Adenomatous Polyp Formation

Healthy intestinal epithelium (colon) tissue is comprised of crypt cells connecting microscopic villi that are responsible for the final uptake of nutrients of digested material before it is removed from the body as waste.  The characteristic polyps that form in the gut lumen of FAP patients are a result of tissue build-up within the villi due to proliferative DNA replication; remember the stem cell-like replication I was talking about earlier?  The tissue build-up begins to puff out from the base of the villi and, over time, fills up the individual villi.  Eventually, the outpocketing tissues cannot be contained within a single villi and begin to spread to neighboring villi causing eruption of villus tissues.  The photo on the right demonstrates adenomatous polyp formation in mice whose APC gene has been experimentally inactivated.


Loss of APC function in FAP patients is a lifelong phenotype.  Thus, at a young age, FAP can be suspected as the cause of the formation of large numbers of polyps in these people.  Hundreds to thousands of polyps form in these people before they reach the age of 20.  Proliferative replication continues to take place throughout a lifetime and, over time, mutations accumulate in other genes.  It is this process of mutation accumulation that ends in cancer formation before the age of 50 in FAP patients.