Excessive trabeculations in noncompaction do not have the embryonic identity

Ventricular noncompaction is characterized by excessive trabeculations and is associated with heart failure. The lesion is hypothesized to result from failed compaction and thus retention of embryonic trabeculations. Here, we assess for the first time the identity of trabeculations in noncompaction to test whether noncompacted hearts show retention of embryonic trabeculations. Using immunohistochemistry, we analyzed cardiac sections of the heart of a control embryo, 3 cases of fetal noncompaction (a set of twins and an unrelated fetus) and 3 fetal hearts without noncompaction. In the embryo, the ventricular trabeculations strongly expressed ANF/NPPA whereas the compact wall did not. In the noncompaction hearts, trabeculations constituted an excessively thick layer. In noncompaction and control fetal hearts alike, however, only a miniscule subset of sub-endocardial myocardium of the trabeculations most proximal to the central ventricular lumen exhibited strong expression of ANF/NPPA, representing Purkinje myocardium. The trabeculations of both fetal control and noncompaction hearts were ANF-negative and orders of magnitude wider than those of the embryo. Both the compact and noncompaction trabeculated myocardium were rich in coronary vasculature. Like embryonic trabeculations, the ANF(+) Purkinje myocardium had little if any vasculature. The excessive trabeculations in noncompaction do not have the embryonic identity and noncompaction is probably not the result of failed compaction. We propose the lesion results from the compact wall growing into the ventricular lumen in a trabecular fashion