Cells within cells (within cells)

One tumor cell can burrow its way entirely inside another. Now, Michael Overholtzer, Joan Brugge (Harvard Medical School, Boston, MA), and colleagues report that, while its fate is usually met in the host's lysosome, in some cases the burrowing cell can divide or pop back out, or even go along for the ride as the host burrows into yet another cell.

Such “cell-in-cell” structures are common in tumors, but the mechanism of invasion was unknown. By labeling human breast cancer cells with different colors, the authors showed that 25% of cells contained other cells within 12 hours of detachment from their substrate. While apoptosis of one cell can drive phagocytosis by another, blockade of either process did not diminish the rate of invasion.

Invasion was suppressed, however, by stopping...