tissue

In biology, any kind of cellular fabric that occurs in an organism's body. It is a group of similar cells that are carrying out a function in a plant or animal. Several kinds of tissue can usually be distinguished, each consisting of cells of a particular kind bound together by cell walls (in plants) or extracellular matrix (in animals). Thus, nerve and muscle are different kinds of tissue in animals, as are parenchyma and sclerenchyma in plants. Tissues of different kinds may be found in a distinct structure, which is then called an organ. The leaf of a plant or the heart of a mammal is an organ.

One example of tissue is the epithelium. Epithelial tissue is made up of epithelial cells of several kinds, such as ciliated epithelial cells and cells producing mucus. Epithelial cells line the air passages and the gut. In the air passages the epithelial tissue protects the walls from dust and micro-organisms that are trapped in the mucus and swept up to the mouth by the beating, hair-like cilia. In the gut, the epithelium protects the wall by producing mucus, but there are no ciliated cells. The mucus also helps food move along. In addition, digestive enzymes which digest food are produced by the epithelium.