As classically described, the precardiac mesoderm of the paired heart-forming fields migrate and fuse anteriomedially in the ventral midline to form the first segment of the straight heart tube. This segment ultimately forms the right trabeculated ventricle. Additional segments are added to the caudal end of the first in a sequential fashion from the posteriolateral heart-forming field mesoderm. In this study we report that the final major heart segment, which forms the cardiac outflow tract, does not follow this pattern of embryonic development. The cardiac outlet, consisting of the conus and truncus, does not derive from the paired heart-forming fields, but originates separately from a previously unrecognized source of mesoderm located anterior to the initial primitive heart tube segment. Fate-mapping results show that cells labeled in the mesoderm surrounding the aortic sac and anterior to the primitive right ventricle are incorporated into both the conus and the truncus. Conversely, if cells are labeled in the existing right ventricle no incorporation into the cardiac outlet is observed. Tissue explants microdissected from this anterior mesoderm region are capable of forming beating cardiac muscle in vitro when cocultured with explants of the primitive right ventricle. These findings establish the presence of another heart-forming field. This anterior heart-forming field (AHF) consists of mesoderm surrounding the aortic sac immediately anterior to the existing heart tube. This new concept of the heart outlet's embryonic origin provides a new basis for explaining a variety of gene-expression patterns and cardiac defects described in both transgenic animals and human congenital heart disease.

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Soon after fertilization, vertebrate embryos grow very rapidly. Thus, early in gestation, a sizeable yet underdeveloped organism requires circulating blood. This need dictates the early appearance of a contractile heart, which is the first functional organ in both the avian and mammalian embryo. The heart arises from paired mesodermal regions within the anterior half of the embryo. As development proceeds, these bilateral precardiac fields merge at the midline to give rise to the primary heart tube. How specific areas of nondifferentiated mesoderm organize into myocardial tissue has been a question that has long intrigued developmental biologists. In recent years, the regulation of Wnt signal transduction has been implicated as an important event that initiates cardiac development. While initial reports in Drosophila and the bird had implicated Wnt proteins as promoters of cardiac tissue formation, subsequent findings that the WNT inhibitors Dkk1 and crescent possess cardiac-inducing activities led to the contrary hypothesis that WNTs actively inhibit cardiogenesis. This seeming contradiction has been resolved, in part, by more recent information indicating that Wnts stimulate multiple signal transduction pathways. In this review, we will examine what is presently known about the importance of regulated Wnt activity for the formation of the heart and the development of the myocardium and discuss this information in context of the emerging complexity of Wnt signal transduction.

Cardiac morphogenesis proceeds from a sequential series of epithelial-mesenchymal transitions which begins by establishing bipotential heart-forming cells and later their segregation into endocardial and myocardial lineages. Cells within each lineage integrate to form two concentric epithelia which inductively interact to transform cells of the inner epithelium, the endocardium, into mesenchymal or ‘cushion’ cells. Noncardiogenic epithelia (dorsal mesocardium, epicardium, neural ectoderm and coelomic mesothelium) undergo transition into populations of extracardiac mesenchyme that combine over time with cushion tissue to remodel the simple tubular heart into a four-chambered organ. Model systems are described for studying the mechanisms of cardiac-related transformations including primary cultures of precardiac epithelia and a differentiation-inducible, avian stem cell line called QCE-6. Focus is centered on the molecular mechanism by which endocardial epithelium transforms into cushion mesenchyme. Experimental findings are reviewed and interpreted in the context of a hypothetical model that seeks to answer why only some cells within an epithelium transform and whether the transformation process is regulated by intrinsic or extrinsic mechanisms. The model proposes that epithelial cells competent to transform to mesenchyme express characteristic markers including receptors for extrinsic signals secreted by stimulator cells (e.g. myocardium). Candidate extrinsic signals include multicomponent complexes called adherons. If applied directly to cultured endocardium, myocardial adherons but not those secreted by L6 myoblasts, induce changes in gene expression within target endocardial cells for proteases and cellxell and celkmatrix adhesion molecules that accompanied transformation to mesenchyme. A main component of myocardial adherons has been identified as ES antigens, one of which, ES/130, has been cloned, found to have a novel sequence and in culture assays shown to be required for endocardium to transform to mesenchyme. The spatiotemporal pattern of ES protein expression within the embryo suggests that common mechanisms may exist for embryonic epithelial-mesenchymal transformations.

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