Connective tissue is composed of cells and an extracellular matrix that binds the cells and organs, integrating all parts of the body. It is the most widespread tissue of the body and can be found in every organ. The nonliving extracellular matrix is usually the most abundant portion of connective tissue and has two components, fibers and ground substance. A variety of cell types are found in connective tissues. The cell type that distinguishes the type of connective tissue exists in immature and mature stages. Immature cells undergo cell division and produce and secrete the extracellular matrix. Mature cells are embedded in the extracellular matrix and act to maintain the matrix.
The types and proportions of fibers of the extracellular matrix determine the features of the connective tissues, whether they are strong and hard, gelatinous, flexible, elastic, or fluid. The three types of fibers found in connective tissues are elastic, collagen, and reticular fibers. An elastic fiber is a type of fiber found in connective tissue that can stretch and recoil. A collagen fiber is a type of fiber found in connective tissue that provides strength. A reticular fiber is a type of fiber found in connective tissue that has a netlike formation and provides support.
The ground substance is composed of extracellular fluid, dissolved substances, and large globular proteins found in the extracellular matrix. The ground substance serves as a diffusion medium, helps cells stick to the matrix, and aggregates proteins to form a gel-like substance. The composition of the ground substance varies between connective tissue types.
The amount of vascularization, or blood supply, of connective tissues ranges from avascular, having no blood supply, to highly vascularized. This disparity is related to the structure, function, and metabolic activity of the tissue. For example, avascular connective tissue such as cartilage is thin and relies on diffusion for nutrient exchange, whereas adipose (fat) and bone are highly vascularized in order to transport materials to and from the tissue.
There are four main types of connective tissue. These include:
1. Connective tissue proper includes loose and dense connective tissues. It contains various cells including fibroblasts to produce fibers of the extracellular matrix, cells that provide an immune function (macrophages, white blood cells, and plasma cells), mast cells to inhibit clotting, and adipocytes that store fat. Connective tissue proper can be classified as loose connective tissue, which contains more ground substance than fibers, and dense connective tissue, which contains more fibers than ground substance.
- Loose connective tissue is highly vascularized and includes areolar tissue, reticular tissue, and adipose tissue. Areolar tissue has a gelatinous matrix with a cushioning and protective function. It is primarily found under the skin and around organs. Reticular tissue contains a high proportion of reticular fibers and makes up the sponge-like tissue of lymph organs. Adipose tissue is a type of loose connective tissue with a high proportion of large fat cells that protects organs and serves as a nutrient reserve. An adipocyte is a cell in connective tissue that stores fat. Adipose tissue is unique because it contains more cells than extracellular matrix. When fat accumulation occurs, the fat-storing compartments of the adipocytes grow larger while the total cell number does not change. White fat is a type of adipose that stores energy in the form of large lipid droplets inside the adipocytes. It functions as energy storage, insulation, and cushioning of internal organs. Brown fat is a specialized type of adipose that stores fats as small droplets and burns them to release heat. It is found primarily in the neck and shoulder region of fetuses and infants, with adults retaining a very small amount.
- Dense connective tissues include regular and irregular forms and are densely packed with collagen fibers, which provides strength. Dense regular connective tissue gets its name from the parallel orientation of the collagen fibers of the matrix. Examples of this tissue are tendons, ligaments, and fibrous membranes surrounding various organs and structures. Dense irregular connective tissues contain randomly oriented collagen fibers and make up the dermis layer (lower layer) of the skin. Both forms of dense connective tissues contain few blood vessels.
2. Cartilage is a firm yet flexible type of connective tissue that consists of cells and a rubbery matrix. A chondroblast is an immature cell that produces the components of the cartilage matrix including collagen fibers and globular proteins, such as proteoglycans. Proteoglycans create a gel-like extracellular fluid that resists compression, an important function of cartilage. Once embedded in the matrix, chondroblasts become mature cells found in cartilage that maintain the extracellular matrix, each called a chondrocyte. There are three main types of cartilage: hyaline cartilage, elastic cartilage, and fibrocartilage. Hyaline cartilage is located in the joints. It prevents adjacent bones from rubbing together as they move, cushions joints, and absorbs the stress of frequent impact. Elastic cartilage contains chondrocytes held by an elastic matrix. This cartilage provides the flexible structure of parts of the ears and nose. Fibrocartilage is an especially tough and dense form of cartilage. It is found between vertebrae. Cartilage is an avascular tissue, and because of this lack of blood supply, it is slow to heal when damaged.
3. Bone (osseous) tissue is a hard, mineralized tissue found in the skeletal system. There are two types of bone tissue, spongy and compact. Spongy bone has a netlike appearance, while compact bone appears more uniform, lacking large, open spaces. Within the hard bone tissue are living cells that maintain the structural integrity of bone. Osteoblasts are immature cells that form the matrix in circular layers called concentric lamellae. Collagen is the major fiber type in bone tissue, and about two-thirds of the matrix is minerals. Lamellae surround tunnels called central canals, which contain the blood vessels and nerves serving the bone. When osteoblasts are embedded in the bone tissue they produced, they mature into a less active cell, the osteocyte.4. Blood is a fluid connective tissue that contains red and white blood cells and platelets in an extracellular matrix called plasma. The plasma is considered ground substance, with fibers only present during blood clotting. The primary functions of blood are to transport materials throughout the body and to protect the body against foreign agents.