Connective tissue is found throughout the body, usually in association with other tissues. As its name indicates, it often serves to connect different tissues together, but it also can serve as a wrapper (in locations where a tough epithelial wrapping is not required), a structural support, cushioning, a storage repository, a protective layer, or a transport medium.
Connective tissue has the most types of subcategories and the most varied functions of all the four major tissue types (epithelial, muscular, nervous, and connective tissues.) Bone and cartilage are connective tissues, as are blood and lymph, fat, ligaments, and tendons. Epimysium, the connective tissue wrapping around skeletal muscles, and periosteum, the connective tissue wrapping around bones, are both connective tissues.
The different types of connective tissue are so diverse, there is no one set of characteristics that encompasses all the different types. However, there are three characteristics that we consider diagnostic of most connective tissue types.
- The cells are dispersed. Connective tissues generally have cells that are not tightly connected to each other, the way the cells in epithelial and muscular tissues usually are. There is usually a fair amount of space between the connective tissue cells. An exception to this is adipose tissue (also known as fat), the rare type of connective tissue in which the cells are packed tightly together.
- The tissue has more extracellular material than cells. Most connective tissues are solid (blood and lymph are the exceptions) because all the volume between the dispersed cells is filled with an extracellular matrix of viscous ground substance and protein fibers.
- An extensive network of protein fibers is found in the extracellular matrix. Protein fibers are complexes of millions of individual proteins threaded into long fibrous structures that provide strength and elasticity to the tissue as a whole. The protein fibers are so large, they are longer than the cells they surround and enmesh. Blood and lymph, being liquid connective tissues, do not have these enmeshing protein fibers, but they still have an extensive liquid extracellular matrix.
A common way of classifying the many different types of connective tissue is to subdivide it into three main sub-categories, and further divide those subcategories into specific types of connective tissue. The three main sub-categories of connective tissue are:
- Connective tissue proper.
These are the types of connective tissue that typically have all three of the defining characteristics listed above. It is further subdivided into dense connective tissue proper, in which the extracellular protein fibers predominate, and loose connective tissue proper, in which the extracellular protein fibers are not so densely woven.
- Supporting connective tissue.
Bones and cartilage are the two types of connective tissue in this sub-category. They both have all three of the defining characteristics listed above, but their extracellular matrix is tougher, denser, and more solid than the various types of connective tissue proper.
- Fluid connective tissue.
Blood and lymph are the two types of connective tissue in this sub-category. Both are fluid, rather than solid, and both lack the network of extracellular protein fibers found in the other types of connective tissue.