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Heart Tube Looping
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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:
  • Blood is pumped through outflow tract.
  • Primitive heart tube
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:
  • Further elongation and bending creates an S-shape:
    • 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:
  • Elongation is complete:
    • 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.
Features of the Adult Heart
Fetal circulation