Tutorial 3 Flashcards
TSCs, placentation, germ cells, and imprinting
- What are trophoblast stem cells?Name two different cell types and stages in vivo from which they can be established.
TSCs are multipotent stem cells of the extra-embryonic trophoblast lineage. They can be established from the polar trophectoderm of the blastocyst (E3.5- E4.5) the extra-embryonic ectoderm of early post-implantation embryos (E5.5- E6.5) or the chorionic ectoderm of E7.5-8.5 conceptuses.
- You want to use trophoblast stem cells in a chimera experiment. What aggregation partner must you use?
They can only be aggregated with early preimplantation embryos (2-cell stage to morula) or injected inside a blastocysts. Although ES cells can form the embryo and primitive endoderm, in ES< >TS chimeras, neither of the partners provide the necessary structure for formation of a blastocyst.
Name the key layers of the mature mouse placenta
In addition to the maternal decidua, the embryo part of the placental consists of the parietal trophoblast giant cell layer, the spongiotrophoblast layer (junctional zone), and the labyrinth (or labyrinthine layer).
Where do exchanges between the maternal and fetal circulation occur?
They occur in the labyrinth.
What does the term hemochorial mean?
The maternal blood is in direct contact with fetal trophoblast cells. The maternal blood spaces (sinuses) of the labyrinth are not lined by endothelial cells.
What is the role of the allantois in placental development?
The allantois is made of extra-embryonic mesoderm. After connection with chorion (trophoblast plus extra-embrionic mesoderm), a process called chorio- allantoic fusion, this allantoic mesoderm will contribute to the fetal blood vessels of the labyrinth layer as well as to the umbilical cord.
What does the term syncytiotrophoblast mean?
A syncitium is a single large cell with multiple diploid nuclei formed by the fusion of several precursor cells. The syncytiotrophoblast is a multinucleated cell type present in the labyrinth layer and separating the maternal blood sinuses from the fetal endothelial cells lining the fetal blood vessels.
The mature blastocyst is composed of three cell lineages, the trophectoderm, the inner cell mass (or epiblast) and the primitive endoderm. Explain how derivatives of these three lineages are implicated in germ line development.
The visceral endoderm sends a signal required to prime the epiblast cells for receiving the BMP4 inducing signal. BMP4 comes from the extra-embryonic ectoderm, derived from the polar trophectoderm, and PGCs themselves are from the epiblast.
Do all migrating PGCs reach the gonads? What is the evidence?
No, some get trapped along the way to the genital ridges. Few are normally visible since they are eliminated by apoptosis, but in BAX mutants, deficient in apoptosis, several ectopic PGCs can be observed in the gut endoderm and mesentery.
What are the differences and similarities between parthenogenetic and gynogenetic embryos?
Both are diploid and contain two sets of maternal chromosomes. However, parthenogenetic (PG) embryos are derived from an activated egg (spontaneously or artificially), whereas gynogenetic (GG) embryos come from manipulated fertilized eggs (zygote) to remove the paternal pronucleus and replace it by a maternal pronucleus.
Several explanations were proposed to explain the lethality of parthenogenetic embryos. Explain two that were eliminated by the generation and analysis of gynogenetic embryos.
Since GG have the same phenotype as PG, sperm contribution to fertilization is not what is missing to PG… GG embryos can be reconstituted from two different mouse strains, so homozygosity seen in PG is not part of the problem…
Name three characteristics of imprinted genes.
- expressed from a single allele, in a parent-of-origin specific way
- carry differential epigenetic marks, such as DNA methylation
- often clustered in large chromosomal domains
- epigenetic marks must be erased in the early germ line and re-established later, depending on the sex of the embryo.
How can DNA methylation regulate a paternally expressed gene?
- Often by direct transcriptional regulation, via methylation of a CpG-rich promoter element, only on the maternal chromosome.
- The DNA methylation (DNAme) mark is directly inherited from the mature egg.
- It is maintained by DNMT1 throughout preimplantation development, as most of the genome gets erased.
- It is only erased in the germ line, in PGCs.
- The DNAme imprint is only re-established in the female germ line, postnatally in the oocyte
Discuss the dynamic changes of DNA methylation at this gene throughout the germ cell and developmental cycle.
- The DNA methylation (DNAme) mark is directly inherited from the mature egg.
- It is maintained by DNMT1 throughout preimplantation development, as most of the genome gets erased.
- It is only erased in the germ line, in PGCs.
- The DNAme imprint is only re-established in the female germ line, postnatally in the oocyte
