Hello. Welcome to Byte Size Med. This video is on the development of the ventricles and the outflow tracts. Of the different aspects in the development of the heart, I found this the hardest to really understand it and learn it. I've tried my best to simplify it and I hope it can help someone else who's trying to learn this. First let's look at some of the structures that need to develop and then we'll go back and see how that happens. There are two ventricles, the right and the left and they are separated by the interventricular septum. This septum has a muscular portion below and a membranous portion above. The ventricles have a rough inflowing part. Rough because they're trabeculated with trabeculae carneae and papillary muscles. They have a smooth outflowing part, that's the infundibulum or the conus arteriosus in the right ventricle and the aortic vestibule in the left ventricle. They lead into the pulmonary trunk on the right and the aorta on the left and we've been drawing the heart since forever. These two vessels they don't emerge straight out. They wind around each other. At the entrance to these vessels are the semilunar valves, the pulmonary valve on the right and the aortic valve on the left. Now let's go back to the heart tube. That's what the heart develops from. A quick recap from part one, the heart tube has dilations. It has an arterial end and a venous end. There's the Sinus venosus, the Primitive Atrium, the Primitive Ventricle, the Bulbus Cordis and the Truncus Arteriosus. The tube bends and I'm gonna group some of the parts together to make it a little easier. The heart loops such that the atria and the sinus venosus move upwards and backwards and the rest come forward and downwards. In this video, we're going to be looking at this part of the heart tube. That's the Truncus Arteriosus, the Bulbus Cordis and the Primitive Ventricle. The distal third of the Bulbus Cordis is the Truncus Arteriosus, the middle third is the Conus Cordis and the proximal third, it doesn't have a special name but it contributes to the formation of the trabeculated part of the right ventricle. First let's see how the outflow tracts develop. That's the ascending aorta the pulmonary trunk and the smooth outflow portions of the two ventricles. The structures we need for that are the Truncus Arteriosus and the Conus Cordis. Remember that the two vessels wind or spiral around each other. As of now they're just a single vessel emerging from the ventricles and the Truncus leads into the aortic sac. This part of the heart tube develops a septum called the spiral septum or the conotruncal septum. It's also called the aorticopulmonary septum, because of the parts that it separates. The walls of the Truncus Arteriosus develops swellings. These are called truncal swellings or truncal ridges. The neural crest cells contribute to the development of these swellings. At the same time, the conus also develops ridges and these grow towards each other and towards the truncal swellings. If we take sections at different levels, the Ridges would be something like that. So we're looking at it from above at different levels and when they fuse they create a spiral course for the two vessels, the aorta and the pulmonary trunk. The relationship between the two tracts changes through the course. First the aortic track is here then here and then here. The pulmonary course also changes. The fusion of those ridges creates this kind of spiral septum and the paths twist around each other. The ridges in the Conus Cordis form an anterolateral pathway which is the outflow tract of the right ventricle leading into the pulmonary trunk, while the posteromedial pathway forms the outflow tract of the left ventricle leading into the ascending aorta. We can see how once they separate by the septum they twist and wind around each other. And that's the formation of the aorticopulmonary septum. So the Truncus Arteriosus forms the ascending aorta and the pulmonary trunk. The Conus Cordis forms the smooth outflow tracts, that's the infundibulum or the conus arteriosus on the right and the aortic vestibule on the left. But the proximal one-third of the Bulbus Cordis forms the trabeculated part of the right ventricle. The Primitive ventricle forms the left ventricle. Between the Primitive ventricle and the Bulbus Cordis of the heart tube is a sulcus called the bulboventricular sulcus. Since this forms the right ventricle and this forms the left ventricle, that would be a site of communication between the two ventricles. So it's also called the primary interventricular foramen. The two ventricles now have to separate. If we assume this is the developing atria and this is the developing ventricles, these are the fused endocardial cushions splitting the atrioventricular canal into right and left. From the apex of the ventricular cavity, there's a muscular growth. This is the muscular part of the interventricular septum. It's partly from the fact that the ventricular chambers are enlarging as they grow. But even with that part of the septum there is a space still left behind between the two chambers. That's the interventricular foramen and it has to close. Remember that the interventricular septum has a muscular and a membranous part. So now the membranous part of the septum has to form. If we look at the heart from this perspective, this is the developing right atrium, the right ventricle, the left ventricle and the left atrium. We're looking into the Chamber of the developing right ventricle. So here we have the right and left atrioventricular canals leading into the atria which are situated behind and in between we've got those fused endocardial cushions. This area would lead into the developing left ventricle So from the floor, the muscular portion of the interventricular septum has formed. The aortico pulmonary septum has fused with the truncal and the bulbar ridges. Now there is a space still left between the chambers. That's the interventricular foramen and unlike the atria, this opening actually has to close. Remember that as the heart develops, things don't happen sequentially. They don't happen one after the other. A lot of stuff happens together at the same time. We just study it one after the other. So as this aorticopulmonary septum is fusing, the atrioventricular cushion proliferates and the two bulbar ridges fused with that proliferation close the gap. That is the membranous part of the interventricular septum. If we put it together, the right and left bulbar ridges and the atrioventricular endocardial cushion together form the membraneous part of the interventricular septum. But now the right and left ventricles are separate and they have their own outflow tracts through the pulmonary trunk and the ascending aorta. Lastly we have the semilunar valves. Again the neural crest cells form valvular swellings at the region where the semilunar valves are going to be located. At the junction between the Truncus Arteriosus and the Conus Cordis. So there are valve swellings and these are the right and left bulbar ridges. The bulbar ridges fuse when the aorticopulmonary septum forms and these two separate, receiving three swellings each which get modified to form cusps. So in the end there are three cusps for the aortic valve and three cusps for the pulmonary valve. Right, left and posterior. Right, left and anterior. And now the ventricles have developed, separated into right and left with their separate outflow tracts, that's the pulmonary trunk and the ascending aorta. I hope this video was helpful. If it was you can give it a like and subscribe to my channel. Thanks for watching and I'll see you in the next one!:)