[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.