The extracellular matrix (ECM) provides a structural framework for the formation of the complex 3-dimensional organization of tissues of the body. The ECM ranges from little more than a fluid medium to a rather rigid scaffolding. The composition and organization of the ECM are very dynamic in most tissues, particularly during embryogenesis, wound repair, and some disease states (Borg & Terracio, 1990; Borg & Burgess, 1993). Cellular interactions with the ECM influence a diverse array of processes not only through the ECM's role as a physical scaffold but also through specific signal transduction mechanisms (see Hynes, 1992, and Juliano & Haskill, 1993, for reviews).
The ECM of the heart is intimately associated with development and maintenance of heart structure and function. The heart ECM is composed of collagens, particularly types I, III, IV, and VI, proteoglycans, and noncollagenous glycoproteins such as laminin and fibronectin (Table 5.1; see also Kitten, Markwald, & Bolender, 1987; Borg & Terracio, 1990; Bashey, Martinez-Hernandez, & Jiminez, 1992; Borg & Burgess, 1993; Carver, Terracio, & Borg, 1993). These ECM components are expressed in precise temporal and spatial patterns in the developing and mature heart (see, e.g., Little et al., 1989; Iruela-Arispe & Sage, 1991; Sinning, Krug, & Markwald, 1992; Zagris, Stavridis, & Chung, 1993). Laminin, for example, appears localized to basement membranes of specific cells of the developing embryo and is particularly abundant in the heart (Figure 5.1). Laminin continues to be localized to the basement membranes associated with cardiomyocytes and vessels in the ventricular wall in the mature heart (Figure 5.1C). Expression of laminin is altered during cardiovascular development and with disease (Lipke et al., 1993).