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 types.
 Epithelium (epithelial tissue) covers body surfaces, lines body cavities, and forms glands. Connective tissue underlies or supports the other three basic tissues, both structurally and functionally.  Muscle tissue is made up of contractile cells and is responsible for movement.
 Nerve tissue receives, transmits, and integrates infor mation from outside and inside the body to control the activities of the body.
Each basic tissue is defined by a set of general morphologic characteristics or functional properties. Each type may be further subdivided according to specific characteristics of its 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; for muscle and nerve tissue, it is primarily functional. Moreover, the same parameters exist in designating the tissue subclasses. For example, whereas 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.
Epithelium is characterized by close cell apposition and presence at a free surface.
Epithelial cells, whether arranged in a single layer or in multiple layers, are always contiguous with one another. In addition, they are usually joined by specialized cell-to-cell junctions that create a barrier between the free surface and the adjacent connective tissue. The intercellular space between epithelial cells is minimal and devoid of any structure except where junctional attachments are present.
Free surfaces are characteristic of the exterior of the body, the outer surface of many internal organs, and the lining of the body cavities, tubes, and ducts, both those that ultimately communicate with the exterior of the body and those that are enclosed. The enclosed body cavities and tubes include the pleural, pericardial, and peritoneal cavities as well as the cardiovascular system. All of these are lined by epithelium.
Subclassifications of epithelium are usually based on the shape of the cells and the number of cell layers rather than on function. Cell shapes include squamous (flattened), cuboidal, and columnar. Layers are described as simple (single layer) or stratified (multiple layers). Figure 4.1 shows epithelia from two sites. Both are simple epithelia (i.e., one cell layer thick). The major distinction between the two examples is the shape of the cells, cuboidal versus columnar. In both epithelia, however, the cells occupy a surface position.
Connective tissue is characterized on the basis of its extracellular matrix.
Unlike epithelial cells, connective tissue cells are conspicuously separated from one another. The intervening spaces are occupied by material produced by the cells. This extracellular material is called the extracellular matrix. The nature of the cells and matrix varies according to the function of the tissue. Thus, subclassification of connective tissue takes into account not only the cells but also the composition and organization of the extracellular matrix.
A type of connective tissue found in close association with most epithelia is loose connective tissue (Fig. 4.2a). In fact, it is the connective tissue that most epithelia rest on. The extracellular matrix of loose connective tissue contains loosely arranged collagen fibers and numerous cells. Some of these cells, the fibroblasts, form and maintain the extracellular matrix. However, most of the cells are migrants from the vascular system and have roles associated with the immune system.