Rectal cancer. Light micrograph of a section through rectal tissue where the mucosa is turning cancerous. The mucosa is the layer that secretes a slime called mucus that lubricates the passage of food through the digestive system. The irregular appearance of the cells in the glandular structure across top is an indication that uncontrolled and cancerous growth may be starting. The cells in the glandular structure across bottom have a regular and healthier appearance. Rectal cancer is thought to be connected to a diet high in fat and low in fibre. Pre-cancerous areas are surgically removed to prevent the cancer developing and spreading.