Fibulin‐1, vitronectin, and fibronectin expression during avian cardiac valve and septa development

Background Extracellular matrix (ECM) proteins have been implicated as mediatorsof events important to valvuloseptal development (reviewed by Little and Rongish, Experentia, 51: 873–882, 1995). The aim of this study was to identify connective tissue ECMproteins present at sites of valvuloseptal morphogenesis, and to determine how their patterns ofexpression change during the developmental process. Methods Immunofluorescence microscopy was used to examine the distribution offibulin‐1, vitronectin, and fibronectin in the embryonic chicken heart over a broad developmentaltime frame (Hamburger and Hamilton stages 14 to 44), emphasizing stages that illustrateendocardial cushion formation, growth, fusion, and development into valvuloseptalcomponents. Results and Conclusions Fibulin‐1 immunolabeling was concentrated in endocardialcushions, notably at boundaries with the myocardium, during stages when the cushions aredifferentiating into valvular and septal components. Fibulin‐1 was detected in the endocardialcushions prior to their seeding with cushion cells, but became undetectable by early midgestation.Vitronectin expression was similar to fibulin‐1, but less restricted in its distribution. Vitronectinwas observed before endocardial cushion cell migration commenced and persisted until theformation of prevalvular structures (early midgestation) in the atrioventricular cushions.Vitronectin remained detectable in the semilunar valves until late midgestation. Fibronectin waspresent in the endocardial cushion region and in portions of the endocardium and myocardiumthroughout the stages presented. Our data suggests that the ECM of the endocardial cushionsundergoes remodelling in a regionally and temporally specific manner which corresponds withmorphogenetic changes during valvuloseptal development. © 1996 Wiley‐Liss, Inc.