Other Talks in the Series: Stem Cells

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.
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.
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.