In cartilage (kar'ti-lii), the cells (chondrocytes), which lie in small chambers called lacunae (lah-ku'ne), 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 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 windpipe. 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 found in the knee joint.
Bone is the most rigid of the connective tissues. It has an extremely hard matrix of mineral salts, primarily 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. In compact bone, bone cells (called osteocytes) are located in lacunae that are arranged in a concentric cylinder called a Haversian system (osteon). Haversian systems form around tiny tubes called central canals, which contain nerve fibers and blood vessels. The blood vessels bring the nutrients that allow bone to renew itself. The nutrients can reach all of the cells because minute canals (canaliculi) containing thin processes 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 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.
Cartilage and bone are support tissues. Cartilage is more flexible than bone because the matrix is rich in protein, rather than the mineral salts found in bone.
Blood is a cormective tissue in which the cells are separated by 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, cells called stem cells continually divide to produce new cells that mature into the different types of blood cells. The rate of cell division is high.
Blood is unlike other types of connective tissue in that the intercellular matrix (that is, 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.
Blood is a connective tissue in which the matrix is plasma.
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