Early Development of the Heart – Blood Vessel and Heart Development

by John McLachlan, PhD

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    00:01 If we look at where the heart itself begins from, we’re looking down in this image at the floor of the embryonic cavity, and we can see the primitive streak at the bottom of the image. The heart mesoderm is actually forming in front of the head, so underlying the ectoderm and forming in a horseshoe shape as the embryo begins to fold over, so the heart mesoderm will come to lie underneath the head as it shifts towards its final position. One interesting property of these cells is that even if they were taken out into culture and isolated, individual cells from this heart mesoderm will begin to beat spontaneously. And as they join up, they will entrain on each other and begin to beat as a sheet. So the fact that heart beats is an intrinsic property of the differentiation state of heart cells. Once the heart is folded over, the horseshoe shape begins to come together in the midline. And in the first image, marked A, we can actually see the two remnants on either side of the horseshoe shape.

    01:09 These are coming together to fuse in the midline. Blood is flowing into the embryo, flowing into the heart from the bottom through the parts marked atrial primordia, and flowing into a region which will be the primitive ventricle. At these stages, the heart is essentially two-chambered. Blood flows into the atrial part. It’s then pumped into the ventricular part and then squeezed out through the outlet tract of the heart and around the rest of the body. At a slightly later stage, the heart tube is beginning to become more clearly identifiable, and we can see it is looking quite markedly to the side. So it’s throwing itself into the side. In this image, we can see the ventricular part, the ventricle as quite a large part, and the blood is flowing into the atrial regions and then being squeezed into the ventricular part, and then squeezed from there into the truncus arteriosus up around the head and through the body. It will of course flow through the body down into the placenta. It will be refreshed with oxygen in the placenta and then flow back into the body again, coming into the atrial parts again after that time. This is the same image seen in the solid view, and you can see the ventricular part, the bulbus cordis which is the truncus, the outflow tract of the heart, and the atrial region down towards the bottom. Now, I know that it’s difficult but we have to try and imagine this diagram in three dimensions wherein the middle one, it’s projecting towards you. And in the third one marked 28 days, the atrial parts are behind the truncus arteriosus. So it’s resting on them from behind, but it’s difficult to grasp that in a two-dimensional diagram. Here, we see the same images. So the truncus is in front and the atrial parts are behind, and the large ventricle is bulging out towards you from the plane of the image. Now, at this stage, the heart is still essentially two chambered, and it has to transform itself into a four-chambered structure while still beating, which as you can imagine, is a difficult and complex trick.

    About the Lecture

    The lecture Early Development 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 Early Development of the Heart – Blood Vessel and Heart Development

     John McLachlan, PhD

    John McLachlan, PhD

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