Maintenance of Gut Lining
After studying the digestive enzymes you would wonder why the gut linings are not digested themselves. This is because animals have several mechanisms that protect their gut lining from auto digestion. The mucous membranes of vertebrates secrete a slightly alkaline mucous that lubricates the food and protects the lining cells from corrosive secretions.
In addition, the lateral surfaces of exposed epithelial cells are joined by tight junctions that prevent the secretions from penetrating between them. Careful studies have also revealed that the entire lining of the gut is renewed every third day in rats and every 2-6 days in humans. Similar mechanisms are present in invertebrates also. In insects, the fore-gut and hind gut are lined by cuticle. This lining is known as intima. Only in the midgut, the epithelial cells are exposed, where most of the digestion occurs. The midgut is lined by a delicate lining the peritrophic membrane in some insects. This corresponds to the mucous lining of vertebrates.