A tissue (from latin texere = weave) is a group of cells that work together to carry out one or several functions in organisms such as plants and animals. The cells of a tissue are joined by direct cell-cell contact or by cell-extracellular matrix-cell binding. Every organ of the body is formed by several types of tissues. The part of science dealing with plant and animal tissues is known as Histology. It is mostly descriptive and use light and electron microscopy to describe tissue morphology. However, the anatomy and organization of tissues are essential to understand the normal physiology and pathology of the organs.
Despite the many types of cells an organism may contain, tissues have usually been classified into four types:
Epithelial tissue. It is made up of tightly packed cells with many cell-cell adhesion contacts. Epithelium covers the external body surfaces and internal cavities. Furthermore, most glands develope from epithelial tissue.v
Connective tissue. Different types of tissues form this group. The majority of them shows a well developed extracellular matrix, and many important functions of connective tissues rely on their extracellular matrices. Connective tissue cells are differentiated from mesenchymal cells and can be found all through the animal body. Support, nutrition and storing are typical functions of connective tissues. Histologists further classified connective tissue in several tissues subtypes: connective proper (loose, dense, mesenchymal, gelatinous), adipose, cartilage, bone, and blood.
Muscle tissue. It is made up of muscle cells, that allow animals to move because these cells have the ability of decrease their size, ie contraction, and recover the original length by stretching.
Nervous tissue. Highly specialized cells form the nervous tissue: neurons and glia. Neurons receive information coming from both internal and external environments. They process and integrate information and send the result to other neurons or to other cell types, typically to muscle cells.