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. These cells communicate and adhere to one another by direct cell-cell contacts and through intermediary molecules like those that form the extracellular matrix. Each 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 a mostly descriptive discipline that uses light and electron microscopy to describe tissue morphology. The anatomy and organization of tissues are essential to understand the normal physiology and pathology of the organs. Histopathology is a branch of histology that deals with alterations and diseases of tissues.
Despite the highly diversity in forms and functions of the cell of an organism, histologists have traditionally classified tissues into four main types:
Epithelial tissue. It is made up of tightly packed cells arranged in layers with many cell-cell adhesion junctions. Epithelium covers the external body surfaces and internal cavities. Furthermore, epithelial cells form most glands of the body.
Connective tissues. They are a diverse group of tissues characterized by the prominence of the extracellular matrix, which performs salient roles in the tissue functions. Connective tissues differentiate from embryonary mesenchymal cells and can be found all through the animal body. Support, nutrition and storing are typical functions of connective tissues. Histologists further classify connective tissue in several subtypes: connective proper, adipose, cartilage, bone, and blood.
Muscle tissue. It is made up of muscle cells that have the ability of decrease their length. The cell contraction produces movement of body parts, and therefore allows the movement of animals from a place to another.
Nervous tissue. Highly specialized cells in information processing (neurons and glia) form the nervous tissue. This tissue receives information coming from both internal and external environments, integrates such information and sends the result to other cells, or typically to muscle cells.