The Germline and Reprogramming Flashcards
(44 cards)
What are primordial germ cells?
PGCs are the first germ cells. 3-5 PGCs can populate the entire lineage. They’re found at the back end of the embryo in extra embryonic tissues, having got there through the primitive streak (mice). In humans they’re found in the yolk sac
How do PGCs travel to their final resting place?
They migrate into the developing gut and head north until they reach the developing gonads (genital ridges). They exit the hindgut and colonise the genital ridges where they continue to develop;
How is the sex specified?
Gonadal somatic cells in male embryos turn on SRY (on Y chromosome) and this triggers a downstream cascade that leads to male gonadal and germ cell development. In females SRY is not expressed so female pathway proceeds
What happens when you force expression/delete SRY?
Forcing expression of SRY in XX embryos, they develop testis but are infertile. If SRY is deleted in XY embryos, they develop ovaries
What are the 3 main processes in the PGC stage of germ cell development?
Specification (induction of PGCs), migration (to the genital ridges), epigenetic reprogramming (sex specification occurs before this)
What is the female pathway of germ cells after epigenetic reprogramming?
They quickly start meiosis. Almost as soon as the process is started, the cells go into mitotic arrest. The female cells at this stage are called oogonia, and are arranged in primordial follicles
What is the male pathway of germ cells after epigenetic reprogramming?
The germ cells begin the process of meiosis - for longer than females. However they also go into mitotic arrest. The male cells are called spermatogonia at this stage and are arranged in primitive tubules called testis cords.
Why is the potency of germ cells hotly contested?
Because germ cells only form one type of cell - gametes, suggesting that they are unipotent. But the gametes can form a totipotent zygote which induces pluripotent cells in the epiblast
What are teratocarcinomas?
They are tumours which contain all 3 germ layer derivatives in them. Caused by germ cells mismigrating
What are EC cells?
EC cells are cells derived from teratocarcinomas, and the first cells discovered with pluripotent properties.
What are embryonic germ cells?
They are PGCs placed in culture, forming a pluripotent cell line - EG cells.
Which important genes do PGCs express?
Many of the most important genes in the pre-implantation epiblast (and ES cells) including Oct4, Sox2, and Nanog
What was the experiment that led to the discovery of imprinted genes and genomic imprinting?
Creating embryos with two maternal/paternal pronuclei which results in failed development. The DNA sequence was identical in all pronuclei so it must have been epigenetic information that differentiated the male/female pronucleus
What is the epigenetic information that is present in developing embryos?
Different DNA methylation marks present on DNA inherited from the mother and father. These DNA methylation marks control gene expression in about 100/20000 genes that we have - this regulation is called genomic imprinting and the genes are imprinted genes
What is the definition of an imprinted gene?
An imprinted gene is one that is monoallelically expressed in a parent of origin specific manner ie. expression depends on which parent it was inherited from, even if the gene is the same.
How do males/females only pass on the male/female imprints if every cell in the body contains both male and female patterns?
When a PGC reaches the genital ridge, epigenetic reprogramming occurs where the majority of histone modifications and DNA methylation is erased. Then, a male/female pattern is established, as the cells go through either spermatogenesis or oogenesis. When the gametes combine, the maternal chromosome has maternal imprints, and paternal has male imprints
What is the second wave of epigenetic reprogramming?
Post fertilisation. The DNA marks are stripped from the genome once more - but not the imprints. Also, 3 imprinted paternal genes survive the ovum attack on the sperm epigenetics
What is the purpose of the second wave of epigenetic reprogramming?
Prepares the embryo to undergo development. Pre-implantation epiblast must be capable to make all cells of the body, so the epigenetic reprogramming gives an epigenetic blank slate.
What is the state of methylation after implantation?
Cells rapidly accumulate DNA methylation as they move forward in development. While this DNA methylation is maintained in somatic cells, it is erased in the epigenetic reprogramming of PGCs
Why is X-inactivation important?
Because if it didn’t occur the female would have too many genes because the X chromosome contains many more genes than the Y chromosome.
What is X-inactivation?
The process that occurs shortly after implantation, where one of the two copies of the X chromosome is inactivated. The process is random in each cell. The inactivated X has its DNA heavily methylated and histones repressively modified. The inactive X chromosome is visualised as a Barr body
What happens in PGCs regarding X inactivation?
In the earliest PGCs, there is an inactive X, like in all other cells at that time. However, during PGC development, the X is reactivated so that each egg has a normal X. This is completed around the time of epigenetic reprogramming
How did John Gurdon prove that each somatic cell in the body retains the full complement of DNA?
Placed a somatic cell nucleus in an enucleated egg and obtained healthy frogs. This also provided evidence that somatic cells can be reprogrammed
What does nuclear reprogramming mean?
A cell increases in potency, ie. the reverse of differentiation
