Some of the most common heritable disorders involve the major connective tissues of the body such as bone, skin, cartilage, blood vessels, and basement membranes. Identification of the causes of these disorders has underscored the important structural role of connective tissue proteins, such as the collagens, fibrillin, and elastin. However, these studies have also uncovered unanticipated defects in other proteins and enzymes involved in cell signaling and protein processing. The literature on connective tissue disorders is vast. Consequently, this chapter will focus on the classification and pathophysiology of these disorders and summarize the clinical features and management of the more common disorders.
The original classification of connective tissue diseases was based on the pattern of inheritance, the cluster of signs and symptoms, as well as radiologic and histologic features. Identification of the mutations causing the diseases has provided a rational framework for understanding the multiple manifestations of the diseases and has led to revisions in the classifications. Also, it has provided tests for prenatal diagnosis and important information for genetic counseling. The usefulness of these tests is likely to increase with the capability to analyze the complete genomes of patients. At the same time, the identification of mutations has thus far provided few new therapies. Also, as with many genetic diseases, the mutations are not always reliable predictors of clinical outcomes. For example, identical collagen I mutations in patients with osteogenesis imperfecta (OI) can be associated with clinical mild or lethal phenotypes for reasons that are not apparent. Also, the current classifications tend to overemphasize the etiologic differences between genetic diseases that are apparent in infants versus similar diseases that appear later in life. For example, small subsets of patients with initial diagnoses of postmenopausal osteoporosis, familial aortic aneurysms, or osteoarthritis have mutations in collagen genes similar to the mutations that are found in patients with OI or chondrodysplasia (CD).
Composition of Connective Tissues
Connective tissues contain a large number of complex macromolecules (Table 363-1). The most abundant components are three similar fibrillar collagens (types I, II, and III). They have about the same tensile strength as steel wires. The three fibrillar collagens are distributed in a tissue-specific manner: Type I collagen accounts for most of the protein of dermis, ligaments, tendons, and demineralized bone; type I and type III are the most abundant proteins of large blood vessels; and type II is the most abundant protein of cartilage.
Table 363-1 Constituents of Connective Tissues in Various Tissues |Favorite Table|Download (.pdf)
Table 363-1 Constituents of Connective Tissues in Various Tissues
|Connective Tissue||Major Constituents||Approximate Amounts, % dry wt||Characteristics or Functions|
|Dermis, ligaments, tendons|
Type I collagen
Type III collagen
Type IV collagen, laminins, and nidogen
Large bundles of fibrils
Form basal laminae under epithelium
|Types V, VI, and VII collagens||<5||V modifies type I fibrils; VI ...|