Selected Video sequences to illustrate the extraordinary behavior of mammalian cells in the presence of pulsating, near-infrared light sources.

Note:

The videos have low contrast and low resolution because the experiments required very low illumination levels.
They are in avi-format and zipped. They were cropped and edited in order to keep the file size manageable, to remove out-of-focus parts, and to eliminate time-lapse jitter. As a result, the time intervals between individual frames is variable.

Experimental conditions:

field illumination: 600 [nm];

pulsating source: 800 [nm];

pulse duration: 1 [s] ON + 1 [s] OFF;

cells: 3T3 mouse fibroblasts;

Only the blinking white spots indicate latex particles that serve as light sources. As the camera looks straight into the irradiating near-infrared light, it sees the diameter of the irradiating beam as a halo around the light-scattering particles. Other white spots are merely the phase-contrast images of unirradiated latex particles that are sitting on the substrate.

The actual light pulsation is completely regular. In contrast, the video sequences show only a certain irregular blinking of the light because

a.the original videos recorded only the beat frequency between the pulsation frequency and the time lapse recording frequency.

b.The avi-sequences contain only a subset of the frames of the originals in order to keep the file size small.
1. The basic phenomenon

A 3T3 cell detects a pulsating near-infrared light source (1.9 MB)
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2a. Exclusion of phototaxis (= Failure of the cell to remain at the point of highest light intensity.)

The cell reaches out to the second light source from which it can receive only the approx. 1000-fold weaker, scattered intensity, while it is exposed to the direct irradiation of the first light source that it had approached earlier.(0.9 MB)
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2b. Exclusion of phototaxis (= Failure of the cell to remain at the point of highest light intensity.)

In this example, the cell does something extremely rare for 3T3 cells: It turns its tail into a leading edge. As a result, it migrates backwards towards the light sources. (This very unusual action of the cell illustrates strongly the point that it obviously detects the pulsating near-infrared light sources). After reaching over to the first light source it seems to detect a second cell that approaches the same light sources from the left lower corner, and extends to it first. Afterwards, it reaches over to the second light source.(1.1 MB)
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3. Cytoplasmic localization of the light detection mechanism.(=light detection proceeds in the absence of the cell nucleus)

Although the nucleus of a 3T3 cell has been removed by treatment with the drug cytochalasin B, the resulting cytoplast is still capable of detecting and reaching out to the pulsating light source. It contacts and pulls the light scattering latex particle towards itself. (0.3 MB)
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4. Activity of the light detection mechanism soon after cell division.

0ne hour after the start of the sequence, the pre-mitotic cell has divided and completed cytokinesis (= cleavage of the cell into 2 daughter cells), and both daughter cells spread. An hour later, the upper daughter cell has successfully contacted the pulsating light source. Only afterwards, the 2 daughter cells separate completely. Considering that the cell cycle of 3T3 cells is about 24 hours, it appears that the light detection mechanism is active very early in the cell cycle. (0.6 MB)
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5. 'Agressive' approach to the light source.

Cell 1, located close to the pulsating light source, extends initially a thin protrusion to the light source. In contrast, cell 2, located further away rapidly approaches the light source. It is not stopped by cell 1, as one would expect, but extends underneath cell 1 in order to contact the light source.
A possible interpretation: It is well known that fibroblasts such as 3T3 cells are attracted to each other. After establishing contact for a while, they move again away from each other, displaying the so-called contact-retraction. Perhaps, cells in such situations detect each other initially at a distance by the emission of light pulses that are similar to the ones used in the experiments. In other words, cells may approach pulsating near-infrared light sources because they mistake them for another cell. If this conjecture is true, the experimental light pulses are hardly identical to the signals emitted from real cell. At best, they share one or the other aspect with real cell signals.
In the above experiment cell 1 and the pulsating light source are both in the 'line of sight'of cell 2. Consequently, the artificial signals from the pulsating light source may be rendered more natural by the close vicinity of cell 1, and increase their attraction for cell 2. (0.4 MB).
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6. Complete turn-around to reach light source.

In order to reach the pulsating light source which is located at its left rear, the cell migrates in a complex circular maneuver towards the light source. Obviously, cells do not necessarily choose the shortest distance to the light source, as one might expect if they were little machines programmed to seek out light sources.(0.5 MB)
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7. Running an 'obstacle' course.

Instead of reaching over to the light source at its right rear side the cell first moves around a cluster of unirradiated particles attached to the substrate. Again, it is obvious that the cells do not necessarily choose the shortest distance to the light source. (0.4 MB)
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8. Multiple attempts to approach the light source.

The cell seems to try twice to contact the light source, before it finally moves around in a circle and touches it. (0.5 MB)
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