Lecture Notes for Wednesday, March 7, 2018


Lessons from the history of cell sorting

Think about, and be ready to discuss examples of the following:

Scientists often misunderstand the phenomena they discovered.

Sometimes they argue strongly in support of alternative explanations.

Consider that in the years when H.V. Wilson did his cell sorting research others had discovered changes in cell differentiation that could be produced by removing or grafting embryonic cells. (Driesch? Spemann? others?)

Discuss whether that might have encouraged Wilson to misinterpret sponge cell behavior as changed differentiation.

Why couldn't Wilson use the same method that Holtfreter did to prove that reaggregated sponge cells were rearranging instead of redifferentiating?

Why couldn't either Holtfreter or Wilson have used the methods Trinkaus did?

Suppose that Trinkaus's experimental result had been that reaggregated cells switched from one cell type to another, depending on their location, would he then have continued to be interested in the phenomenon?

If you were trying to develop methods to cause regeneration of human fingers, arms and legs, which of the following would you try first?

    a) Seek a method to put undifferentiated cells on the cut off surfaces? (And get them to differentiate into muscles at the positions where muscles should be, differentiate into skeleton at places where cartilages should be, etc.)

    b) Graft already-differentiated skeletal cells, muscle cells etc. onto the cut surface.
    (And try to stimulate them to rearrange into normal anatomical patterns.)

Holtfreter, Moscona and Trinkaus all three disdained Malcolm Steinberg's Differential Adhesion Hypothesis, considered it a waste of time, but also did not design or do experiments to try to prove it true or false. Why do you suppose not?

As more different cadherins were discovered, wasn't their existence a disproof of Steinberg's theory that sorting out was caused by differences in amount of cell-cell adhesion, rather than differences of kinds of cell-cell adhesion proteins.

Were other scientists more (or less) likely to be persuaded by Steinberg's theory if they had less (or more) knowledge about thermodynamics?

For example, Scott Gilbert's textbook says that Steinberg's theory was "based on thermodynamic principles"

Can you suggest what thermodynamic principles he may have meant?
Can you list two or three actual thermodynamic principles?

I think he vaguely recalled that when systems go to the same end result by any of several intermediate pathways, that has something to do with thermodynamics.

Please consider these possibilities:

A) Maybe cadherins cause heart cells to get together with other heart cells, liver cells to get together with other liver cells, etc. but that differences in strengths of surface contraction are what cause relative positioning of cell types, heart cells interior to liver cells, etc. and that people were dealing with mixtures of different causes?

B) The ability to reach the same end result by two or more different pathways is caused by homeostasis. Usually homeostasis is considered as stabilizing quantitative variables (like body temperature and blood pressure), but is there any reason why negative feedback cycles can't just as well create and stabilize geometric shapes and arrangements of parts?

C) Muddled beliefs about thermodynamics have deflected embryologists from explaining anatomy in terms of homeostasis. (Negative feedback cycles instead of minimization of free energy.) What do you think?

D) Wayne Brodland is a research scientist at the University of Waterloo in Ontario, who is interested in the birth defect Spina Bifida, which results from failure of the neural tube ectoderm to close completely. He has published some important papers about cell sorting, which include arguments against the DAH. Would improved understanding of cell sorting help avoid or treat spina bifida?
[link to Brodland's web site. He has some nice animations and other material.]

E) Many web sites and textbooks continue to regard Steinberg's DAH as true, in contrast to the last paper that Steinberg co-authored before he died, which says sorting results from mixtures of adhesion and contraction.
[link to this paper]


Regarding limb bud regeneration:

Nardi and Stocum discovered the amazing fact that if you cut off one salamander leg and graft it onto the side of another salamander leg, the graft will move along the other leg until wrist is juxtaposed to wrist, or elbow moves next to elbow, or shoulder next to shoulder. Grafts move until like is next to like. The term they have used for this is "affinopheresis".

Is this analogous to cell sorting? Nardi and Stocum believe that it is. They believe legs have gradients of cell-cell adhesiveness, with maximum adhesiveness at the hand or foot. Can you figure out what evidence could detect differences of adhesiveness? Could gradients of contractile tension produce the same movement of grafts? Would such knowledge help cause regeneration of legs.

In what sense do grafted wrists sort out? They tried to explain this phenomenon in terms of Steinberg's Differential Adhesion Hypothesis.

A Google search for "Nardi Stocum affinophersis" brings up many images.
Their classic paper is Nardi, James B., and David L. Stocum (1983). Surface properties of regenerating limb cells. Evidence for gradation along the proximo distal axis. Differentiation 25, 27-31.