The origin of the heart and early morphogenesis
The origin of the heart has been classically studied in the developing embryo by producing prospective maps of the early blastoderm (DeHaan, 1965; Rosenquist & DeHaan, 1966). Recently, fate mapping has traced the origin of the heart back to the primitive-streak stages (Garcia-Martinez & Schoenwolf, 1993). Precardiac cells ingress through the primitive streak and migrate toward the anterior embryonic pole to form a loose but cohesive epithelial sheet. Then, precardiac cells condense in crescent-shaped areas of the splanchnic mesoderm. The formation of the precardiac mesoderm is the first morphological indication of the heart anlage. The anlage is formed of premyocardial cells, preendocardial cells, and associated extracellular material (Figure 10.1). Later fusion of the paired primordium under the endodermal foregut results in formation of the primitive tubular heart (Figures 10.1-10.2). This process has been reviewed in Manasek et al., 1986; Icardo, 1988; and Icardo, Fernandez-Teran, and Ojeda, 1990.
The mechanisms that regulate condensation of the precardiac cells to form the cardiogenic crescent are poorly understood. The use of cell adhesion molecules (CAMs) has provided some information on this matter. N-cadherin, a calcium-dependent CAM that shows an even distribution on the surface of the early mesodermal cells, changes to an apical localization as the mesoderm splits into the parietal and splanchnic layers, the pericardial coelom forms, and mesodermal cells condense into the cardiogenic crescent (Linask, 1992). Similarly, integrin (a cell surface receptor for fibronectin) redistributes to a basal localization, and Na+, K+- ATPase activity becomes restricted to the lateral surfaces of the premyocytes (Linask, 1992).