More Sets of Alternative Theories   Unsolved Problems   Sept 23, 2016  Albert Harris What force causes re-aggregating masses of cells to "round up" = become spherical? Is the answer? a) Maximization of cell-cell adhesions? b) "Reversible Work of Adhesion" c) Total area of cell-cell adhesions? d) Contraction of cell surfaces. e) Stronger contraction of cell surfaces where they are in contact with the surrounding water (or tissue culture media). In other words, contraction of cell surfaces, combined with weakening of contraction where cells touch each other. f) If Area of cell-cell contact + Area of cell medium contact = Total Area Then maximization of one equals minimization of the other. According to Professor Malcolm Steinberg's "Differential Adhesion Hypothesis", "b" above is true. That would mean that measuring the force with which cell aggregates resist rounding up would be a measure of the "reversible work of adhesion". (Which wouldn't exist unless cells were pulled by reversible forces of cell-cell adhesion). Criticisms: * Cells crawl by contractions, instead of being pulled by forces of adhesion. * Formation or enlargement of cell-cell adhesions don't necessarily exert a force. * Even if adhesion exerted a force, it wouldn't necessarily be reversible (= conservative). (Springs and weights exert conservative forces; motors, friction, viscosity and acto-myosin exert non-conservative (= non-reversible) forces). Aggregates of liver cells engulf (surround) aggregates of heart cells; what does this tell us? g) That heart cells are more strongly adhesive than liver cells? h) That heart cells are more strongly contractile than liver cells? Clumps of cells from embryonic limb buds also engulf one another. Cells cut from the distal (hand) end engulf cells taken from the more proximal end of the limb bud. What does this tell us? i) Limb buds have a proximo-distal gradient in amount of cell-cell adhesiveness , with strongest adhesions at the distal (hand) end. j) Limb buds have a proximo-distal gradient in strength of cell-surface contraction, with strongest contraction at the distal (hand) end. If you put aggregates of different cell types (heart, liver, nerves, etc.) next to each other, then one cell type will consistently engulf, or be engulfed by the other, in the following pattern: "transitivity." * If cell type Z engulfs cell type Y, and if Y engulfs X, then Z will always engulf X. What does that prove? k) Engulfment is caused by differences in cell-cell adhesion. (If X is more adhesive that Y, and Y more adhesive than Z, then X has to be more adhesive than Z) l) Engulfment is caused by differences in strengths of cell contractility. (If X is more contractile that Y, and Y more contractile than Z, then X has to be more contractile than Z) m) Engulfment is caused by differences in some thermodynamically reversible force. n) Engulfment is caused by differences in some quantitative (not qualitative) variable. (difference in amount, not difference of kind; but cadherins differ qualitatively (in kind)) From the Wikipedia article on sponges   Questions: Regarding the directionality of water flow, what possible causes can you think of that would cause water to flow in the sides and out the top, rather that the reverse, or flowing around in circles? Can you find any part of the Wikipedia article where the authors seem to be aware of this problem? How would directionality be affected if the flagellated cells act like "solenocytes" ("Protonephridia") in the sense of pulling water between adjacent cells toward the flagellated cells?

 

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