Extracellular matrix (ECM) proteins have been implicated as mediators of 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 ECM proteins present at sites of valvuloseptal morphogenesis, and to determine how their patterns of expression change during the developmental process.

Immunofluorescence microscopy was used to examine the distribution of fibulin‐1, vitronectin, and fibronectin in the embryonic chicken heart over a broad developmental time frame (Hamburger and Hamilton stages 14 to 44), emphasizing stages that illustrate endocardial cushion formation, growth, fusion, and development into valvuloseptal components.

Fibulin‐1 immunolabeling was concentrated in endocardial cushions, notably at boundaries with the myocardium, during stages when the cushions are differentiating into valvular and septal components. Fibulin‐1 was detected in the endocardial cushions 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. Vitronectin was observed before endocardial cushion cell migration commenced and persisted until the formation of prevalvular structures (early midgestation) in the atrioventricular cushions. Vitronectin remained detectable in the semilunar valves until late midgestation. Fibronectin was present in the endocardial cushion region and in portions of the endocardium and myocardium throughout the stages presented. Our data suggests that the ECM of the endocardial cushions undergoes remodelling in a regionally and temporally specific manner which corresponds with morphogenetic changes during valvuloseptal development. © 1996 Wiley‐Liss, Inc.