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

Table 2.02 - How Early Heart Growth Is Controlled:

  • Intrinsic factors not related to functional demands.

    A. Synthesis of contractile machinery - not dependent upon presence of hemodynamic forces.

    B. Looping - linear tube to "C" shape:
    Therefore, not dependent on hemodynamic forces (high K+ stops heart beat, but not morphogenesis).
    Critical mass of myofibrils required (e.g. inhibition by cycloheximide).
    {conclusion: morphogenesis is due to intrinsic properties of heart itself!}

  • Extrinsic factors which can be related to functional demands.

    C. Post-looping:
    Decreased inflow of blood thru atrioventricular canal results in abnormally small LA and ventricle (hypoplasia); modification of aorta by selective occlusion of aortic arches.
    {conclusion: embryogenesis after looping is affected by hemodynamic forces!}

    D. Postnatal:
    Pressures, ie. workload of RV & LV about equal before birth; increased total systemic peripheral resistance post-birth results in faster growth of LV relative to RV.
    Increased workload on either ventricle results in compensatory hypertrophy, either normal or abnormal growth.

    Therefore, dependent on hemodynamic forces.

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