A metaphor has been emerging among biophysicists: cells have rheostats or dimmer switches.  The metaphor implies that some cellular regulatory processes are not just on or off; they have continuous ranges of values that can be finely tuned for the need of the organism.  It’s been years since our first report that gene expression is controlled by molecular rheostats (01/10/2003), but this week, the metaphor appeared twice: once for embryonic spinal development in tadpoles, and another for DNA repair response that is “a matter of life or death.”

In PNAS, the metaphor was right in the title: “Development of a spinal locomotor rheostat.”1 Scottish scientists studied Xenopus tadpole spinal cord development.  They found that the first pools of neurons are undifferentiated, but rapidly sort out into ventral and dorsal domains within a day.  As the tadpole encounters more and more need to swim with finesse, the neurons become more specialized.  “This unfolding developmental plan, which occurs in the absence of movement, probably equips the organism with the neuronal substrate to bend, pitch, roll, and accelerateduring swimming in ways that will be important for survival during the period of free-swimming larval life that ensues.”  In other words, the developmental program has a kind of foresight into what the tadpole will need, and fine-tunes the “rheostat” of neural specialization to permit the tadpole to interact with its environment….

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