The wall of the colon and rectum is made up of several layers (see the illustration in the
staging section). Colorectal cancer starts in the innermost layer and can grow through some
or all of the other layers. Knowing a little about these layers is important, because the stage
(extent of spread) of a colorectal cancer depends to a great degree on how deeply it grows
into these layers. For more detailed information, please see the section “How is colorectal
cancer staged?