Cell sorting: some things to think about   Consider these three videos: sponge aggregation cells on a grid of differential adhesiveness cells on an adhesion gradient The debate about the cause of cell sorting includes whether crawling cells are pulled by forces of cell-cell adhesion ("reversible work of adhesion"), or whether the cells crawl actively by each mesenchymal cell being pulled by actin "treadmilling" in different directions, so that each cell continually undergoes a "tug of war" between different sides of itself. The net result of these tug-of-war counter-balances of each cell is to pull them preferentially onto more adhesive surfaces (which appear gray in these videos). Are the two cell aggregates being pulled together to become a single sphere by forces of cell-cell adhesion ("passive locomotion"), or by active contractility of the cells, with cells at the aggregate surfaces pulling more strongly than cells in the interior? By watching these videos, see whether you can find specific reasons to distinguish whether cells are actively crawling in opposing directions, as if playing "tug-of-war", or whether they are being pulled by adhesive forces. The gray areas and small gray squares were created by vacuum evaporation of heated atoms of the inert metal palladium. Blurred boundaries of these gray areas are adhesion gradients.

 

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