Heart Tube Looping

Heart Tube Looping

~ Days 22-28
Brings the chambers into alignment.
After alignment is established, further development of the chambers, outflow tracts, and valves can occur.

~Day 22

Key Points:

Anatomical Regions, from caudal to cranial:

Sinus venosus.
Primitive atrium and ventricle, which are linked via the atrioventricular canal.
The bulbus cordis and truncus arteriosus, which comprise the outflow tract.

~Day 23

Key Points:

On day 23, progenitor cells from the second heart field (SHF) are added to the tube; as it elongates, the tube bends to form a C-shape.

Elongation shifts the ventral surface to the right edge of the C-curve, while the dorsal surface of the heart is displaced to the left inner edge of the curve.
Shifting ruptures the dorsal mesocardium, which anchored the heart to the dorsal body wall; the rupture creates the transverse sinus of the pericardium.

~Day 24

Key Points:

Growth displaces the ventricle to the left, as the atrium is pushed dorsally and cranially.
Incidentally, blood circulates throughout the entire embryo by this stage.

~Day 28

Key Points:

The tube approximates a U-shape, so that the atria and ventricles are properly aligned for blood flow.
Cranially, the truncus arteriosus gives rise to the aortic sac, which later gives rise to the aortic arches.
The bulbus cordis can be subdivided into the conus cordis, which becomes part of the outflow tract, and,

caudally, the developing primitive right ventricle.

The bulboventricular sulcus is the external indentation that marks the internal primary muscular fold (formerly called the bulboventricular fold).

As the atria are pulled cranially and dorsally, the sinus venosus also shifts dorsally.

It undergoes a series of reconfigurations to form, most notably, portions of the right atrium and the orifices of the caval veins and coronary sinus.

Clinical Correlations

A key aspect of heart tube looping is right-left orientation along the crainiocaudal axis, which is established early in development.

Failure to establish sidedness leads to laterality defects, including dextrocardia (right-sided heart), ventricular and atrial septal defects, and outflow defects.

Fetal circulation