Can Cells Sense Gravity?

Summary:

The structure and arrangement of centriole-pairs which occur in animal cells are well suited for sensing the direction of surrounding fields. I suggest that in addition to sensing local chemical gradients, the centrioles may also be involved in sensing gravity field gradients. Sensing chemical gradients are presumed to play a role in cell mitosis. Sensing gravity gradients could provide the cell with the ability to tell time and perhaps latitude. [See Remarks for updated information.]

The thesis - the centriole structure may function as a "gravity antenna"

Animal cells contain centriole bodies which occur in pairs, aligned at right angles to each other. Centrioles are best known for their role in mitosis where they help the cell to separate into two daughter cells. My speculation is that the structural form of centriole-pairs also allows them to function as antenna-like sensors of the local gravity field.

These speculations are not based on my knowledge of the biology of cells (which is scant). Rather they stem from an engineer's "Top-Down Approach" which seeks to infer the "functional block diagram" of the centriole's function by examining its structural form. The approach depends on recognizing similarities between the forms of different structures, and speculating on how those similar forms might perform analogous purposeful functions.

An engineer looking at the geometry of centriole structures would see a form that is well suited to function as an "antenna" for sensing the local gravity vector. In examining the geometry of centrioles I was led to consider the following questions:

• Why do centrioles occur at right angles to each other, and what function might be served by that orthogonal configuration?
• What function might be served by the particular structure of the centriole walls, consisting of nine triplets (of three microtubules each), all skewed at equal angles from the centriole axis?
• As I considered possible answers to these questions, they led to other questions about how molecular sensing "mechanisms" might depend on the centriole's structural form.

Introducing the centriole pair

 Biology texts provide drawings and micrographs of cells and their internal structures. Consider the enlarged view of the inside of a cell (see figure to left) which shows microtubules radiating outward from the centriole region near the cell nucleus. The microtubules are the backbone of the cytoskeleton that determines the shape of the cell.

 

Two cylindrical centrioles, near the circular nucleus of the cell where the microtubules converge, are arranged at right angles to each other. These are not clearly visible in the previous figure, but can be seen more easily to the figure to the left which shows an illustration of how the two centrioles in every pair are always arranged in an orthogonal configuration near the nucleus in a cell.

This right-angle orientation of one centriole cylinder with respect to the other is a striking feature. Right angles, or orthogonality, is often used in engineering structures to obtain two views of a three dimensional field which can then be used to calculate all three components of the three dimensional field.

Each element of a pair of orthogonal elements can sense two components of the full field, with one of the components common to both elements (see the table further below).

Sensing three dimensions with a centriole pair (the geometry of orthogonality)

Consider the centriole as a simple cylinder, lying with its axis along the horizontal axis of a three dimensional coordinate system. Let's define the coordinate system in terms geographical directions, and place the centriole's long axis on the East-West axis. The two other axes are then designated North-South, and Up-Down (the vertical direction).

We can now draw a "gravity vector" passing through the cylinder at an arbitrary angle with respect to the centriole coordinate system. The force of gravity does not actually exist as the single gravity vector shown in the figure. An infinite number of lines would be needed to show the force on each particle of the centriole. However, the direction of those lines would be parallel to the gravity vector shown in the figure.

Notice that the gravity vector passes through the centriole at points (a) and (b). Those points are projected on the plane that is perpendicular to the East-West axis, i.e., the orthogonal plane defined by the North-South and Up-Down axes. This means that a centriole lying along the East-West axis would detect the North-South and Up-Down components of the gravity vector.

Similarly, an orthogonal centriole (of that pair) lying along the North-South axis would detect the gravity components projected on its orthogonal plane, i.e., the one defined by the East-West and Up-Down axes. In this way, any pair of orthogonal centrioles would detect all three gravity vector components, as follows.

If a centriole is to detect the direction of a gravity vector, it must provide detection-features that are uniformly distributed around its circumference. The circumference of a centriole is composed of structures called "triplets." The circular distribution of the centriole's triplets seem well suited for such 360-degree coverage.

Structure of the centriole - nine walls of triplets (of three microtubules each)

Figure 3 below shows how the walls of the centriole consist of nine triplet walls, or faces. A micrograph of the cross section of a centriole is shown on the left, and an illustration of the triplet arrangement is shown on the right. Notice that the nine triplets are each skewed at an angle with respect to the centriole axis. Each triplet is composed of three microtubules, fused to each other to form a plate-like structure. For convenience I shall use the word "face" to refer to the "three-microtubule-wide" flat side of the triplet, and "edge" to refer the "one-microtubule-thick" edge of the triplet.

