A neonate has a ventricular septal defect in the muscular (trabecular) part of the interventricular septum. The muscular septum develops by the upgrowth of which structure?

• A Upgrowth of the muscular floor of the ventricular chambers from below (trabecular muscle growth) ✓
• B Endocardial cushion tissue from both AV canal walls
• C Neural crest cells migrating through the outflow tract
• D Epicardial cells proliferating into the ventricular wall

Explanation

The interventricular septum has two parts: the larger muscular (trabecular) part, which develops by the upgrowth of myocardium from the floor and walls of the ventricles (trabecular growth and remodeling), and the smaller membranous part, which is completed by fusion of tissue from the endocardial cushions, the bulbar ridges (aorticopulmonary septum contribution), and the muscular septum itself. Muscular VSDs result from deficient trabecular muscle growth. The membranous VSD (most common overall) is due to incomplete fusion of the endocardial cushions with the muscular septum and bulbar ridges.

Reference: BD Chaurasia's Human Anatomy, 8th ed.

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