelium amoebae change into long cylindrical "slugs"?

16) What combinations of symmetry do each of the following have?

    Dictyostelium slugs
    Limb buds before they develop an apical ectodermal ridge
    Apical ectodermal ridges
    Would you consider that formation of an apical ectodermal ridge is an example of "symmetry breaking"? (Hint: If a long narrow ridge is less symmetrical than either a flat place or a hemisphere, then yes)

17)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...

18) What is metamorphosis?

19) What are at least three different examples of metamorphosis (e.g. Caterpillars change into what? Plutei change into what? Tadpoles change into what?)

20) What are at least 7 different changes that occur when a tadpole metamorphoses into a frog?

21) What is thyroxine? What functions does it serve? (You don't need to know the molecular structure)

22) What is ecdysone?

23) What is meant by molting, in relation to the growth of insects and other arthropods?

24) What is Juvenile Hormone? (again, what is its function, not its chemical structure)

25) Does Juvenile Hormone participate in the control of molting and metamorphosis of insects? What does it cause? What does it prevent?

26) A common misconception is that people today live much longer than they did a few hundred years ago. Explain why they are wrong.
Hint: What is the meaning of "life expectancy"?

27) What if aging were caused by effects of somatic mutations specifically in the genes that code for DNA repair and replication. What statistical patterns would be / could be predicted in changes in death rates as a function of age?
Hint: would you expect the symptoms of aging to accelerate with time? Explain why or why not.

28) What if aging were caused by defects in telomerase enzyme:
First: Could you predict any observable statistical pattern in death rates as a function of age?
Second: Could you predict which specific genes would first become defective?

29) Suggest several different possible explanations why primordial germ cells don't age.

30) Draw a graph showing the changes in death rates over the human life span. What is meant by "Gompertzian"?


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