Talk:Intermediate - Heart Tube Looping
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--Phoebe Norville 11:09, 22 September 2009 (EST) Content added (no text/formatting):
Cardiac looping occurs from late in the fourth week to early in the fifth week. The straight heart tube begins to elongate with simultaneous growth in the bulbus cordis and primitive ventricle. This forces the heart to bend ventrally and rotate to the right forming a C-shaped loop with convex side situated on the right. The second stage of looping creates an S-shape as the ventricular bend moves caudally and the distance between the outflow and inflow tracts diminishes. The atrial and outflow poles converge and myocardial cells are added forming the truncus arteriosus. The following animations portray these concepts from the traditional ventral view of the heart as well as right and left sided views.
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.
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).