Rectal cancer cells. Coloured scanning electron micrograph (SEM) of rectal carcinoma cells. The cells' nuclei are brown, within the nuclei are nucleoli (circles). These are particularly active in cancer cells, secreting enzymes that allow the rapid and immortal division of cells. Carcinomas are cancers that arise from the lining of an organ. The cells have grown very close together, forming a sheet. Colorectal cancer is one of the most common cancers in the western world. Risk factors include a diet high in animal fat and low in fibre and a sedentary lifestyle.