Ventricles and outflow tract

The ventricles develop from the intermediate part of the heart loop, which mainly becomes the left ventricle. The left ventricle is followed by a narrow passage between the interventricular sulcus on the outside and the muscular interventricular septum on the inside. This passage leads to a new enlargement of the tubular heart, the right ventricle. At S11, both ventricles are already recognizable by their trabeculation. The next structures up from the right ventricle are the smooth-walled conus cordis (proximal outflow tract) and then the truncus arteriosus (distal outflow tract), which are collectively known as the conotruncus. By around S12 (3.5 mm embryo), they are already bent against each other. By S16-17, mesenchymal pads develop on the inside of this bent part of the outflow tract, which develop into the semilunar valves.