Page 3, Early Development, Devel. Physiol., Dr. D. Penney

Factors Controlling Early Heart Development:

Growth of the heart is controlled by both intrinsic and extrinsic factors. Intrinsic factors are not related to functional demands, while extrinsic factors are (Table 2.02).

Synthesis of the contractile machinery (myofibrils, myosin, actin, etc.) is not dependent upon the presence of hemodynamic forces. Looping, when the cardiac tube changes from a linear to a "C" shape is also not dependent upon hemodynamic forces. For example, high K+ stops the heart beat but does not stop morphogenesis. A critical mass of myofibrils is required for this process to occur. It can be stopped by cycloheximide, an inhibitor of protein synthesis.

Thus, morphogenesis is due to intrinsic properties of the developing heart.

On the other hand, during the "post-looping" stage a decreased inflow of blood through the A-V canal results in an abnormally small left atrium and ventricle, a condition known as hypoplasia. It also produces modification of the aorta. We conclude that embryogenesis after looping is affected by hemodynamic forces.

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