Development of the Vertebrate Heart and the Extracellular Matrix*

Abstract

Abstract Three major topics in vertebrate cardiogenesis were surveyed with special attention given to the cellular migration and the extracellular matrix (ECM).1. Prior to primitive tubular heart formation, cardiogenic (lateral mesoderm) cells migrate on the endoderm to the definitive position ventral to the foregut. SEM observations of newt embryos revealed a subtle network of fine fibrils on the endodermal cell surface. These ECM fibrils were proved to be composed of fibronectin by immuno‐SEM method using latex beads.2. The epicardium does not differentiate in situ from so‐called “epi‐myocardium”, but is formed in such a way that tips of mesothelial cell clusters extending from the septum transversum region attach to the dorsal side of the cardiac loop and spread gradually over the myocardium, eventually covering the entire heart. Using ruthenium red‐containing fixative, precipitated ECM was found specifically on the myocardium.3. Cells of the cushion mesenchyme are known to be liberated from the endothelial cells and migrate into the cardiac jelly. There is growing evidence that ECM affects the cellular behavior in this process. Furthermore, neural crest cells were shown to contribute to mesenchyme formation in the outflow tract. Disturbed formation of the cushion tissue likely results in various malformations of the heart.