Septation of the Heart – Blood Vessel and Heart Development

by John McLachlan, PhD

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    00:01 Let’s look at this process of septation of the atrium. In this view, we can see the atrial chamber from the right hand side, leading down to the atrio-ventricular junction between the atrium and the ventricle. What we can see here is a septum, the septum primum, which is growing towards the atrio-ventricular junction. The opening is known as the first opening or foramen primum. A little later, it would have grown further towards the atrio-ventricular junction. But now we can see some holes beginning to appear in the dorsal part of the septum primum. A little later still, the foramen primum has completely disappeared, but the holes have coalesced to form a second hole, the foramen secundum. Now, what will happen is that the second septum, the septum secundum will begin to develop on the right hand side of the septum primum. This will also grow forward but will never completely reach the atrio-ventricular junction. It looks as if the septum secundum is covering the hole in the septum primum. That hole is known, you’ll remember, as the foramen secundum.

    01:13 But in fact, blood can still cross from right to left. This is because the septum primum and the septum secundum are separated in space. If we look at this from a different angle, we can see that blood can flow under the edge of the septum secundum and through the foramen secundum.

    01:30 This arrangement forms the foramen ovale. Things will remain in this arrangement until the point of birth. At that time, the pressure in the left atrium will increase and this will press the septum primum against the septum secundum, cutting off the blood supply from right to left in the atrial cavity. Now we’ve looked at the heart itself, let’s look at the outflow tract. This is difficult and complex to visualize because it twists in space, and therefore, the membranes which are closing off the two sides are actually arranged in a very complex manner. That’s really all I’m going to say about at the moment.

    02:11 It’s complex and difficult to understand and it would be advanced cardiology I think to have a good understanding of all that’s happening here. So let’s review our terminology because it’s admittedly complicated. The first septum to grow down in the atrium is called the septum primum. It originally has a gap or opening at the foramen primum. Gradually, the foramen primum will disappear, but at the same time, the foramen secundum begins to appear as perforations which extend and finally join up to form the definitive foramen secundum.

    02:47 Meanwhile, the interventricular septum is growing up towards the endocardial cushions.

    02:52 The second septum, the septum secundum, is growing down parallel to the original septum primum.

    03:00 The septum secundum never completely closes off in the direction of the endocardial cushions, and blood can flow down underneath the septum secundum through the foramen secundum and into the left atrium. So that’s the root of blood as it short circuits the pulmonary circulation. Now, I promise you that it was fairly complicated. I think I’ve delivered one that promised. But what we’ve looked at so far is the formation of blood cells, the origin and looping of the early heart, and then the process of septation which divides the two-chambered heart into the four chambered heart. Thank you very much.

    About the Lecture

    The lecture Septation of the Heart – Blood Vessel and Heart Development by John McLachlan, PhD is from the course System-Specific Embryology with John McLachlan.

    Author of lecture Septation of the Heart – Blood Vessel and Heart Development

     John McLachlan, PhD

    John McLachlan, PhD

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