Hatching and Implantation – Week 1 of Embryogenesis

by John McLachlan, PhD

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    00:01 It’s a very important moment in your life as a whole. Then about five or six days after fertilization, the process of hatching takes place. I imagine you probably didn’t realize that the humans hatched in a very different sense than a hen might hatch from an egg. Nonetheless, what has to happen is that this trophoblast surrounded embryo has to emerge from the zona pellucida.

    00:27 What happens is it actually pumps some fluid into its inside, and as it expands, eventually, the zona pellucida will rupture. In the picture in the extreme right, you can see that the developing structure of the embryo itself is beginning to hatch out to emerge through the zona pellucida.

    00:50 This is an important stage. If this was to happen prematurely, as we’ll see, then there can be difficult consequences as a result. These pictures, incidentally, are from an in vitro fertilization laboratory. This is an individual who may well have developed into an adult at some point. Now, we’re moving to a diagram. It’s often convenient to use diagrams rather than photographs, because, in the diagram, you can color things making it easier to follow which cells give rise to which. So here in this diagram, I’ve colored the original inner cell mass as red and yellow. That’s to indicate that it’s divided again into another two different sorts of cells. So the cells shown in red are primitive ectoderm cells. The cells shown in yellow are the primitive endoderm cells. But you can probably also see that the cells of the trophoblast have distinguished themselves into two different sorts as well. The cells from the original trophoblast around the outside are now known as cytotrophoblast cells, but you can see that up at the top where it’s beginning to implant into the uterine wall, then the cells have lost their cell membranes.

    02:15 So you have nuclei which are effectively freely distributed within one mass of cytoplasm, and this is called syncytiotrophoblast. Now, it sounds a long complicated name, but it’s doing a very important purpose because it is the syncytiotrophoblast that will help the developing embryo implant into the uterus, into the wall of the uterus. As you can see in the picture, it’s beginning to kind of invade its way and to eat its way into the underlying tissue. You’ll know that in the uterine tissue, I’ve also sketched in some blood vessels surrounded by endothelial cells and with red blood cells within them.

    02:59 And of course, the uterus is also rich in uterine glands. At this stage, the total overall size is still very much like the original fertilized egg. Growth will only start to take place as it begins to receive nutrients from the mother from the uterus and were about six days after fertilization at this point. The inner cell mass, as I said, has given rise to ectoderm and endoderm, and is only a few of the primitive ectoderm cells that are going to give rise to the baby that will eventually be born. So you can see that at these early stages, most of the cells will contribute to the support network to the placenta and support structures that will help the embryo and then the fetus develop along the way that ought to do subsequently. So these can be seen as the support cells for the developmental process.

    03:57 Here, we have two days later, about eight days after fertilization. Now, the whole pre-embryo is beginning to disappear under the surface of the uterine wall.

    04:09 So it’s almost invisible from the outside. There’s perhaps just a little scar, just a little plug of tissue if you’re to look inside the uterus where the implantation process has taken place. But otherwise, it’s borrowed its way under the surface of the uterine wall and in doing so; you can also see it has engulfed some of the blood vessels that were present in the wall of the uterus. In fact, it’s actually effectively dissolved the walls of these blood vessels so that the maternal blood, the red cells shown as red dots in the diagram, are now directly in contact with that tissue that we called the syncytiotrophoblast.

    04:52 Our red cells, the primitive ectoderm, they’ve also begun to hollow out. They had begun to form another hollow cavity. And that cavity is called the amniotic cavity. That will, in fact, be the amniotic cavity of the later developing baby. So it will become very large and significant subsequently.

    05:16 Equally, if we’re to look at the yellow primitive endoderm cells, they are beginning to move round the inside of the trophoblast shell, and eventually, they will meet and they will form another structure. So we can look at that in another diagram, and here, you can see the primitive ectoderm that is forming the amniotic cavity. And the primitive endoderm marked in yellow, endoderm cavity closed over, and then that will be the yolk sac.

    05:47 It’s called the yolk sac as an evolutionary marker. Long ago in our evolutionary history, when we were still developing inside eggs, then we had yolk, and it’s the process of evolution that moved us away from being inside eggs to being inside the mother. But we still have a yolk sac. So it’s still a marker of our long evolutionary history. If we look at the blood cells of the uterus, we can see now that they have been entirely engulfed by the syncytiotrophoblast. In fact, some of the trophoblast cells shown in the diagram in dark brown are beginning to develop finger-like projections or villi and this is an early marker for what will be the placenta. Now, what they are doing at this stage is to increase the surface area across which nutrients can flow and wastes can be exchanged. So it’s forming some of the functions of the placenta in this very early on a primitive form.

    06:51 And of course, note that it’s connected to the mother then growth can begin to take place.

    06:56 It’s taking nutrients from the mother so the whole system can begin to expand.

    07:01 There’d be a very large number of changes of arrangements in morphology. And before it will get to anything, we can even begin to recognize as looking at it like a human embryo. So at the moment, it would be almost impossible to spot where development is going to take place.

    07:18 This did not look like a human in any sense, whatsoever at these stages. Now, I mentioned ectopic pregnancy.

    About the Lecture

    The lecture Hatching and Implantation – Week 1 of Embryogenesis by John McLachlan, PhD is from the course Embryology: Early Stages with John McLachlan.

    Author of lecture Hatching and Implantation – Week 1 of Embryogenesis

     John McLachlan, PhD

    John McLachlan, PhD

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