The Questions of Developmental Biology

Specification of the heart cells occurs gradually, with the ventricular cells becoming
specified prior to the atrial cells (Markwald et al. 1998).
Cell differentiation occurs independently in the two heart-forming primordia that are
migrating toward each other (Figure 15.3). As they migrate, the cells begin to express N-cadherin
on their apices and join into an epithelium. A small population of these cells then downregulates
N-cadherin and delaminates from the epithelium to form the endocardium, the lining of the heart
that is continuous with the blood vessels. The epithelial cells form the myocardium (Manasek
1968; Linask and Lash 1993; Linask et al. 1997). The myocardium will form the heart muscles
that will pump for the lifetime of the organism.
The endocardial cells
produce many of the
heart valves, secrete the
proteins that regulate
myocardial growth, and
regulate the placement of
nervous tissue in the
heart.
As neurulation proceeds, the foregut is formed by the inward folding of the splanchnic
mesoderm (Figures 15.3 and 15.4). This movement brings the two cardiac tubes together,
eventually uniting the myocardium into a single tube. The bilateral origin of the heart can be
demonstrated by surgically preventing the merger of the lateral plate mesoderm (Gräper 1907;
DeHaan 1959). This results in a condition called cardia bifida, in which a separate heart forms
on each side of the body (Figure 15.4E). The two endocardia lie within this common tube for a
short while, but these will also fuse. At this time, the originally paired coelomic chambers unite to
form the body cavity in which the heart resides.