There are a series of events that have to occur in order for a benign tumor to develop into a malignant tumor. Speaking at a presentation during the "2001 NIH Mini-Med School," Dr. Alan Rabson, Deputy Director of the National Cancer Institute, explains:
"First the tumor cell - there's a basement membrane that contains it and it
stays where it is. But the first step is to activate enzymes that will chew
a hole in the basement membrane. Now, it turns out that in normal cells there
are enzymes like that. Then the second thing you need are motility factors.
If the cell gets through the basement membrane, it's not going to go anywhere
unless it's got some sort of factors that steer it to move in a direction. And
if it gets up to the edge of a blood vessel, then it can stick onto the blood
vessel with things that are called adhesion molecules. And then it can activate
its digesting enzymes again and chew a hole in the vessel, wiggle its way in
with its motility factors, and then it can go all sorts of other places, like
the lungs or the liver. And when it gets there it gets out of the vessel just
like it got in, by digesting the hole."
This is Calvin Jackson, the National Institutes of Health, Bethesda, Maryland.