Are centrioles the 'eye' of the cell?

[See ref 9, ref 14, ref 16, and Appendix ]

During my study of the motile behavior of cells I came across several observation that raised the startling possibility that cells may be able to detect objects at a distance as if they could see them. Among them were the frequent observation that cells reach out for each other across a distance of one cell diameter or more. To be sure, this observation could also be explained by assuming that cells secrete and detect each other by certain 'recognition' molecules. I found it difficult, though, to understand how cells were able to find the source with such certainty regardless of the cell type. However, no chemical substances would be secreted from intersections between guiding lines. Nevertheless, the cells recognized their arrival at such an intersection and started to probe the optional directions of the guiding 'roads' as if they had a 'global image' of their spatial environment. Cells could not have eyes, or could they? Would we recognize them as eyes, if we saw them? What would eyes look like in the thermally chaotic, microscopic world of cells?

Eyes map the directions of signal sources in a one-to-one fashion.

A cellular eye will hardly look like anything that we have ever experienced in our macroscopic world. Before trying to identify it, we have to define in very abstract terms what an eye is. Consider the schematic of a human eye in the figure below.

Its main feature is the ability to generate different images (1',2') on the retina if the objects (1,2) lie in different directions. If the objects (1, 3) lie in the same direction but at different distances from the eye the images (1', 3') coincide. In other words, eyes are devices to map source directions in a one-to-one fashion. By themselves they cannot tell distances. For example, it takes the processing of data concerning the parallax, relative movement, or bluriness of images by the brain before humans can tell distances of light sources.

The structure of a pair of centrioles suggests their function as cellular eyes.

It is not difficult to show that the a cellular eye should have the same geometric features like a pair of centrioles. would look like the pair of centrioles shown in the electron micrograph below [See ref 14 ]


Consistent with this conclusion, all cells that have centrioles have exactly this structure regardless whether they belong to humans at the top of the evolutionary tree or to protozoa at its roots. Cells which never migrate into unknown territory, such as higher plant cells which stay next to their sister cell all their lives, also have no centrioles. If a plant needs motile cells (e.g. sperms cells), this cell will it will make centrioles de novo.

Centrioles are able to map the direction of light sources in a one-to-one fashion.

Assuming that each slanting blade of a centriole
  1. absorbs or deflects light of certain wavelengths,
  2. carries a photoreceptor at its base (see red spots in the figure below)
  3. the lumen of the centriole is opaque for the light.
the light source is visible for one and only one of the photoreceptors (see blue arrow). The blades cast 'shadows' on all other photoreceptors.

Therefore, if the source moves by more than the angle of resolution, another photoreceptor will be irradiated as shown in the figure below.

(The illustration is animated.Click here for a minimal strip of frames.)
Similarly, if the angular distance between 2 different sources exceed the limit of resolution, then 2 and only 2 receptors are irradiated. The figure below illustrates this situation by alternating between 1 and 2 sources.


If the first centriole measures the 'longitude' of the source, the second is oriented perpendicular to the first in order to measure the 'latitude'.

Centrioles come in pairs that are oriented perpendicular to each other. From the point of view of the theory presented here that makes a lot of sense. Each centriole can only map the angle of the source in a plane perpendicular to its axis (e.g. the longitude). In reality, however, the light sources will be distributed in three dimensions. Therefore, we need a second centriole at right angles to the first in order to determine the latitude of the source, too. The figure below illustrates how a pair of centrioles measures the longitude (red lines) and the latitude (yellow lines) of a light source.


Cellular 'vision' can only make sense for near infrared wavelengths

One can guess which light, if any, cells would not use for their 'vision'.
  1. UV, or gamma-rays because they would be mutagenic.
  2. Visible light, because inside the body of large animals like whales, there is no visible light (But there is, of course, infrared light equivalent to the the body temperature).
  3. Infrared light with wavelength above 10 [µm], because the black body radiation peaks there and makes everything glow with the same high intensity.
  4. Infrared light with wavelength above 1.5 [µm], because cells live in water and water absorbs this light heavily.
Therefore, we are left with near infrared light in the range of 750 - 1500 [nm] as the only viable candidate for light that cells might see with their centrioles as 'eyes'. Our experiments, indeed showed that cells were able to detect microscopic near-infrared light sources at a distance.

Problems: angular resolution,diffraction, absorption, and signal-to-noise ratio

Angular resolution: So far the angular resolution AR of the device is AR=360°/N (N=number of blades= number of photoreceptors).In view of the centriole structure we used as number of receptors N=9 which yielded a resolution of AR=40°. In other words, the cells could not tell sources apart if their directions are closer than 40° which would give cells only a very crude map of their locations. However, as explained in ref. 14 and in the Appendix, there is a very simple and elegant way to make the angular resolution continuous by adding a pitch to each blade. And, indeed, actual centrioles have blades with this pitch. However, it should be noted that pitched blades may pose a problem for the requirement of one-to-one mapping (see Appendix).

Diffraction: How can a blade of 70 [nm] width cast a shadow for near-infrared light that has a much larger wavelength of 800 [nm]? Should light of 10 times the wavelength not simply diffract around the blade? Yes, and no. If the photoreceptor is placed right behind the blade, there will still be a shadow even if the wavelength of the light has a much larger wavelength. However, if the photreceptors would move away from the blade, the amplitude of the diffracted light would rise rapidly and reach it. This little known fact about diffraction can easily be verified with a metal plate playing the role of the shielding blade, and a small transistor radio that plays the role of a photoreceptor. AM stations transmit at frequencies of 100 kHz corresponding to wavelengths of 300 [m] which are much larger than the metal plate and the transistor radio. Still, one can locate the station by pressing the radio against the back of the plate while turning the plate into different directions.

Absorption: How can a blade of only 20-30 [nm] thickness absorb or reflect enough light to cast a shadow? It can if it is metallic, i.e. if it had a high electrical conductivity. For example, a layer of 10 [nm] gold, aluminum or copper is quite opaque, because it reflects incident light very efficiently. Therefore, it will be important for the above theory to test whether centriolar blades or any other microtubular arrays have high electrical conductivity. Human technology has produced already a number of organic polymers with high electrical conductivity. Maybe, nature knows the same trick.

Signal-to-noise ratio: No matter what specific light sources the cellular 'eye' is supposed to detect its near-infrared wavelength is also a component of the black body radiation that is emitted everywhere by virtue of the ambient temperature. Therefore, the ratio between the signal and the background noise must be very unfavorable for the signal detection. In similar situations human engineers resort to a very effective method. They modulate the signal intensity with specific pulsation frequencies which allow the detectors to ignore the background noise as it does not pulsate with a regular frequency. Therefore, we expect the cellular near-infrared light sources to pulsate.

As pointed out in the Appendix, pulsating sources are also required if cells have to distinguish between 2 sources that are located in the same sector of size AR=360°/N (N=number of blades= number of photoreceptors)..

Significance for cell intelligence:

If there are eyes, there is a data integration system.
As pointed out in the Summary the existence of eyes in cells would imply the presence of a data integration system that would justify to call cells intelligent. The rest of the chapter will present the experimental evidence to support the hypothesis that the centrioles are the near infrared 'eyes' of cells.