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The anterior lateral plate mesoderm has the capacity to form distinct cardiomyogenic cell lineages soon after gastrulation. A minimal defined culture system in vitro is adequate to maintain the lineages' positionally dependent maturation. Cardiogenic mesoderm and endoderm were removed from the chicken embryo at Hamburger and Hamilton stages 4-8 to determine whether undifferentiated cardiogenic tissue could diversify into distinct anterior and posterior lineages before the heart was formed.

A study of the cellular sources of heart cells using the QCE-6 cell line isolated from a differentiating cardiomyogenic cell from avian cardiogenic mesoderm reveals that retinoic acid and growth factors added to the culture medium activate the expression of cardiac myogenic and endothelial cell phenotypes by QCE-6 cells at high frequency. Several cardiac- and muscle-specific proteins are expressed by the activated cardiomyogenic cells. Differentiation of QCE-6 cells is similar to the formation of an initially differentiated precontractile cardiac myocyte from cardiogenic mesoderm.

Analysis of cardiac myocyte progenitors' commitment during gastrulation reveals that pregastrulated cells undergo restricted cardiac myocyte differentiation and that postgastrulated anterior lateral-plate mesoderm consists of committed cardiac progenitors with unequal differentiating capability. Bromodeoxyuridine (BrdU) fully inhibits the progenitor cell differentiation from the 4th and 5th stages but not from stages 7 and 8. The progenitors acquire BrdU resistance during their migration and postgastrulation.

Indirect histochemical analysis of an embryotic heart revealed the presence of DNA-binding proteins found exclusively in the nucleus of cardiac cells. Samples from different stages of development were immunoprecipitated and incubated to test for the presence and characteristics of the proteins. Data indicated that at least one protein occurs transiently through the various stages of heart cell development. Furthermore, this protein reacts with MyoD1 and myogenin.

We have used a subtractive method to clone novel messages enriched in the heart. Here we show that one such message, byes (blood vessel/epicardial substance) is a novel protein that is highly conserved between chicken and mouse. The byes message is detected at high levels in early chick hearts. Using anti-Byes antibodies, we show expression in cells of the proepicardial organ, migrating epicardium, epicardial-derived mesenchyme, and smooth muscle of the developing intracardiac arterial system, including the coronary arteries. Our data suggest that Bves is an early marker of developing vascular smooth muscle cells. In addition, the expression pattern of Bves protein reveals the patterning of intracardiac vascular smooth muscle and possible insights into the cellular regulation of smooth muscle differentiation during vasculogenesis.