Physical forces and form

Verworn (1899) in General Physiology sketched amoeboid motion from a spherical cell. Oxygen entering the biogen-molecule would lower surface tension locally at one point on the periphery, and the protoplasm would bulge out there¹. Bayliss (1915) asked where the energy of surface tension comes from. He judged that it is ultimately chemical, since it differs with the chemical constitution of the liquid². He cited Hardy's experiments, in which liquids insoluble in water spread as thin films because they lower surface tension³. Stable hydrocarbons hardly spread, while esters and glycerides spread widely³.

Hill (1926) in Muscular Activity tested a surface-tension account of muscle force. He supposed that lactic acid forms a monomolecular layer and calculated that surface tension would then have to rise by 4100 dynes per centimetre. He compared this with water, alcohol and mercury and called it clearly impossible⁴. He found it hard to imagine a change much larger than about 50 dynes per centimetre. To reach the needed surface energy, the acid would have to occupy only about one eightieth of the surface, which he thought impossible to suppose⁴. He judged the explanation unreasonable⁴.

Thompson, in On Growth and Form, argued that the view of cell division turns on whether local, symmetrically placed differences of surface tension occur in the liquid film of a small spherical cell⁵. If they do not, he wrote, changes of shape that lead to division must come from changes in internal pressure, or from mechanical forces due to an induced surface distribution of electrical potential⁵. He allowed that a cell may be treated, with limits, as a liquid drop or vesicle⁶. He said that tissues and cell aggregates need other theoretical considerations, and that he touched only the fringe of the subject⁶.

Recent authors connect mechanical force to bioelectric signals. Golding and colleagues (2016) used a silk hydrogel device in limb regeneration. They found a rise in stiffness that was small but statistically significant, and wrote that the change in material and mechanical forces can affect cell responses in wound healing⁷. Schofield and colleagues (2020) report that some ion channels respond to local mechanical forces in the membrane, which links membrane potential, cell physiology and mechanical force⁸. George and Bates (2022) note that many ion channels are sensitive to mechanical stress. They put forward the hypothesis that forces between cells may guide ionic signaling, and they list the question as unanswered⁹.