Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

1102 Chapter 20: Cancer
cells grow as benign tumor in epithelium
cells become invasive and enter capillary
normal epithelium
basal lamina
capillary
travel through bloodstream
(fewer than 1 in 1000 cells will survive
to form metastases)
adhere to blood vessel
wall in liver
escape from blood vessel
to form micrometastasis
colonize liver, forming
full-blown metastasis
Figure 20–16 Steps in the process of metastasis. This example illustrates the spread of a tumor from an organ such as the
bladder to the liver. Tumor cells may enter the bloodstream directly by crossing the wall of a blood vessel, as diagrammed here,
or, more commonly perhaps, by crossing the wall of a lymphatic vessel that ultimately discharges its contents (lymph) into the
bloodstream. Tumor cells that have entered a lymphatic vessel often become trapped in lymph nodes along the way, giving rise
to lymph-node metastases.
Studies in animals show that typically far fewer than one in every thousand malignant tumor cells that enter the bloodstream
will colonize a new tissue so as to produce a detectable tumor at a new site.
wall of a blood or lymphatic vessel. Lymphatic MBoC6 vessels, m20.17/20.16 being larger and having
more flimsy walls than blood vessels, allow cancer cells to enter in small clumps;
such clumps may then become trapped in lymph nodes, giving rise to lymphnode
metastases. The cancer cells that enter blood vessels, in contrast, seem to
do so singly. With modern techniques for sorting cells according to their surface
properties, it has become possible in some cases to detect these circulating tumor
cells (CTCs) in samples of blood from cancer patients, even though they are only a
minute fraction of the total blood-cell population. These cells, in principle at least,
provide a useful sample of the tumor-cell population for genetic analysis.
Of the cancer cells that enter the lymphatics or bloodstream, only a tiny proportion
succeed in making their exit, settling in new sites, and surviving and proliferating
there as founders of metastases. Experiments show that fewer than one
in thousands, perhaps one in millions, manage this feat. The final step of colonization
seems to be the most difficult: like the Vikings who landed on the inhospitable
shores of Greenland, the migrant cells may fail to survive in the alien environment;
or they may only thrive there for a short while to found a little colony—a
micrometastasis—that then dies out (Movie 20.3).
Many cancers are discovered before they have managed to found metastatic
colonies and can be cured by destruction of the primary tumor. But on occasion,