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The MBL’s Friday Evening Lecture series got off to a great start last week with “Building Brain Circuits” by Hollis Cline of Scripps Research Institute who co-directs the MBL Neurobiology course. Advanced imaging technology lets us see directly and in exquisite detail the “bush-like” character of individual nerve cells in living brains. That nerves are bushy has long been known through images of fixed (non-living) preparations. But what was not fully appreciated was that these images represented only snapshots in time–that the bushes were constantly in the process of being remodeled. Holly’s revelatory experiments with live nerve cells showed that the branches were continually growing and dying back–I nearly wrote “pruned” but that would raise the question of who was doing the pruning. And that brings me to the topic of this blog.

The verb “build” in common usage implies a “builder.” The wonder of living things is that objects of great complexity, such as brains, are “built” without a “builder.” How is this possible? Doesn’t construction require blueprints?

An alternative approach is variation and selection. This, of course, is the fundamental principle discovered by Darwin that underlies evolution. Death is as necessary as reproduction for the origin of species. Pruning is as necessary as branching for the shaping of a bush. In my own field of molecular cell biology, the shortening of microtubules is as important as their growth for the shaping of cells. So why should we be surprised that deconstruction is as necessary as construction for the self-building of a brain?

Dual Innervation of the Tadpole Optic Tectum. Image taken by Ed Ruthazer

Dual Innervation of the Tadpole Optic Tectum. Image taken by Ed Ruthazer