LOOPING OF THE LINEAR HEART TUBE
As a consequence of its formation, differentiation, and rudimentary
functionality, the linear heart tube is mostly postmitotic. During the fourth
week of human gestation, growth and elongation of the linear heart tube occur
by means of contribution and division of second heart field cells at both the
sinus venosus and truncus arteriosus (posterior and anterior poles,
respectively). Concurrently, an embryo-wide genetic program breaks the final
axis of symmetry the left-right axis. Asymmetrical intercellular signaling on
the left side of the embryo governs the migration and division of second heart
field cells in the lengthening heart tube, leading to two major morphological
cardiac asymmetries. First, the entire linear heart tube displaces to the right
and rotates 90 degrees about its
anterior-posterior axis, so that the original ventral surface of the linear
tube is now the left side of a C-shaped tube (Fig. 1.3). Second, asymmetrical
mitotic expansion of the second heart field contributions leads to localized
“ballooning” of the primitive atrial and ventricular regions of the heart tube,
transforming the C-shaped tube into an S-shaped heart (Fig. 1.3).
 |
| FIG 1.3 Formation of the heart loop. |
Further gross morphogenetic movements of the embryo bring the two poles
in close apposition, anterior to the primitive chambers. This repositioning
prepares the inflow and outflow tracts for appropriate connections to the
developing vasculature, thereby contributing to proper segregation of oxygenated
and deoxygenated blood flow among the heart, lungs, and body. By 30 days of
gestation, the prospective atria are repositioned
anterior to the ventricular region, marking the first
resemblance of the embryonic heart to its future adult structure.
Formation of the S-looped heart overlaps with the beginnings of
ventricular and outflow tract septation and valve development as endocardial
cushions emerge within the atrioventricular junction and the outflow tract.
