Embryology   Biology 441   Vertebrate Embryology, Spring 2016   Albert Harris


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

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).

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.

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

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"?

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)

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...


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