By CARL ZIMMER (New York Times)
It’s easy to imagine the cells in our bodies like bricks in a house, all cemented into place. But we are actually seething with cells that creep, crawl, and squirm. They start wandering soon after conception, and, throughout our lives, our bodies continue to hum with cellular traffic.
Some cells burrow into old bone so that new bone can be laid down in their wake. The tips of new blood vessels snake forward, dragging the cells behind along with them. White blood cells race along on flickering lobes to chase down bacteria before they can make us sick.
The fact that cells can move is old news. How they move is just now being understood. In the mid-1600s, Antonie van Leeuwenhoek built one of the first microscopes and observed single-cell organisms making what he called “pleasing and nimble” movements. But he had no idea what was going on inside those cells, and three centuries later, scientists were still baffled.
Thomas Pollard, a biochemist at Yale, started studying crawling cells in the 1960s, when, he said, “Exactly zero was known.” Today Dr. Pollard and his colleagues have identified many of the key proteins that work together to let cells navigate through our bodies. Scientists can even see some of these proteins at work in living cells and measure their forces.
“My dream was always to be a little gremlin, to get inside the cell and watch all this stuff,” Dr. Pollard said. “This is almost like being a little gremlin.
“We’ve gone from a black box to chemistry and physics.”
One of the chief reasons for these advances is the technology that scientists can now use to watch cells in motion. When developmental biologists first began to study how embryos grow, for example, they could only look at different stages under a microscope.
Today, they make high-resolution videos of embryos and track the movement of thousands of cells — videos that overturn some traditional ideas.
“There’s tremendously more migration than we thought,” said Scott Fraser, the director of the Biological Imaging Institute at Caltech.
To undertake the migrations that form an organism from an embryo, cells need to know where to go. An embryo is awash in signals that can guide them. Different kinds of cells respond to different signals. Cells that will give rise to skin, blood vessel walls and other linings of the body — epithelial cells — are attracted to a signaling protein called epidermal growth factor. Released by white blood cells in the embryo, this protein draws the cells crawling toward their source.
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