Preimplantation embryo development

I'm Jay Baltz. I'm from the University of Ottawa and the Ottawa Hospital Research Institute, and we will be speaking about Preimplantation Embryo Development.

These are preimplantation embryos, most of what I'll show are mouse embryos, although there are a few other species that will appear here and there. But if I don't say what they are, then they're mouse. This is a set of mouse embryos from the fertilized egg stage up through till the very end where they're about to implant in the uterus and form the pregnancy. They go through a set of stages that are named for the number of cells, a one cell, two sell, four cell, eight cell, and then some more specialized stages towards the end, which we will talk about. The morula, which is a ball of cells, as you can see, and the blastocyst where different parts of the embryo have started to differentiate, to become different lineages that are going to become other parts of the embryo and the fetus and the placenta.

Here are the features of these that are especially important, so in the fertilized egg and through all of these stages, up to the very end, the embryo is surrounded by a shell essentially. It's a extracellular matrix shell. It's made out of glycoproteins and it's called the zona pellucida, which is simply Latin for the clear zone from how it appeared in microscopes and that contains the cells and protects them. Earlier it was what the sperm bound to and then burrowed its way traverse to get to the egg to fertilize it. But after fertilization, it will no longer bind sperm and it's merely there to contain the embryo and protect it and keep it from sticking to the fallopian tubes also called the oviduct or the uterus, until it's supposed to. The pieces of the fertilized egg that are important other than the zona pellucida are the polar body, which you can usually see one or two of these. Those are leftover from when the oocyte spit out got rid of the excess chromosome so it could become haploid. That's essentially a little garbage cell that is going to break down. The pro nuclei are the genetic material of the egg, the female pronucleus and the sperm, which is the male pronucleus, and those don't combine into a single regular nucleus until the next stage. The next stage is the two-cell stage that the next slide over and the nucleus, now there are nuclei in each of those cells and there are two cells, each of which are half the size of the one before. We're just going to go through then these early stages where the two cells then become four cells because each cell cleaves and then eight cells and then that stage that's called the morula, which will talk about how this happens at the very end of the top. Instead of being individual cells, you can now see that this is a blob of cells. They're all stuck together and you can't see the individual cells easily. If you took this zona pellucida off and shook that around, they would all stick together. While the earlier stages, if you took the zone pellucida off and shook it around, those cells aren't stuck to each other. They would just float away. This is the first time you're getting a tissue integrated entity and this is called compaction and these are compacted cells. Then the very next stage, which in the mouse is the next day you now have the blastocyst, which you can see looks very different here. It's got a fluid filled cavity called the blastocoel, and then two different parts. One is the spherical shell around the outside of cells which are very tightly stretched. The glass, the seal is fluid under a little bit of pressure so it stretches this out. You can see the zone pellucida is much thinner now, and that's because it's being stretched. It's elastic in it. It gets stretched out and thin due to the pressure of the growing blastocoel. What contains all of that are the trophectoderm cells and those again, we'll talk about it towards the end. But those are a set sphere of epithelial cells that form the closure of the embryo.. Then inside of there is something called the inner cell mass, which is stuck to the inner wall of the trophectoderm and is bathed in the blastocoel fluid that's going to make the fetus. Those are the embryonic stem cells basically. The inner cell mass is going to stay together like that until after implantation and will form the fetus. The next stage is, that's the final preimplantation stage, after that, it hatches from the zone pellucida. You can see that the bottommost picked micrograph shows a partially hatched blastocyst where on the left side it's poking out of the zone pellucida and the inner cell mass is that little ball on the right, and that's going to go on then to implant in the uterus. This is what we can see in the microscope now. We didn't always know about that, and I think it's interesting to keep in mind how our knowledge over the centuries has grown.