From heart-forming region to ballooning chambers

This chapter describes the formation of the adult four-chambered heart from the precardiac mesodermal cells. The precardiac mesoderm develops into a linear heart tube by the process of folding. The subsequent increase in size of the heart by the addition of precursor cells derived from the first and second heart fields is discussed. For the sake of clarity, the chapter describes the addition of precursor cells to the inflow and outflow, separately. Next, the formation of the ventricular chambers with respect to ballooning and differentiation into a compact and trabecular layer is discussed. Finally, the formation of the septa in the heart tube is described, creating the adult four-chambered heart.

From heart-forming region to ballooning chambers

This chapter describes the formation of the adult four-chambered heart from the precardiac mesodermal cells. The precardiac mesoderm develops into a linear heart tube by the process of folding. The subsequent increase in size of the heart by the addition of precursor cells derived from the first and second heart fields is discussed. For the sake of clarity, the chapter describes the addition of precursor cells to the inflow and outflow, separately. Next, the formation of the ventricular chambers with respect to ballooning and differentiation into a compact and trabecular layer is discussed. Finally, the formation of the septa in the heart tube is described, creating the adult four-chambered heart.

Cells that express MyoD mRNA in the epiblast are stably committed to the skeletal muscle lineage

The epiblast of the chick embryo contains cells that express MyoD mRNA but not MyoD protein. We investigated whether MyoD-positive (MyoDpos) epiblast cells are stably committed to the skeletal muscle lineage or whether their fate can be altered in different environments. A small number of MyoDpos epiblast cells were tracked into the heart and nervous system. In these locations, they expressed MyoD mRNA and some synthesized MyoD protein. No MyoDpos epiblast cells differentiated into cardiac muscle or neurons. Similar results were obtained when MyoDpos cells were isolated from the epiblast and microinjected into the precardiac mesoderm or neural plate. In contrast, epiblast cells lacking MyoD differentiated according to their environment. These results demonstrate that the epiblast contains both multipotent cells and a subpopulation of cells that are stably committed to the skeletal muscle lineage before the onset of gastrulation. Stable programming in the epiblast may ensure that MyoDpos cells express similar signaling molecules in a variety of environments.

Regulation of avian cardiogenesis by Fgf8 signaling

The avian heart develops from paired primordia located in the anterior lateral mesoderm of the early embryo. Previous studies have found that the endoderm adjacent to the cardiac primordia plays an important role in heart specification. The current study provides evidence that fibroblast growth factor (Fgf) signaling contributes to the heart-inducing properties of the endoderm. Fgf8 is expressed in the endoderm adjacent to the precardiac mesoderm. Removal of endoderm results in a rapid downregulation of a subset of cardiac markers, including Nkx2.5 and Mef2c. Expression of these markers can be rescued by supplying exogenous Fgf8. In addition, application of ectopic Fgf8 results in ectopic expression of cardiac markers. Expression of cardiac markers is expanded only in regions where bone morphogenetic protein (Bmp) signaling is also present, suggesting that cardiogenesis occurs in regions exposed to both Fgf and Bmp signaling. Finally, evidence is presented that Fgf8 expression is regulated by particular levels of Bmp signaling. Application of low concentrations of Bmp2 results in ectopic expression of Fgf8, while application of higher concentrations of Bmp2 result in repression of Fgf8 expression. Together, these data indicate that Fgf signaling cooperates with Bmp signaling to regulate early cardiogenesis.