Heart Tube Looping Cardiovascular System > Embryology > Embryology

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.

1 / 3 - 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

FULL-LENGTH TEXT

Here we will learn how the heart tube elongates and loops to bring its chambers into alignment; after alignment is established, further development of the chambers, outflow tracts, and valves can occur. To begin, denote that we will illustrate the changes that occur by days, 22, 23, 24, and 28; bear in mind that these days are approximations, and that intertextual and interindividual variation exist. Let's draw the primitive heart tube at day 22: From caudal to cranial, label the regions: Sinus venosus, primitive atrium and ventricle, which are linked via the atrioventricular canal, the bulbus cordis and truncus arteriosus, which comprise the outflow tract. Denote that, by day 22, blood is pumped through the outflow tract. 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. Again, label the regions: sinus venosus, atrium, ventricle, bulbus cordis, and truncus arteriosus. Draw arrows to indicate that 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. Denote that shifting ruptures the dorsal mesocardium, which anchored the heart to the dorsal body wall; the rupture creates the transverse sinus of the pericardium. By day 24, further elongation and bending creates an S-shape: Indicate that 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. Denote that, by day 28, elongation is complete: The tube approximates a U-shape, so that the atria and ventricles are properly aligned for blood flow. Cranially, indicate that the truncus arteriosus gives rise to the aortic sac, which as we'll learn elsewhere, gives rise to the aortic arches. Then, indicate that the bulbus cordis can be subdivided into the conus cordis, which becomes part of the outflow tract, and, where the bulbus cordis shifted caudally, it gives rise to the developing primitive right ventricle. Label the left primitive ventricle, and, the bulboventricular sulcus, which is the external indentation that marks the internal primary muscular fold (formerly called the bulboventricular fold). Indicate the where the right and left atria will ultimately reside. Write that the sinus venosus shifts dorsally; as we'll learn elsewhere, it

2 / 3 undergoes a series of reconfigurations to form, most notably, portions of the right atrium and the orifices of the caval veins and coronary sinus. A key aspect of heart tube looping is right-left orientation along the crainiocaudal axis, which is established early in development. Denote that failure to establish sidedness leads to laterality defects, including dextrocardia (right-sided heart), ventricular and atrial septal defects, and outflow defects.