Distribution of connective tissue proteins during development and neovascularization of the epicardium

The epicardium is the site of initial cardiac neovascularization and formation of the coronary circulatory system. Recent evidence indicates that vascular progenitor cells are influenced by the connective tissue proteins of their extracellular environment, yet little is known about the composition or function of the embryonic epicardial extracellular matrix (ECM). This study examines the distribution of ECM proteins during the migration, growth and maturation of epicardial cells and also during the development of the coronary vascular network. Immunofluorescence microscopy was used to determine the distributions of vitronectin, fibronectin and a newly described fibrillin-like protein, the JB3 antigen, in the embryonic chicken heart. Immunoblot analysis was performed to compare the relative electrophoretic mobilities of the JB3 antigen and fibrillin-1. The data show that vitronectin and fibronectin are present at sites of initial migration of the epicardial cells. The expression of vitronectin (and also fibronectin) becomes more pronounced as the epicardium thickens, undergoes remodeling and differentiates. The JB3 antigen is prominently expressed in the coronary arteries, allowing visualization of their connection to the systemic circulation and to the heart muscle, as well as vessel wall formation and organization. Immunoblot analysis suggests that the JB3 antibody recognizes a fibrillin-like polypeptide that is distinct from fibrillin-1. The observed distributions of vitronectin and fibronectin are consistent with roles in migration of epicardial cells, in remodeling of the epicardium and as substratum components during blood vessel formation. The observed distribution of the JB3 antigen indicates a structural/organizational role in coronary arterial wall assembly and suggests that the JB3 antibody be considered an early marker for maturing coronary arteries.