Evidence for an extracellular matrix bridge guiding proepicardial cell migration to the myocardium of chick embryos.

During heart development, the proepicardium (PE) gives rise to cells of the epicardial epithelium, connective tissue of the subepicardium and the myocardium, and smooth muscle, endothelium, and connective tissue of the coronary arteries. The PE arises as an outgrowth of the pericardial serosa at embryonic day 2 (Hamburger and Hamilton stage [HH] 14) of chick development. Between stages HH14 and HH17, multicellular villous projections extend from the PE toward the dorsal aspect of the lesser curvature of the myocardium. On reaching the atrioventricular (AV) junction, the cells spread over the myocardium, eventually enveloping the complete heart surface as a simple squamous epithelium. Although the lineage of the PE cells is well established, it remains uncertain how cells of the PE reach the myocardial surface and specifically target the AV junction. By using a combination of serial section reconstructions, immunofluorescence, and electron microscopy, we have identified an extracellular matrix bridge (ECMB) spanning the coelomic cavity between the PE and the myocardium. The ECMB is first detectable at HH14 and persists until the PE contacts the bare myocardial surface. This ECMB stains intensely with ruthenium red and Alcian blue, contains heparan sulfate and fibronectin, and exhibits both fibrillar and globular ultrastructure, reminiscent of proteoglycans. After PE attachment to the myocardium (HH16-HH17), the subepicardium exhibited strong staining for heparan sulfate. Heparinase injection into the pericardial coelom at HH15 resulted in aberrant development of the primordial epicardium. On the basis of these studies, we suggest that the ECMB may participate in migration and targeting of the PE to the myocardium. Copyright 2003 Wiley-Liss, Inc.