There are no obvious structural differences along the length of the centriole's triplets. Hence, there are probably no differences in a triplet's response to gravity along the centriole's length. 

Since the three fused microtubules effectively represent a plate-like structure, I saw the triplet as analogous to a narrow plate. Such a plate is more likely to vibrate more freely in the direction normal to its face than to its edge.

That was the first premise, i.e., that the triplets' thermal vibrations would be primarily normal to the faces, with little vibration normal to the edges.

The second premise was that the gravity vector would have some modulating effect on these vibrations, depending on its angle with respect to the triplet face. If this were the case, the centriole could report the direction of the gravity vector by determining which triplet experienced the greatest modulation.

 With that in mind, I referred to the micrograph of the centriole and drew a schematic of its cross section to see if there were any interesting relations suggested by the geometry of the triplets (see figure to the right).

Based on the micrograph, I placed the nine triplets 40-degrees apart around the circumference of the centriole. I then depicted each triplet by three small adjacent circles representing the fused microtubules. The triplets were skewed at a 45-degree angle from their radii (or circumference of the centriole), as shown in the micrograph.

How the geometry of triplet structures might support gravity detection.

The next step was to examine the idealized centriole's cross section to determine if any geometric features suggest how the triplet "plates" might provide information about the direction of a gravity vector. I assumed that the gravity vector's influence would be greatest when it is perpendicular to the face, and minimal when it is perpendicular to the edge of the triplet, with intermediate effects between those extreme angles.

Consequence of triplet spacings: It was immediately apparent that the 40-degree radial spacing, and the 45-degree skew angle, provided full 360-degree coverage, such that no two triplets face in exactly the same direction.

Consequence of 45-degree skew angle: Each triplet is at 45-degrees with respect to its radius to the centriole's axis. Hence, the intensity of a field gradient impinging on its face might be transduced into equal radial and tangential components relative to that radius. Those components might be then transmitted through its associated radius-structure to the centriole axis. This would represent a simple one-triplet sensing mechanism. The triplet with the strongest response would indicate the radial direction of the gravity vector.

Consequence of comparing responses from different Triplets: On the assumption that the response of each triplet depends on the angle between its face and the direction of the gravity vector, the question arises as to whether a more precise determination of the direction of the gravity vector may be obtained by comparing the responses from different triplets.

Such a comparison can be based on examining the consequences of the angular differences between the directions of the triplet face-planes. Each triplet face is offset from its neighbor by 40 degrees. These are identified by the nine letters A, B, C,...to I, in the previous figure. The face-plane direction is represented by the lines passing through the circles that represent microtubules.

Two such comparisons are described below. (It will help to print out the figure and draw lines on it as described below.)

"Once-Removed" Triplet Configurations (Almost orthogonal): The faces of two triplets that are "once-removed" from each other, are offset by 80-degrees (almost orthogonal). For example Face C and Face H are offset by 80 degrees from their "once-removed" companion, Face A (see the figure above).

If you were to draw lines parallel to a gravity vector that is perpendicular to Face A, they would intersect Face C and Face H at 10 degrees. In this case, Face A would experience the strongest response to the perpendicular gravity vector, while triplets C and H would experience small but equal responses to that shallow-angled 10 degree gravity vector.

In this way, a comparison of the magnitude of a gravity response of one triplet, against its companion triplets, could provide the information required to identify the direction of the gravity vector.

Consider another example of the comparison between a triplet and its companions.

Four-times Removed Triplet Configurations (Almost Parallel): Triplets that are four-times removed from each other are offset by 20-degrees from each other (almost parallel). For example the direction of Face E and Face F are offset by 20 degrees from their "four-times removed" companion, Face A (see figure above.)

If you were to draw lines parallel to a gravity vector that is perpendicular to Face A, they would intersect Face E and Face F at 70 degrees. (70 degrees is the complement of 20 degrees). In this case, Face A would again experience the strongest response to the perpendicular gravity vector, while triplets C and H would experience strong (and equal) responses because of the large 70 degree angle of the gravity vector.

These two examples illustrate how comparisons between several triplet faces might provide redundant information about the direction of the gravity vector. In this case the gravity vector would be in the direction normal to Face A if the triplets once-removed indicated equal but small responses, while triplets four-times removed indicated strong but equal responses.

