Symmetry of sister cells

[See ref 1, ref 2, ref 4 ]

Why cell movement may appear to be random.

For many years people have believed that cell migration is random, and many still do. There are understandable reasons for this belief. For example, the complex shape changes of migrating cells are quite confusing in the short run. In order to recognize the non-randomness of cell migration much longer observation times are required. Long periods of live cell observation in turn require keeping the cells alive and moving inside a temperature controlled, sealed observation chamber under the glaring lights of a microscope. Often, researchers placed too many cells in the microscope field, which added the complexities of cell-to-cell collisions to the situation. Finally, it is always tempting to call something 'random' even if it is merely 'unpredictable' by the knowledge of the time.

Phagokinetic tracks

Many years ago I found a new technique that allows cells to migrate in the controlled, protected and and dark environment of their normal culture incubator for days and weeks, while the experimenter can still observe their movement. This is possible because the technique, called phagokinetic tracks works like a cloud chamber in which the cells leave tracks of their movements in a carpet of tiny gold particles on their substratum. The phagokinetic tracks can be viewed by scanning electron microscopy like in the illustration above, but it is more convenient to use darkfield light microscopy like in all the illustrations below.

The related tracks of sister cells

When a cell divides, the track of the mother cell branches as the 2 sister cells go their different ways. If there are no other cells around which might disturb the branching pattern, it shows an amazing property: In 40% of the cases the track of one sister forms the mirror image of the other. Below is an example of such symmetry that even pertains to the tracks of the 'grandchildren'.

(The illustration is animated.Click here for a minimal strip of frames.)
In 20% of the cases, the track of one sister cell is identical to the track of the other. Below is an example of such an identity.

(The illustration is animated.Click here for a minimal strip of frames.)
The montage below shows the 2 sister tracks in the same orientation to demonstrate how closely they resemble each other.

Considering the complex shapes of the tracks, it can be shown by Monte-Carlo simulation that the odds for an accidental occurrence of the relationships between sister cell tracks are astronomically small. Therefore, we must conclude, that cells are programmed to turn at certain times in their lives at certain angles. The directional change program which a mother cell gives to one sister cell is normally the mirror image of the program given to the other sister cell. The programs themselves appear to be epigenetic because mothers and sisters leave different tracks, and different cells of the same genetic origin, migrate along different tracks, as well.

Not only are the tracks of sister cells related, but the shapes and internal architecture of their bodies (i.e. their cytoskeleton) appear as symmetrical or identical as their tracks are [See ref 1, ref 2 ]. This suggests that the programs that determine the future movements of the cells are implemented by building and re-building the inner architecture of the cells [See ref 3 ]. .

Significance for cell intelligence:

Cells can 'measure' angles and time intervals

If cells are able to program directional changes at certain times in their life cycle they must be able to measure angles and time durations. This implication is strengthened by their ability to override these programs in sophisticated ways if they collide with other cells, encounter guiding lines or participate in group migration.