128
The Body Electric
BONE IS A RECTIFIER—A PN JUNCTION DIODE
We could now follow the entire control system of Wolff's law. Me-
chanical stress on the bone produced a piezoelectric signal from the
collagen. The signal was biphasic, switching polarity with each stress-
and-release. The signal was rectified by the PN junction between apatite
and collagen. This coherent signal did more than merely indicate that
stress had occurred. Its strength told the cells how strong the stress was,
and its polarity told them what direction it came from. Osteogenic cells
where the potential was negative would be stimulated to grow more
bone, while those in the positive area would close up shop and dismantle
their matrix. If growth and resorption were considered as two aspects of
one process, the electrical signal acted as an analog code to transfer infor-
mation about stress to the cells and trigger the appropriate response.
Now we knew how stress was converted into an electrical signal. We
had discovered a transducer, a device that converts other forces into elec-
tricity or vice versa. There was another transducer in the Wolff's-law
system - the mechanism that transformed the electrical signal into ap-
propriate cell responses. Our next experiment showed us
something
about how
this one worked.