"We grow, mostly, by dying."

A startling sentence, found in "Runs in the Family/New findings about schizophrenia rekindle old questions about genes and identity," by Siddhartha Mukherjee (in The New Yorker). Context:

Throughout the brain—particularly in the parts involved in cognition, memory, and learning—synapse pruning continues into our first three decades, which suggests that it may be responsible, in part, for the starburst of adaptive learning that characterizes the first decades of human life. We are hardwired not to be hardwired, and this anatomical plasticity may be the key to the plasticity of our minds.

In the winter of 2004, having joined the laboratory of Ben Barres, a neuroscientist at Stanford, [Beth] Stevens began to study the pruning of synapses in the visual system. “When I began my work in Ben’s lab, little was known about how specific synapses are eliminated,” she told me. “The pruning phenomenon was thought to be quite general.” There was evidence of synapse pruning in the cortex of the brain during learning, cognition, and the formation of memories. But Stevens and Barres focussed their attention on visual neurons, because they were the easiest to study: the eye would be the eye to the brain.

In 2007, they announced a startling discovery. Stevens was trying to identify the proteins that recognized and eliminated neuronal synapses during visual development. “The strangest finding was that a protein that usually tags and removes pieces of dead cells, bacterial remnants, or cellular debris was also being reworked to tag and remove the synapses,” she said. Mice designed to lack tagging proteins—called complement proteins—had problems both in clearing cellular debris and in tagging and pruning their synapses.

The Stevens and Barres study, published in the journal Cell in 2007, documented one of the most arresting instances of repurposing in biology: a protein designed to ticket germs and junk for destruction had been co-opted by the nervous system to ticket synapses for destruction. “It reinforces an old intuition,” my psychiatrist friend Hans, in Boston, told me. “The secret of learning is the systematic elimination of excess. We grow, mostly, by dying.”