Connective tissue binds structures together, provides support and protection, fills spaces, produces blood cells, and stores fat. The body uses this stored fat for energy, insulation, and organ protection. As a rule, connective tissue cells are widely separated by an extracellular matrix composed of an organic ground substance that contains fibers and varies in consistency from solid to semifluid to fluid. Whereas the functional and physical properties of epithelial tissues are derived from its cells, connective tissue properties are largely derived from the characteristics of the matrix (Table 4.2). The fibers within the matrix are of three types. White fibers contain collagen, a substance that gives the fibers flexibility and strength. Yellow fibers contain elastin, which is not as strong as collagen but is more elastic. Reticular fibers are very thin, highly branched, collagenous fibers that form delicate supporting networks.
Fibrous Connective Tissue
Fibrous connective tissue includes loose connective tissue and dense connective tissue. The body’s membranes are composed of an epithelium and fibrous connective tissue.
Loose (areolar) connective tissue commonly lies between other tissues or between organs, binding them together. The cells of this tissue are mainly fibroblasts-large, star-shaped cells that produce extracellular fibers (Fig. 4.5). The cells are located some distance from one another because they are separated by a matrix with a jellylike ground substance that contains many white (collagenous) and yellow (elastic) fibers.
Figure 4.5 Loose (areolar) connective tissue. This tissue has a loose network of fibers.
The white fibers occur in bundles and are strong and flexible. The yellow fibers form a highly elastic network that returns to its original length after stretching. Adipose tissue (Fig. 4.6) is a type of loose connective tissue in which the fibroblasts enlarge and store fat, and there is limited extracellular matrix.
Dense connective tissue (Fig. 4.7) has a matrix produced by fibroblasts that contains bundles of white collagenous fibers. In dense regular connective tissue, the bundles are parallel as in tendons (which connect muscles to bones) and ligaments (which connect bones to other bones at joints). In dense irregular connective tissue, the bundles run in different directions. This type of tissue is found in the inner portion of the skin. The fibroblasts of reticular connective tissue are called reticular cells, and the matrix contains only reticular fibers. This tissue, also called lymphatic tissue, is found in lymph nodes, the spleen, thymus, and red bone marrow. These organs are a part of the immune system because they store and/or produce white blood cells, particularly lymphocytes. All types of blood cells are produced in red bone marrow.
Figure 4.6 Adipose tissue. The cells are filled with fat droplets.
In cartilage, the cells (chondrocytes), which lie in small chambers called lacunae, are separated by a matrix that is solid yet flexible. Unfortunately, because this tissue lacks a direct blood supply, it heals very slowly. The three types of cartilage are classified according to the type of fiber in the matrix.
Hyaline cartilage (Fig. 4.8) is the most common type of cartilage. The matrix, which contains only very fine collagenous fibers, has a glassy, white, opaque appearance. This type of cartilage is found in the nose, at the ends of the long bones and ribs, and in the supporting rings of the trachea. The fetal skeleton is also made of this type of cartilage, although the cartilage is later replaced by bone.
Elastic cartilage has a matrix containing many elastic fibers, in addition to collagenous fibers. For this reason, elastic cartilage is more flexible than hyaline cartilage. Elastic cartilage is found, for example, in the framework of the outer ear.
Fibrocartilage has a matrix containing strong collagenous
fibers. This type of cartilage absorbs shock and reduces friction between joints. Fibrocartilage is found in structures that withstand tension and pressure, such as the pads between the vertebrae in the backbone and the wedges in the knee joint.
Bone is the most rigid of the connective tissues. It has an extremely hard matrix of mineral salts, notably calcium salts, deposited around protein fibers. The minerals give bone rigidity, and the protein fibers provide elasticity and strength, much as steel rods do in reinforced concrete. The outer portion of a long bone contains compact bone.
Compact bone consists of many cylindrical-shaped units called an osteon, or Haversian system (Fig. 4.9). In an osteon, matrix is deposited in thin layers called lamellae that form a concentric pattern around tiny tubes called central canals. The canals contain nerve fibers and blood vessels. The blood vessels bring nutrients to bone cells (called osteocytes) that are located in lacunae between the lamellae. The nutrients can reach all of the cells because minute canals (canaliculi) containing thin extensions of the osteocytes connect the osteocytes with one another and with the central canals. The ends of a long bone contain spongy bone, which has an entirely different structure.
Spongy bone contains numerous bony bars and plates called trabeculae separated by irregular spaces. Although lighter than compact bone, spongy bone is still designed for strength. Like braces used for support in buildings, the solid portions of spongy bone follow lines of stress. Blood cells are formed within red marrow found in spongy bone at the ends of certain long bones.
Figure 4.7 Dense regular connective tissue. Parallel bundles of collagenous fibers are closely packed.
Figure 4.8 Hyaline cartilage. The matrix is solid but flexible.
Figure 4.9 Compact bone. Cells are arranged in a cylindricalmanner about a central canal.
Table 4.2 Classification of Connective Tissue
Blood (Fig. 4.10) is a connective tissue composed of cells suspended in a liquid matrix called plasma. Collectively, the blood cells are called formed elements. Blood cells are of two types: red blood cells (erythrocytes), which carry oxygen, and white blood cells (leukocytes), which aid in fighting infection. Also present are platelets, which are important to the initiation of blood clotting. Platelets are not complete cells; rather, they are fragments of giant cells found in the bone marrow. In red bone marrow, stem cells continually divide to produce new cells that mature into the different types of blood cells. Blood is unlike other types of connective tissue in that the extracellular matrix (plasma) is not made by the cells of the tissue. Plasma is a mixture of different types of molecules that enter blood at various organs.
Figure 4.10 Blood. When a blood sample is centrifuged, the formed elements settle out below the plasma. Plasma is the liquid portion of the blood. Red blood cells, white blood cells, and platelets are called the formed elements.