Distribution of vitronectin in the embryonic chick heart during endocardial cell migration

In the early phase of heart development, the endocardial cells migrate into the truncal swellings and atrioventricular (AV) cushions, and become mesenchymal cells. Vitronectin is a glycoprotein which is thought to mediate cell migration. The present study demonstrates by immunohistochemistry the distribution of vitronectin in order to elucidate its contribution to endocardial cell migration in the developing chick heart. At Hamburger and Mamilton's stage 23, the network of fibrillar material filled the extracellular space of both truncal swellings and AV cushions. The fibrillar network has been thought to be a matrix for endocardial cell migration. The network was stained with the anti-vitronectin antibody. At stage 29, the swellings and cushions were packed with mesenchymal cells, though immunoreactivity to the antibody was still observed in the extracellular matrix. The myocardium facing the AV cushions reacted to the antibody, but the myocardium surrounding the truncus arteriosus did not. The intensity of the immunohistochemical staining of the myocardium facing the AV cushions increased and reached a peak at stages 24 to 26, and then became weak by stage 29. The endocardial sheet, aortico-pulmonary septum and developing tunica media of the great arteries were not stained by the antibody at any stage. These results strongly suggest that vitronectin is involved in the migration of endocardial cells, and that the myocardium facing the AV cushions produces vitronectin.