RELATIONSHIP BETWEEN FIBRONECTIN EXPRESSION DURING GASTRULATION AND HEART FORMATION IN THE RAT EMBRYO

By utilizing myosin immunostaining, we were able to identify early rat myocardium as a thin epithelial sheet and realized that its cohesive movement toward the midline leads to the straight heart tube formation . Localization study of fibronectin mRNA and protein was, therefore, c arried out to investigate its tissue origin and possible roles in faci litating mesoderm migration and heart formation, Fibronectin mRNAs wer e first detected throughout the mesoderm during the early primitive st reak stage, suggesting that the mesoderm is the source of fibronectin. By pre-head fold (pre-somite) and head fold (early somite) stages, th e mesoderm became largely down-regulated for fibronectin mRNAs, while it was also at these stages when myosin-positive myocardium formed its elf into the epithelium and was subsequently folding toward the midlin e, Thus, there appears to be little fibronectin synthesis during and d irectly relevant to early heart tube formation, Later, during the earl y straight heart tube stage (5 somite and older), endocardium became h ighly positive for fibronectin mRNAs, suggesting that the endocardium is the major source of fibronectin for the cardiac jelly. Based on the results, we present a map for the early mammalian heart in which the heart is a single crescentic band lying in front of the prechordal pla te. We also suggest a process for heart tube formation based on the co hesive movement of the myocardial epithelium, During heart tube format ion, fibronectin protein had been deposited previously by the mesoderm and was found uniformly in the ECM and not newly produced by any adja cent tissue. The data contradict the endodermal guidance of heart migr ation by fibronectin gradient and suggest, instead, a permissive role for the fibronectin substrate. (C) 1995 Wiley-Liss, Inc.