Overview Of Tissues

Tissues are aggregates or groups of cells organized to perform one or more specific functions

At the light microscope level, the cells and extracellular components of the various organs of the body exhibit a recognizable and often distinctive pattern of organization. This organized arrangement reflects the cooperative effort of cells performing a particular function. Therefore, an organized aggregation of cells that function in a collective manner is called a tissue [Fr. tissu, woven; L. texo, to weave].

Although it is frequently said that the cell is the basic functional unit of the body, it is really the tissues, through the collaborative efforts of their individual cells, that are responsible for maintaining body functions. Cells within tissues communicate through specialized intercellular junctions (gap junctions, page 101), thus facilitating this collaborative effort and allowing the cells to operate as a functional unit. Other mechanisms that permit the cells of a given tissue to function in a unified manner include specific membrane receptors and anchoring junctions between cells.

Despite their disparate structure and physiologic properties, all organs are made up of only four basic tissue types

The tissue concept provides a basis for understanding and recognizing the many cell types within the body and how they interrelate. Despite the variations in general appearance, structural organization, and physiologic properties of the various body organs, the tissues that compose them are classified into four basic tissues:

• Epithelium (epithelial tissue), which covers body surfaces, lines body cavities, and forms glands

• Connective tissue, which underlies or supports the other three basic tissues, both structurally and functionally

• Muscle tissue, which is made up of contractile cells and is responsible for movement

• Newe tissue, which receives, transmits, and integrates information from outside and inside the body to control the activities of the body

Each of these basic tissues is defined by a set of general morphologic characteristics or functional properties. Each type may be further subdivided according to specific characteristics of their various cell populations and any special extracellular substances that may be present.

In classifying the basic tissues, two different definitional parameters are used. The basis for definition of epithelium and connective tissue is primarily morphologic, whereas for muscle and nerve tissue, it is primarily functional. Moreover, the same parameters exist in designating the tissue subclasses. For example, while muscle tissue itself is defined by its function, it is subclassified into smooth and striated categories, a purely morphologic distinction, not a functional one. Another kind of contractile tissue, myoepithelium, functions as muscle tissue but is typically designated epithelium because of its location.

For these reasons, tissue classification cannot be reduced to a simple formula. Rather, students are advised to learn the features or characteristics of the different cell aggregations that define the four basic tissues and their subclasses.

Reverse Testicular Atrophy
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