Other useful geometric relationships that can be considered, but these examples indicate the concept of examining the geometry of combinations of triplets, to infer the direction, and changing direction, of the gravity field.

Mechanisms by which triplet-faces might convert gravity gradients into signals.

It's one thing to recognize that a structural form might be suitable to perform useful functions, but quite another to understand how those functions are actually performed. In this case, it requires speculations about the transformation of gravity's potential energy into suitable mechanical, electrical, chemical or thermal processes which can represent information about the gravity vector and its changing values.

Again using an engineer's "Top-Down" approach, I considered analogies which might suggest mechanisms and processes for making such transformations. This can best be characterized as the "Brainstorming" approach, during which no speculation is dismissed because of obvious lack of merit. Instead, all ideas are considered, most of which will be easily dismissed after more careful consideration, while others might trigger insights in those who have different perspectives, and different domains of scientific knowledge.

With that being said, let me now offer some brainstorming bits for your consideration.

An interplay of thermal and potential energies.

It can be safely assumed that the triplets vibrate randomly in response to thermal energies. By analogy, I suggest that the fused microtubules provide structural rigidity such that the triplet can resonate more easily in the direction normal to its face than it can in the direction of its edge. Visualize a flat board that can be bent more easily against its flat side that it can against its edge.

The speculation then is that a gravity vector would provide a greater "modulation" (or influence of the random thermal vibrations) if it were normal to the face of the triplet, than if the gravity vector were normal to the edge of the triplet.

But what sort of modulation might that be? One possibility may be found in an earlier suggestion by Ilya Prigogine who argued that the "ratio of potential to thermal energy" can cause the gravitational field to be "perceived" at the molecular level.

Quoting from his book (Emphasis mine.): (See references).

"We have recently discovered a striking example of the fundamental new properties that matter acquires in far-from-equilibrium conditions: external fields, such as the gravitational field, can be "perceived" by the system, creating the possibility of pattern selection."

"How would an external field -- a gravitational field -- change an equilibrium situation? The answer is provided by Boltzmann's order principle: the basic quantity involved is the ratio of potential energy/thermal energy."

"Non equilibrium magnifies the effect of gravitation (see Prigogine's Ref.¹¹ below). Gravitation obviously will modify the diffusion flow in a reaction diffusion equation. Detailed calculations show that this can be quite dramatic near a bifurcation point of an unperturbed system. In particular , we can conclude that very small gravitation fields can lead to pattern selection."

I have to confess that while I found the argument very exciting, I lack the theoretical background to delve into its promising perspective. I must therefore leave this task for others who are familiar with the required scientific background.

Mechanical-Chemical Analogs

Producing a molecular soup: Consider again the analogy of the triplet as a flat board, which bends more readily along its face than along its edge. Consider further that somehow such bending may open apertures in the triplet face which then more easily discharge molecular materials (amino acids? peptides? proteins?), depending on the extent of the bending. This mechanism would be micro-version of how a Pacinian cell "opens its pores" to release chemicals, in response to physical pressure on its surface.

In this way, a larger gravity gradient normal to the triplet face would cause the release a larger concentration of molecular materials. If each triplet were to release a triplet-specific protein, then the concentration of triplet molecules would provide an indication of which triplet was most stimulated. Considering the earlier discussion about how combinations of triplets can indicate the direction of the gravity vector, the proportions of the protein mix can provide even finer discrimination of the direction and magnitude of the gravity vector.

Producing Acoustical Standing Waves: As an alternative to the discharge of molecular materials, consider that each triplet's motion generates acoustic waves that represent the vibrational pattern of the triplet, which would depend on its orientation to the gravity field. The interaction of triplet waves in the surrounding milieu would then generate patterns representing the integrated response to a gravity vector. The readout of such patterns may be accomplished by their effects on the cell's cytoskelton which radiate outward as a web from the orthogonal centrioles near the nucleus.

In this way, networks of filamentous proteins become candidates for performing the integrative functions of sensing and storing information about the gravity vector that modulates the chemical messages which determine the changing patterns of cell configurations.

Last Minute Brainstorming about sensing "mechanisms"

Just as I was posting this speculation page, I came across the following information on the state of the art in measuring very small gravitational fields:

"Researchers at the Jet Propulsion Laboratory (JPL) in Pasadena, California...can detect an acceleration of one hundred-millionth the force of gravity -- a record breaking zero to 60 mph in nine years." ("Probable Tomorrows," Marvin Cetron and Owen Davis, 1997, St. Martin's Press, page 92).

