A kind of “intravital” microscopy, developed at John Condeelis’s lab at the Albert Einstein College of Medicine, reveals that macrophages from within the tumor pair off with cancer cells to enter a blood vessel that would otherwise be impenetrable to the cancer cells. The macrophage has the chops, so to speak, to pry apart two adjacent blood vessel cells and make a hole through which the cancer cell can escape to colonize other parts of the body.12 And the cancer cells are desperate to escape, since their own reproductive success creates a suffocatingly crowded environment within the tumor,
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