Biology 441, Spring 2014


Third set of review questions for the first hour exam

DON'T BE FRIGHTENED by any of the following questions. Not many of them will be on the exam; they are much easier than they seem, and for many of them both "pro" and "con" can be marked correct, depending on the thoughtfulness of your argument.

Salamanders can regenerate their legs, which means re-forming a new humerus, a new radius and ulna, new carpals and phalanges, and also new muscles and tendons, new blood vessels and skin.

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 (the phenomenon discovered by H. V. Wilson, and further studied by Holtfreter, Trinkaus, Steinberg and others).

When legs or other organs regenerate, three kinds of hypotheses are (always) made to explain how they re-form the correct geometric arrangement of differentiated cells:

Possibility A) Remaining cells switch from being one cell type to another (e.g. cells that had been muscle cells change into chondrocytes), depending on their location.

Possibility B) Differentiated cells, and the mitotic descendants of differentiated cells, rearrange to re-form the correct geometric arrangement.

Possibility C) Previously undifferentiated "stem cells" or archeocytes differentiate according to location, and the regenerated tissues are formed from them, not from descendants of cells that were already differentiated before the amputation.

Compare or contrast these three with the three kinds of explanations that scientists have advocated as explanations for sorting out of dissociated cells. (Specifically Wilson's first interpretation, Scott Gilbert's current explanation, and Malcolm Steinberg's explanation.)
[hint: are there logical similarities or equivalences in the theories, and in the kinds of evidence that are needed to prove which explanation is correct?]

If you dissociate a neural tube into its individual cells, and then randomly mix these with dissociated cells from the "somatic ectoderm" (i.e. the cells that will form the outer layer of the skin), then the neural tube cells will sort out to the internal position.
Argue pro and/or con whether this means that the cause of cell sorting must have the same underlying mechanism as the cause of normal neurulation.

Embryos of mammals, salamanders, and frogs form their neural tubes by folding of a sheet of ectodermal cells, and sealing the edges together, leaving a fluid-filled space inside, called the neurocoel. In contrast, embryos of teleost fish form their neural tube from a solid rod of cells, which somehow hollows itself out down the middle (that is, instead of folding and sealing the edges).
Argue pro and/or con whether this means that teleost neurulation has a different causal mechanism than the neurulation of mammal emmbryos, frog embryos and salamander embryos.

Briefly describe and/or discuss whether your opinion or conclusions are changed by each of the following facts.

1) When sorting-out occurs between neural tube cells and somatic ectoderm cells (in any vertebrate), the mass of aggregated neural tube cells hollows out to form a neurocoel-like cavity.

2) The anterior 80% of a bird embryo's neural tube forms by active folding and sealing of the edges, just as occurs in the whole neural tube of mammal and amphibian embryos; but the rear-most 20% of a bird embryo's neural tube forms by hollowing out of what starts out as a solid rod of cells.

3) If a solid ball of aggregated neural tube cells is put in tissue culture media, next to and in contact with a solid ball of aggregated somatic ectodermal cells, then the latter will extend around the outer surface of the mass of cells derived from the neural tube, until it forms a continuous surface layer. (i.e. the same arrangement as gets produced when these two kinds of cells sort out).

Argue pro or con: Cells of the eye can be observed to sort out from cells of limb buds (even though they normally never come in contact), therefore we can conclude that the mechanism by which cells sort out cannot be important in normal embryonic development. (There is nothing to learn from cell sorting about the causes of normal embryonic development?)

According to material posted on the web page, which particular scientist reached this conclusion while stopped at a traffic light?

How would (and did) Malcolm Steinberg interpret the ability of dissociated and randomly-mixed cell types to sort out from each other, regardless of whether any given pair of cell types ever normally come in contact in normal development?

Several more argue-pro-and/or con questions:

Because normal animal development never consists of formation of 5 or 6 concentric spheres of differentiated cells, layered around each other, therefore the causal mechanism of cell sorting can't be the same as (or over-lap with?) the causal mechanism of normal development. (?)

Because no kind of animal develops from an embryo consisting of a random mixture of all kinds of differentiated cell types, which then sort out so as to create anatomical structures, therefore the mechanism of cell sorting can't be the same as the mechanisms of normal embryonic development.

Because mixtures of differentiated cells often rearrange by means of several different sequences of geometrical intermediates, on their way to form the same eventual anatomical shapes, that tells us what?

1) That different causes can form the same end result?
2) That the same causes can produce different intermediates?
3) That cell sorting is caused by lack of counter-balance between opposed forces?
4) That these forces eventually become balanced?
5) That there are many arrangements in which opposed forces are imbalanced?
6) That (in such a case) there was just one balance in which forces are stably counter-balanced?
7) That the forces producing the rearrangement must all be thermodynamically reversible?
8) That only reversible forces can produce the same end result from different beginnings?
9) That different kinds of cell-cell adhesion proteins must cause sorting out.
10) That different total amounts of adhesion proteins (regardless of kind) must cause sorting out.

Which are true?
Which are true and can be logically deduced from the observed ability of cells to reach the same eventual results by two or more different sequences of intermediates?
Which are not true?
Which are not true, but believed by many scientists?
Which could or would be true, if some particular additional fact were observed or otherwise known?

What is the difference between osmosis and electroosmosis? hint: don't forget the semi-permeable membrane


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