Formation of Blood – Blood Vessel and Heart Development

by John McLachlan, PhD

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    00:01 Hello, and welcome to this lecture on the development of the heart and blood vessels.

    00:06 Now, I should tell you honestly right to the start that heart development is complicated and difficult to understand. So, you may have to watch this one several times before you grasp it fully. It probably wouldn’t do any harm if you revised the circulation of blood through the adult heart before starting in order to make clear of some of the processes that we’re going to be looking at. However, we can summarize what we’re going to see as we’ll look, first of all, at the formation of the blood cells, then we’ll look at the origin of the early heart, then we’ll look at looping of the heart, and then finally, division of the two-chambered heart into the four chambers of the adult heart, a process known as septation. We’re beginning very early in the embryonic process, not long after gastrulation. Up at the top of the image, you can see the embryo running across the screen. Hanging down from that is the yolk sac. On that yolk sac, little spots of red begin to appear, and these are actually blood islands where red blood cells are differentiating from the mesoderm of the yolk sac. So these are endothelial bags. They are lined with endothelium, and we’ll look at the histology in a second, and each of them contains red blood cells. Gradually, these islands will begin to join up and they form a loose meshwork or net through which blood begins to flow as the heart begins to beat. Where the blood is flowing fastest, there, those turn into major blood vessels. So the formation of the blood vessels is driven by the circulation driven by the heart. So they will flow through and the dynamic forces will make blood vessels form. If we are to section through a blood island, now as you can see in the illustration, marked B, we can actually see hemangioblasts as primordial germ cells of blood cells beginning to form within the species, and these are also lined with endothelial cells. Gradually, these are the little islands that will begin to join up to form the loose meshwork that makes up the early circulation system.

    02:19 As we’ve indicated, blood begins to form on the yolk sac. But then it begins to shift in the developing embryo to a region known as the AGM, the aorta, gonad, and mesonephros region of mesoderm. Blood will be formed there for a while, and then it will start to shift beginning in the liver and then in the spleen. Finally, it will start to move to the bone marrow, and later on in the adult, of course blood production is confined to the bone marrow.

    02:48 Each of the different developmental stages requires different kinds of hemoglobin, and that’s because the embryonic blood has to extract oxygen from the adult maternal blood, and therefore, it has to have a higher affinity for oxygen than the adult blood does.

    03:04 As a result, the composition of the chains which make up hemoglobin shifts during the course of development. Let’s have a look at that as a diagram. In this one, we see, first of all, blood cell production beginning in the yolk sac, but some also in the placenta, and then beginning in the AGM, the aorta, gonad, mesonephros region. Next to kick in is the liver, followed by the spleen. So the spleen actually produces red blood cells during the course of development but this is before birth. Liver enlarges massively. The liver in the developing embryo in fetus is much larger proportionately than it is in the adult, and then it begins to diminish in the liver as well as the bone marrow begins to take over.

    03:50 You can see that by the time of birth, the liver is almost cyst producing red blood cells.

    03:55 The bone marrow is taking over as the major, the sole source of red blood cell production.

    04:01 Something similar is happening with the hemoglobin chains. So to begin with, we have embryonic hemoglobin chains, and these are then associated with fetal hemoglobin chains. But these also begin to diminish towards birth and adult hemoglobin begins to kick in. But again, you can see that after the baby is born, there is still some fetal hemoglobin present for a little while before that diminishes, disappearing by six months after birth. If we look at where

    About the Lecture

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

    Author of lecture Formation of Blood – Blood Vessel and Heart Development

     John McLachlan, PhD

    John McLachlan, PhD

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