Embryonic Period – Weeks 3-8 of Embryogenesis

by John McLachlan, PhD

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    00:00 Now, just looking again at some of the early stages, in this diagram, we can see the amniotic cavity in green on the top, and then below that, the yellow yolk sac and if we were inside the amniotic cavity looking down at the floor, we would see the primitive streak forming, as it’s described in more detail in the lecture on gastrulation. This moment is the time of individuation. It’s when the individual is formed and twins can no longer form, and also determine the main body axis, left and right, head and tail. As we saw in the lecture in gastrulation, it also leads to the formation of a third germ layer, the mesoderm. So, it’s a very significant point in development.

    00:49 Recapping that with the illustrations which are labeled, we can see the amniotic cavity, the yolk sac, and between the two, the bilaminar disc in which the embryo will begin to form and then in the lower illustrations in the picture, you can see the primitive streak which determines where the main body axis actually is. Looking at the process of development of the embryo, again, from inside the amniotic cavity, imagine that you’re a tiny diver swimming inside the amniotic fluid looking down at the floor of the amniotic cavity.

    01:25 What we would then see is the neural tube beginning to form, as the ectoderm rose up, fuses in the middle, and then begins to extend towards the head, towards the brain end, and towards the tail, gradually fusing to form the neural tube and on either side of that are the somites. It’s actually quite helpful to look at a photograph, a scanning electron micrograph, in fact, of a real human embryo at a stage that corresponds to those previous diagrams. Off at the head, you can see the developing brain. The neural tube runs down the midline, and on either side, there are little blocks of mesodermal tissue called somites, which will contribute to the vertebrae among other things. The spine leads down to the tail. And in the background of the picture, the little sac-like structure that you can see is, in fact, the yolk sac. So you’re looking down on the yolk sac from above.

    02:29 During this time, there’s a considerable expansion mostly of the extra-embryonic membranes, in particular, the amniotic cavity. So the amniotic cavity will expand, becomes filled with amniotic fluid, and gradually, it will take up most of the spaces in the uterus.

    02:48 We’ll look at that in more detail in another lecture. If we’re to look at the embryo from the side, so this is a lateral or side view of the embryo, during the embryonic period, you can see that it gradually becomes more curved as we move from left to right.

    03:05 The head becomes much larger, and a significant amount of growth takes place, and the embryo is rising into the amniotic cavity. In the third illustration on the right, and the embryo is projecting into the amniotic cavity and is surrounded by amniotic fluid at that point.

    03:25 In this electron micrograph, we can see a human embryo seen from the side. This one is perhaps about 27, 28 days after fertilization. So, it’s in the earlier parts of the embryonic period and the bulge of the heart is marked with an H in the diagram, and the umbilical cord is marked with a U. You can see the curve of the somites sweeping around the back, and the arm bud and the leg bud are visible as well. Now, it can be difficult to interpret these images in three dimensions. So, I brought along a model embryo to help us understand what the three dimension arrangement is. Here’s the model embryo. Here we can see the curve of the head, coming round to where the mouth will be. This is the bulge of the heart.

    04:16 This is the arm bud and this is the leg bud. We can see the somites sweeping round and ending up in a distinct tail. Now, normally, the tail will diminish and disappear. But in a very few human babies, perhaps, one per a hundred thousand live births, the tail persists, and the baby is born with a persisting prehensile tail. As I say, that’s very rare, and perhaps that’s a shame. It might be fun if we had tails again. Looking at the whole structure that’s being conceived, the conceptus as it were as a whole, here we can see the embryo inside its chorionic cavity, and it’s surrounded by its amniotic membrane. And the little sac that you can see to the side of the embryo, just to the right of it, that’s the yolk sac as it steadily diminishes in relative size. All roundabout, the cloudy structures are the developing placenta as they engage with the uterine tissue and the limb buds in this slightly later picture are beginning to take on more recognizable form as the arm and leg of the human. This is a particularly significant diagram because it summarizes the time when different body systems are at their most sensitive. So you could see, for instance, marked in dark green, the heart is highly sensitive between weeks three and six after fertilization. It remains fairly sensitive for the rest of the embryonic period, but after that time, you’re unlikely to get a major heart abnormality, a major abnormality of the heart’s form. You may disturb its function as the heart matures, but it will still be an intact heart. The major defects of heart development are initiated during that three to six week period. Similarly, you can see the sensitive periods for the central nervous system, the eyes, ears, limbs, and so on. These timings are actually very helpful in working out what may have gone wrong during the course of a pregnancy.

    06:31 If, for instance, we were to observe that a baby had a heart defect and a central nervous system defect but its eyes and ears and upper limbs were normal, that might lead us to investigate whether or not something had happened during the third to fourth week after fertilization.

    06:52 So that would be weeks five to seven of pregnancy, which might have brought about these defects, might have caused them in some way. So you can use this diagram retrospectively by looking at the nature of abnormalities, which often come not singly but in combination to find out when something might have gone wrong during the course of development. And here’s an image of an embryo to reinforce this view of the embryo in this embryonic period with the major body structures being formed and laid down at that point.

    About the Lecture

    The lecture Embryonic Period – Weeks 3-8 of Embryogenesis by John McLachlan, PhD is from the course Embryology: Early Stages with John McLachlan.

    Author of lecture Embryonic Period – Weeks 3-8 of Embryogenesis

     John McLachlan, PhD

    John McLachlan, PhD

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