Mixed cultures of avian blastoderm cells and the quail mesoderm cell line QCE-6 provide evidence for the pluripotentiality of early mesoderm.

During the early stages of embryogenesis, the mesoderm gives rise to cells of the cardiovascular system which include cardiac myocytes and vascular endothelial and red blood cells. We have investigated the development of these cell phenotypes using aggregate cultures of avian blastoderm cells, which replicated mesodermal cell diversification. The cell phenotypes expressed by the blastoderm cells were dependent upon the age of the blastoderm cells, with Hamburger-Hamilton stage 3 or 4 cells giving rise to endothelial and red blood cells and stage 5 cells producing endothelial and myocardial cells. To begin to understand the stage dependency of the cellular diversification of these aggregate cultures, we treated the cultures with various signaling factors that have been shown to be present in the early avian embryo. These experiments showed that stem cell factor and TGF alpha altered cell phenotypes by stimulating red blood cell and myocardial differentiation, respectively. The ability of these growth factors to shift the differentiation profile of aggregate cultures demonstrated the plasticity of early embryonic cells. To explore the diversification of individual mesodermal cells, labeled QCE-6 cells were incorporated within these blastoderm aggregate cultures. Previous studies have shown that this quail mesodermal cell line possesses characteristics of early nondifferentiated mesodermal cells and can be induced to express either myocardial or endothelial cell phenotypes (C. A. Eisenberg and D. M. Bader, 1996, Circ. Res. 78, 205-216). In the present study, we show that when these cells were cultured as a component of blastoderm cell aggregates, they differentiated into fully contractile cardiomyocytes or endothelial or red blood cells. Moreover, QCE-6 cell differentiation was in accordance with that displayed by the blastoderm cells. Specifically, QCE-6 cells differentiated into red blood cells when cultured within stage 3 or stage 4, but not stage 5, blastoderm cell aggregates. Accordingly, the differentiation of QCE-6 cells into beating cardiomyocytes only occurred when these cells were incorporated into stage 5 blastoderm cell aggregates. The identical sorting and differentiation patterns that were exhibited by QCE-6 and blastoderm cells suggest that expression of differentiated cell types within the early mesoderm is directed by the surrounding environment without immediate cellular commitment. In addition, these results provide further evidence that QCE-6 cells are representative of a multipotential mesodermal stem cell and that they possess the potential to exhibit fully differentiated cell phenotypes.