Discussion questions about engulfment of cell aggregates and mechanism of cell rearrangement.

a) Why is the cause and functional significance of H. V. Wilson's cell sorting still not solved, more that a century after Wilson discovered the phenomenon?

After all....* The experiment is easy to repeat.
** The experiment has been repeated and observed many thousands of times!

b) Maybe cell sorting does not have any functional significance? Please argue pro or con.

There is no species that develops by rearrangement of randomly mixed differentiated cell types.
     (Therefore, you conclude what......?)

But rearrangement of differentiated cells IS what happens in gastrulation, neurulation, and regeneration of leg skeleton (and maybe also in somite formation??).

If randomly-mixed differentiated cells can sort out to form relatively normal anatomical and histological patterns, does that prove that gastrulation could be caused by the same mechanism(s) as cause cell sorting?

If sponge cells constantly sort out by differentiated cell type , except during their embryonic development , then how do you interpret that?

c) If you dissociate pigmented retina cells into a single-cell suspension, and mix them with randomly dissociated heart cells, first all cells will stick together into a random mixture, and later the pigmented cells will sort out into inner blobs, surrounded by heart cells.

What could be the embryological significance of this sorting out of retinal cells interior to heart?

? Because retinal cells never contact heart cells in normal development, therefore nothing can be learned from this (or any other cases) of sorting out of differentiated cells ? (J.P. Trinkaus)

?? This observation means that pigmented retinal cells are more strongly adhesive that heart cells ? (Steinberg)

??? The observation is caused by stronger contractility of retina cells than heart cells ? (Harris)

???? Heart and retinal cells differ by the same property that causes gastrulation ? (Steinberg and Harris agreed!)

d) Could Wilson's cell sorting could have the same cause as gastrulation?
The same cause as gastrulation in sponges?
The same cause as gastrulation in vertebrates?

The same mechanism as skeleton regeneration?

The same forces as move skeletal muscle cells to their correct locations?
.... during regeneration of salamander legs?
.....during the original development of limb muscles and cartilages?

e) When animals or plants regenerate tissues or organs, is it likely that they use the same combinations of molecular signals and mechanical forces as were used to create the original tissues or organs?

? Or is it more likely that entirely different mechanisms were evolved (to regenerate structures) different from the mechanisms used in original development?

? By what experiments or other criteria could it be proved whether or not the same mechanism is used for regeneration, as compared with the mechanism used in the original embryonic development?

Consider the following experiment: Graft the distal tip of a regenerating limb bud to the base of an embryonic limb bud.
(This was tried by Prof. Kenneth Muneoka at Tulane University)
When he reversed the anterior-posterior axis of the (Embryonic) base relative to the (Regenerating) tip, the result was the same as when you graft an (axis-reversed) embryonic tip to an embryonic base. (branching of the tip to form 3 distal legs!)

Does this result persuade you that leg regeneration must use the same mechanisms? Does it suggest other experimental tests whether regeneration uses the same mechanism as embryonic development?

f) Why did Nardi and Stocum conclude that a gradient of increasing cell-cell adhesiveness must form in developing or regenerating limb buds?

g) Based on what experimental evidence did the short review paper by Green conclude that textbooks should rewrite how they explain the experimental observations of H. V. Wilson and Johannes Holtfreter?


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