Stem cells from the early embryo

Published on June 2, 2014   39 min
0:00
Hello. My name is Janet Rossant. I'm a senior scientist at the Hospital for Sick Children in Toronto and Professor of Molecular Genetics at the University of Toronto. This talk is going to be about stem cells from the early mammalian embryo.
0:15
Early mammalian development is the stage of development of which the zygote, a single fertilized egg, develops through to the blastocyst. The blastocyst is the stage of development from which we can derive embryonic stem cells. And as I'll show you later, in the mouse we can also derive other stem cells that represent some of the extra embryonic lineages that a mammalian embryo uses to survive in the uterus. If we look at the stages of mouse development shown here, these stages from a single cell to the blastocyst take four days. And we see that the embryo undergoes a process known as cleavage in which the cells continue to divide, but don't specialize until it begins to cavitate to form a blastocyst with three distinct cell types. The outer, trophectoderm, enclosing a group of cells at one end called the inner cell mass, which then go on to form epiblast and the primitive endoderm.
1:10
In this slide, we see some real images of mouse embryos during these stages of preimplantation development. And in fact, these images go right back to the oocyte stage through to the blastocyst. We see a process where the oocyte matures, fertilization occurs to form the zygote, and then the embryo starts to undergo cleavage. All of these embryos are shown without their encompassing zona pellucida, which is kind of an egg shell that protects the embryo in the uterine environment. However, it allows us to see the cells very clearly. The next event that occurs around a two-cell to four-cell stage is a process of genome activation where the zygote genome becomes active. The embryo continues to divide and undergoes a process called compaction at the 8- to 16-cell stage where instead of seeing these single cells we now have something that's called a morula where all the cells are very compacted against each other. As we will see later, this is a very important event as it starts to establish an inside environment and an outside environment leading up to the formation of the blastocyst.
2:17
Here is a beautiful image of a mouse blastocyst stained with a number of genetic markers that demonstrate very clearly that by the time the embryo is about to implant in the uterus, these three specialized images of the blastocysts have become established. We see the outer trophectoderm shown here with a blue staining, which is an integrin molecule that's specific to this lineage. And enclosing them, the inner cell mass, which contains the pluripotent epiblast. Those pink cells that you see are pink because they are stained with an antibody, Oct4, which is a very famous transcription factor involved in pluripotency. And then on the surface, we see the primitive endoderm. So we can see that these three cell types are apparently different by their gene expression patterns, by their morphology, and they are, indeed, distinct lineages that have their fate committed for future development. Experimental embryology over the years and currently still today is trying to ask three questions about these lineages. First of all, what do they give rise to in later development? What other lineages that arise at the blastocyst? Then importantly, how are they established during that process from fertilization to blastocyst stage? And underlying all of that as we move into the stem cell era, can we derive lineage-specific stem cell lines from all three types of the blastocyst?
3:45
Addressing the lineages of the blastocyst and what they give rise to requires us to be able to follow the fate of those cells somehow during later development. And it's with that in mind that we can turn back the clock and look at some of the early processes that have been developed to study mouse development. And the most important one of those is a generation of an embryonic chimera. So Kristoph Tarkowski in Poland and Beatrice Mintz in the US were early pioneers who were able to show that if you took eight-cell embryos and pushed them together in a Petri dish and then put them back into the maternal environment, if those eight-cell embryos carry different pigment markers you could generate the kind of mouse shown on the bottom right. A nice mixture of black and white cells in this case. And every tissue in those animals will be a mixture of the two original embryos. So a chimera. Richard Gardner in 1968 developed very important methodology, which also generates chimeras in which cells can be introduced into the blastocyst. And here you see a picture of inner cell mass cells being injected into the blastocytic cavity, also generating these mixed chimeras. With that in mind, with the right genetic markers you can begin to ask, what happens to those injected cells? What do they give rise to in later development? So let's start with the first question. We make the trophectoderm and we produce the inner cell mass. How can we use chimeras to find out what those cells produce?
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Stem cells from the early embryo

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