The device is based on the physics of Scanning Tunneling Microscopes, where a quantum tunneling current between two extremely close conductors will vary when there are nanometer changes in the distance between the conductors. One conductor is a spring which can move relative to the other, when it experiences an acceleration. That movement causes a change in current which is then sensed as an indication of the acceleration.

This raises the question of whether a microbiological "quantum tunneling" phenomenon might be found which produces changes in current flows as a function of the changing gravity field. Such current flows could then produce chemical changes that can serve as indicators of the changing gravity field. The chemical changes could, in turn, be used by the cell's extensive internal network of microfilaments to reflect that gravitational state by some appropriate change in the cells intricate configurations.

Consequences of cellular ability to sense gravity

My interest in exploring the mechanisms for the cellular sensing of gravity was triggered by the chance perusal of some textbook illustrations showing the orthogonal configuration of centriole pairs. Those illustrations seemed analogous to engineering configurations used for sensing three-dimensional fields. It was therefore natural for me to speculate that the centriole structures were probably involved in sensing three dimensional fields (probably the internal chemical gradients used for triggering mitosis).

But then I remembered an earlier interest. Years ago, I tried to understand how the changing gravitational fields could have an effect on the biological behavior of organisms. There were many reports of animals responding adaptively to tidal conditions, but there were no descriptions of the "mechanisms" responsible for those behaviors.

Recalling those earlier interests, I wondered whether the cell's centriole structures might provide them with the ability to sense gravity. Those considerations led me to the speculations which I just described in this article. As a result, I now feel strongly that cellular sensing of gravity may be a possibility, and worthy of further investigation.

But then the question arises, "How can the ability to sense gravity changes provide an organism with useful biological survival information?"

Again, because of other earlier interests, it soon occurred to me that if cells could sense gravity changes, they might be able to tell the time of day, the time of the month, and the time of the year, and perhaps their geographic latitude. For that discussion see "Can Cells Tell Time."

Notes:

1. About the figures: These were scanned from "Biological Science," 4th Edition, William T. Keyton and James L. Gould, Norton 1988; where Figure 1 was 5.20 pg 127 where Figure 2 was Figure 5.27 pg 132, and Figure 3 was Fig5.23 pg 129. In each case I cropped the figures to focus on the areas of particular interest. ( Back to where you came from)

2. It should be noted that the following discussion is relevant to the detection of both chemical and gravitational field gradients. The arguments would differ in some respects, but the general concepts apply to both. It may turn out that the role of the centriole in performing cellular functions may depend in part on the ability of the triplets to act as directional chemical gradient detectors. (Back to where you came from).

References:

1. (Ilya Prigogine, "Order Out of Chaos," Bantam Books, 1984, page 163.)

2. Ref ¹¹: D.K. Kondepudi and I. Prigogine, Physica, Vol. 107A (1981), pp. 1-24; D.K. Kondepudi, Physica, Vol. 115A (1982) , pp. 552-66. It could even be that chemistry may bring to the macroscopic scale the violation of parity in weak forces; D.K. Kondepudi and G.W. Nelson, Physcial Review Letters, Vol. 50, No. 14 (1983), pp. 1023-26.

Updated information concerning article "Can Cells Sense Gravity?"

Sometime after posting my article "Can Cells Sense Gravity?" I discovered the excellent Internet Home Page of Dr. Guenter Albrecht-Buehler of Northwestern University Medical School, Chicago. That page, "Cell Intelligence," provides an excellent description of a cell's ability to detect infrared light emitted by other cells, and explains how such an ability can support "intelligent behavior."

An important cell feature, that contributes to such detection capability, is the longitudinal twist of each of the nine microtubule triplets of the centriole wall. My arguments were based on the assumption that the centriole walls remained "flat" along the length of the triplets. My earlier reviews of biology references showed the centriole walls to be untwisted triplet "plates" (or "blinds" as Dr. Albrecht-Buehler refers to them).

The figure below shows how Dr. Albrecht-Buehler describes the nine plates of the centriole walls (from "Cell Intelligence" by Guenter Albrecht-Buehler).

This configuration does not support my arguments which are based on having each of the nine triplets facing in a unique direction, so that they could sense the gravitational gradient from that direction.

As a result of this updated representation of the nine triplets I am now back to the drawing board, in search of other mechanisms by which cells might sense the changing gravitational vectors.

However, I will leave my original posting of "Can Cells Detect Gravity?" online, in the hope that some of the arguments might stimulate someone else into thinking about this question from a fresh perspective.

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