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.