Electro-mechanical Factors Regulating Bone Architecture 83 



sidered. Now, the mechanisms by which stress-generated electrical potentials might 

 effect changes in osseous architecture will be discussed. In 1962, Bassett and Becker 

 postulated "it is probable that these (electrical) potentials influence the activity of 

 osseous cells directly. Furthermore, it is conceivable that they may direct, in some 

 manner, the aggregation pattern of the macromolecules of the extracellular matrix." 

 Investigations in these laboratories since 1962 have demonstrated that these postulates 

 may be true. Osteogenesis in vivo was found to be affected by weak, artifically induced, 

 direct currents and alignment of collagen fibers was affected in vitro by similar 

 means. The amount of current employed in these studies was comparable to that 

 calculated to occur in fresh bone in response to deformation. While it was not 

 surprising that collagen, a charged molecule, would migrate in an electric field, it was 

 notable that it moved so rapidly and into such an orderly pattern at current values 

 as low as those employed. Drops of soluble collagen, derived from acetic acid 

 extraction of rat tail tendons, and icthyocol were subjected to currents of 0.2 to 

 1 // amps, for periods ranging from 1 — 30 minutes. Within 1 — 5 minutes, a bire- 

 fringent band of collagen was formed at right angles to the electric field and near 

 the cathode. Collagen fibrils in the drop could be reconstituted in this band by the 

 addition of salts of proper ionic strength. Once reconstituted, the fibrils remained 

 stationary after the current was stopped and were found to be well oriented, parallel 

 to one another, and perpendicular to the lines of the electric field in which they were 

 developed. Alteration of the current pattern from continuous to intermittent did not 

 change the result. In fact, collagen drops wired with a multivibrator in the circuit to 

 produce an on-off square-wave cycle at intervals of 1 — 2 seconds seemed to produce 

 uniform bands somewhat faster than drops in which continuous current was employed 

 (Becker et ai, 1964; Bassett, in press). 



On the basis of these in vitro results, it seems possible that molecules, having a 

 net charge, may migrate and align themselves under the influence of currents of the 

 magnitude found in vivo. In view of this likelihood, the nature of the electrical 

 signal being produced is of utmost importance. If the signal or pulse is biphasic, with 

 equal positive and negative components, similar to that produced by a classic piezo- 

 electric crystal such as quartz, a charged molecule would be moved equally in opposite 

 directions, thereby resulting in no change in position. One exception to this behavior, 

 however, might occur. Should the time constants of the signal and the rate of 

 molecular migration fall within a certain range, it is possible that the molecule might 

 be involved in a chemical reaction, such as polymerization, during a half cycle of the 

 biphasic pulse. During the reverse phase of the pulse, the molecule would be unable 

 to migrate. On the other hand, if the signal is purely or essentially uniphasic, it is 

 not necessary to invoke such an exception. 



Is it possible that extracellular macromolecules, by generating electrical potentials 

 in response to stress, possess an "auto-control" mechanism which can direct not only 

 their size and orientation, but influence others in their neighborhood? Certainly the 

 high degree of orientation evident in the collagen fibers of osteones and lamellae of 

 bone and in basement lamellae of adult lamprey skin, implies that a very precise 

 control mechanism must exist. Perhaps the orientation of extracellular molecules may 

 be cell-mediated, as suggested by Porter (1964) or mechanically mediated (Bassett, 

 1964). Certainly, it seems likely that there may be a close interrelationship between 

 the electrical characteristics of a cell's membranes, organelles and macromolecules and 



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