Collision behavior

[See ref 2 ]

Colliding cells seem to rebound like colliding billiard balls

Sister cells can only move along symmetrical or identitical tracks if they do not run into obstacles or other cells. What happens if they do collide? Do they literally stop them in their tracks, or do they run around erratically? Neither of these possibilities occur: The cells seem to bounce off each other like colliding billiard balls. The figure below shows an example of the tracks of 2 colliding cells that produce remarably symmetrical paths in the vicinity of the impact area For more examples see ref 2.

(The illustration is animated.Click here for a minimal strip of frames.)

Rebounding must be reprogramming

In spite of the appearance of the tracks, the collision between 2 cells cannot be elastic like the collision between billiard balls. Cells do not fulfill the minimal requirements of an elastic collision whose hallmark is the conservation of momentum and kinetic energy. Their extremely slow crawling movements resemble moving through molasses because it dissipates all momentum and kinetic energy. More importantly, they have no defined, hard surface from which they could bounce off. The sequence below shows complex the shape changes are and how tenuous the contacts are if an epithelial cell (on the left) collides with a fibroblast.

Note: the fast moving cells in the experiments described here are always fibroblasts. Most epthelial cells migrate very little, and if they collide with other epithelial cells they remain together.

(The illustration is animated.Click here for a minimal strip of frames.)
Therefore, the symmetry between the inbound tracks and the outbound tracks of 2 colliding cells must be the result of a reprogramming of their movements. For example, if cells would simply run their pre-collision instructions in reverse order they could produce the observed collision patterns.

Significance for cell intelligence:

Cells can read or modify their internal 'programs of movement' at will.
There is no physically defined interface between the colliding cells. Therefore, the mirror image relationship between their in- and outbound tracks means that they have reoriented their movement. This, in turn, means that they either have the freedom to read their internal programs in reverse or that they modified them by a well-defined rule of reorientation. Either way, such actions imply the existence of elaborate data integration systems.