What is connective tissue?

Structure and Function

Cells, the structural and functional units of life, are organized into tissue, a group of different types of cells and their nonliving intracellular matrix, or glue, that performs a specialized function. The four groups of tissues are epithelial (covering and lining tissue; also glands); connective (adipose, blood, bone, cartilage, ligament, and tendon); muscle (skeletal, cardiac, and smooth); and nervous (brain and spinal cord).

Connective tissue typically has cells widely scattered throughout a large amount of intracellular matrix (that is, a substance in which the cells are embedded), unlike epithelial tissue that typically has cells arranged in an orderly manner and has a limited amount of intracellular matrix.

Connective tissues are categorized as loose (areolar), dense, and specialized. Some connective tissues are difficult to classify, with the distinction between “loose” and “dense” not clearly defined. Also, dense connective tissue may be called fibrous connective tissue because of the large amount of collagen or elastin fibers contained.

Because a tissue is defined as a collection of different cells, several types of cells may be found in various types of connective tissue: fibroblasts, which secrete collagen and other elements of the extracellular matrix, thereby creating and maintaining the matrix; adipocytes, which store excess caloric energy in the form of fat; and mast cells, macrophages, leukocytes, and plasma cells, which have immune functions and, therefore, an active role in inflammation. The components of the matrix are different in the various types of connective tissue and may include fibers, amorphous ground substances (glycoproteins, proteins, and proteoglycans), and tissue fluid. Each type of connective tissue has a characteristic pattern of cells and a distinctive amount and type of matrix. For example, bone matrix includes minerals, while blood has plasma for a matrix.

Loose connective tissue is the most common type of connective tissue; it holds organs in place and attaches epithelial tissue to underlying tissues. Loose connective tissue can be further categorized based on the type of fibers and how the fibers are arranged: collagenous fibers, which are composed of collagen and are arranged as coils; elastic fibers, which are composed of elastin and are able to stretch; and reticular fibers, which join connective tissue to other tissues. Loose connective tissue has a relatively large amount of cells, matrix, or both, and a relatively small amount of fibers. Loose connective tissue is found in the hypodermis and fascia (the connective tissue that loosely binds structures to one another).

Dense connective tissue is identified by the high density of fibers in the tissue and a low density of cells and matrix. The type of fiber that predominates determines the type of dense connective tissue. Dense collagenous connective tissue, for example, contains an abundance of collagen fibers and is found in structures where tensile strength is needed, such as the sclera (white) of the eye, tendons, and ligaments. Dense elastic connective tissue contains an abundance of elastin fibers and is found in structures where elasticity is needed (for example, the aorta).

Specialized connective tissues include adipose tissue, cartilage, bone, and blood. Adipose tissue is a form of loose connective tissue that stores fat. It is found in the fatty layer around the abdomen, in bone marrow, and around the kidneys. Cartilage is a form of fibrous connective tissue. It is composed of closely packed collagenous fibers embedded in a gelatinous intracellular matrix called chondrin. While the skeleton of human embryos are composed of cartilage, cartilage does not become bone but rather is replaced by bone. The replacement is not universal; cartilage provides flexible support for ears (external pinnae), nose, and trachea. Bone is a type of mineralized connective tissue, and it contains collagen and calcium phosphate. Cells found in bone include osteoblasts, which form new bone for growth, repair, or remodeling, and osteoclasts, which break down bone for growth and remodeling. The living cells are found in spaces in the calcified matrix. These spaces are called lacunae and are interconnected by small channels called canaliculi that eventually join up with blood vessels in the bone organ. Thus, even in a solidified matrix, living cells are able to obtain nutrients and expel wastes.>

Blood too is a type of specialized connective tissue. Blood may seem to be an unlikely connective tissue, but it fits the definition: different cells widely dispersed in intracellular matrix, working together to perform a specific function. Unlike other connective tissues, blood has no fibers. Blood does have several types of cells: red blood cells or erythrocytes, white blood cells or leukocytes (with subdivisions of monocytes, macrophages, eosinophils, lymphocytes, neutrophils, and basophils), and platelets or thrombocytes. The matrix is liquid and contains enzymes, hormones, proteins, carbohydrates, and fats.

Disorders and Diseases

Connective tissue, like any other tissue, is subject to disorders and diseases. Some disorders are inherited (passed from one generation to the next by means of DNA in chromosomes), while other disorders are related to environmental factors (such as a lack of specific nutrients).

Some inherited connective tissue disorders are Marfan syndrome and osteogenesis imperfecta. In Marfan syndrome, connective tissue grows outside the cell, having deleterious effects on the lungs, heart valves, aorta, eyes, central nervous system, and skeletal system. People with Marfan syndrome are often unusually tall with long, slender arms, legs, and fingers. In osteogenesis imperfecta, or brittle bone disease, the quantity and quality of collagen is insufficient to produce healthy bones. People with this disorder have multiple spontaneous bone breaks. Other connective tissue diseases are environmental, such as scurvy, which is caused by a lack of vitamin C required for the production and maintenance of collagen. Without sufficient vitamin C in the diet, and subsequent lack of collagen, the patient will develop spots on the skin, particularly the legs and thighs; will be tired and depressed; and may lose teeth. Osteoporosis has many factors, but lack of vitamin D and calcium in the diet will lead to a thinning of the bone, subjecting the patient to fractures, primarily of the hip, spine, and wrist.

Connective tissue diseases may also be classified as systemic autoimmune disease and may have both genetic and environmental causes. In these situations, the immune system is spontaneously overactivated and extra antibodies are produced. Examples of systemic autoimmune diseases include systemic lupus erythematosus and rheumatoid arthritis. Systemic lupus erythematosus can damage the heart, joints, skin, lungs, blood vessels, liver, kidneys, and nervous system. More woman than men are diagnosed with lupus, and more black women than other groups. Rheumatoid arthritis is caused when immune cells attack the membrane around joints and destroys the cartilage of the joint; it can also affect the heart and lungs and interfere with vision.

Bibliography

Gordon, Caroline, and Wolfgang Gross. Connective Tissue Diseases: An Atlas of Investigation and Management. Oxford: Clinical Publishing, 2011.

Lundon, Katie. Orthopedic Rehabilitation Science: Principles for Clinical Management of Nonmineralized Connective Tissue. Boston: Butterworth-Heinemann, 2003.

"Mixed Connective Tissue Disease." Mayo Clinic, May 30, 2012.

Price, Sylvia Anderson, and Lorraine McCarty Wilson, eds. Pathophysiology: Clinical Concepts of Disease Processes. St. Louis: Mosby, 2003.

"Questions and Answers about Heritable Disorders of Connective Tissue." National Institute of Arthritis and Musculoskeletal and Skin Diseases, October 2011.

Royce, Peter M., and Beat Steinmann, eds. Connective Tissue and Its Heritable Disorders: Molecular, Genetic, and Medical Aspects. New York: Wiley-Liss, 2002.