Advanced - Cardiac Looping
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Cardiac looping occurs from late in the fourth week to early in the fifth week and has been studied in great detail in chick embryos whose initial, straight heart tube is representative of that developing in humans. There are two animations showing advanced cardiac looping. The first is shown below and represents heart looping from a ventral view. The second animation of cardiac looping shows the heart looping from a left, lateral perspective. A written description of the process is outlined below. (Click image to play on current page or Play video on new page).
<html5media height="720" width="560">File:Heart looping 006.mp4</html5media>
The second stage of looping, creating an S-shape, occurs as the ventricular bend moves caudally and the distance between the outflow and inflow tracts diminishes. At this time the dorsal mesocardium degenerates, forming the transverse pericardial sinus (a point of communication across the pericardial coelom). In addition, the atrial and outflow poles converge and myocardial cells are added, forming the truncus arteriosus. The final stage of cardiac looping is the wedging of the aorta between the atrioventricular (AV) valves. This occurs during septation and is dependent on the retraction and rotation of the myocardium by 45°.
The heart is the first organ in the body to express left-right asymmetry in the form of looping. The left-right axis of the heart is established during gastrulation. Shh is expressed in the left side of Henson’s node in the primitive streak but is inhibited by the activation of Activin IIa receptors in the right. Shh leads to the expression of Nodal and Pitx2 in the left sided mesoderm. Pitx2 expressed in the left side of the heart tube is then responsible for asymmetric organogenesis.
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