US – Cancer is particularly feared because of its ability to metastasize. When this process has started, you can often find small clumps of twenty to one hundred tumor cells in blood. Since these so-called circulating tumor cell clusters are larger than the tiny capillaries of the body, it is unclear whether they are in fact the cause of metastases or whether the latter emerge from individual tumor cells.

Actually, the tumor cell clusters would get stuck because of their size in the capillaries. This however does not make sense that they can be detected in the venous blood. Blood is removed from the arm vein, has passed through the capillaries of the lungs and the fingers and should actually contain no lumps, if they were too large for the capillary vessels.  Sam Au from Massachusetts General Hospital in Boston and his colleagues have now found an explanation for this paradox.

As they report in their study published in PNAS (Proceedings of the National American Academy of Sciences), the tumor cell cluster dissolve before passing through the capillary vessels on which cells pass individually the vessel to subsequently clump again. This is demonstrated by flow experiments in artificial microchannels, through computer simulations and by working with transparent zebrafish.  There are indications that it can stop the orderly passage of tumor cell clusters through the capillaries and thus may prevent metastasis.