Intermediate - Heart Tube Looping

From Embryology
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Begin Intermediate: Primordial Heart Tube  Heart Tube Looping  Atrial Ventricular Septation  Outflow Tract  Heart Valves  Cardiac Abnormalities  Vascular Overview


Cardiac Embryology     Begin Basic     Begin Intermediate     Begin Advanced  


HeartILP draft itimeline.jpg


Looping of the heart tube allows the straight heart tube to form a more complex structure, reminiscent of the adult heart. Most of cardiac looping occurs during the fourth week and completes during the fifth week of development. The steps in looping can be summarised as:

  1. The straight heart tube begins to elongate with simultaneous growth in the bulbus cordis and primitive ventricle.
  2. This forces the heart to bend ventrally and rotate to the right forming a C-shaped loop with convex side situated on the right.
  3. The ventricular bend moves caudally and the distance between the outflow and inflow tracts diminishes.
  4. The atrial and outflow poles converge and myocardial cells are added forming the truncus arteriosus.

Hence, an S-shape is formed with the first bend of the 'S' being the large ventricular bend, while the bend at the junction of the atrium and sinus venosus forms the second 'S' bend.

The following animation portrays these concepts from the traditional, ventral view of the heart. You can also view this animation from a lateral left or right view.


<Flowplayer height="564" width="720" autoplay="false">Heart looping 002.flv</Flowplayer>


Throughout the morphological changes of the heart tube during looping, the locations of the divisions of the heart tube change, as described above.

Movement of the parts of the heart tube during looping

The following scanning electron micrographs illustrate the same morphological changes in the heart tube described above. The straight heart tube can be seen to form a C-shape, with the convex bulge on the right side of the embryo representing much of the expansive conotruncus and right ventricle. By day 25, it is possible to see the early formation of the S-shaped loop where the left-sided predominant bulge indicates the left ventricle and the atrium and sinus venosus have moved dorsally.

File:HeartILP draft SEMseries.jpg
Series of scanning EMs showing the rapid change in the appearance of the heart tube

Note that previously, the heart was attached to the dorsal pericardial wall by the dorsal mesocardium. During heart looping, the dorsal mesocardium degenerates forming the transverse pericardial sinus (a point of communication across the pericardial coelom).


Back to the Primordial Heart Tube Next: Heart Septation
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