Inside the egg
Now it’s time to wind the developmental clock back even further -all the way back to the egg. Courtney Hanna, a postdoc at the Babraham Institute, is investigating the curious characteristics of mammalian egg cells and the special cells, known as oocytes, that they come from...
She’s using mice as a model and focusing particularly on DNA methylation - an epigenetic mark found around the control switches, or regulatory regions, on DNA that tells cells important information about whether certain genes are active or not, forming a kind of programme for how cells should behave.
As Courtney explained to Kat Arney, taking a closer look at the changes, or reprogramming, of these epigenetic marks as oocytes change into eggs and are then fertilised she’s hoping to understand the events that happen at the very earliest moments of life.
Courtney - So the oocyte differentiates of course as an egg, and it looks like an egg, and has the genes to regulate its function. But it also needs to contain all the information for the embryo when it gets fertilised by a sperm. So we’re really interested in how does it possibly set up these two sets of instructions at the same time.
Kat - It’s remembering that it needs to become an egg cell, so from a precursor cell it’s got to become an egg, and then also it has to then go from being an egg to being a baby.
Courtney - Yup! And we know that the very first steps of an embryo, so after the sperm fertilises the egg, the egg reprograms all of the sperm DNA with maternal proteins and the egg is full of maternal transcripts which create proteins for these early processes. But also, we think that epigenetic information is passed as well, which might instruct the use of the genes in the very first cell divisions.
Kat - What do you mean by epigenetic information and how does that kind of fit with genes and the genetic information that this egg cell is carrying?
Courtney - When I'm talking about epigenetics, there are few different layers of regulation. But in general, you can kind of see it as a set of instructions that either dictates whether a region of DNA is open and accessible or closed and tightly packed. So we look at the different layers of epigenetic information and try to infer whether this is involved in gene regulation itself or in setting up the chromatin state in an open or close state which then might be useful in the embryo.
Kat - So you’ve got two processes going on at once. You’ve got to set up the epigenetic marks for the genes that say, “I'm going to become an egg” and then you’ve got to set up the epigenetic marks that say, “Okay, this is what you need to get the process of building a baby started.”
Courtney - Yup, exactly.
Kat - You’ve got to go from an oocyte, a precursor cell, to making an egg. How do you see the methylation patterns and the epigenetic patterns changing during that process?
Courtney - So, that also hasn’t been looked at that extensively, but when oogenesis starts, there is no methylation throughout the genome and we don’t really understand how the oocyte turns on its set of genes that are required for the oocyte. But as oocyte growth progresses and then an oocyte matures and becomes ready to be ovulated, methylation is acquired during that oocyte growth.
It’s been quite interesting because actually, all of the ground-breaking technologies in low cell numbers have been required to look at eggs because we have to hand collect them so you can only get a few hundred at a time. So, we’ve been really trying to be at the forefront of technology development to try and use molecular tools in these low cell numbers.
And interestingly, everything we seem to look at is abnormal or unusual compared to all other cell types. And so, it’s been quite an interesting road.
Kat - You're taking a handful of mouse egg cells, mouse oocytes, and they look weird to you? What do they look like in epigenetic terms? What's kind of going on in there?
Courtney - So they look really unlike any other mammalian cell type. Actually, the DNA methylation is restricted to such a confined compartment of the genome. It actually really resembles more like insects or plants because they have methylation along gene bodies and that’s what we see in the oocyte. So, it’s thought that this might be really ancestral form of where methylation is laid down.
Kat - Wow! So going way, way, way, way back in our evolutionary history, like a billion years or something, just put those marks on the genes and not on any of the control switches or anything like that, that’s like right at the start of life and right at the start of evolution.
Courtney - Yes, exactly. And so, it is quite the question - how does the egg regulates gene expression if it doesn’t have methylation anywhere at the beginning of oogenesis, but then at the end of oogenesis, we really only see these blocks of methylation over the gene bodies and not at any regulatory regions.
Kat - So then what is going on? You’ve got egg, egg meets sperm, they fall in love, they start the process of making a baby. Then what happens to that pattern that you see where you’ve just got the marks over the genes themselves? What happens next?
Courtney - There's been a few studies published this last year looking at that exact question. So when a sperm enters the egg, what it seems like is almost everything and the sperm DNA is reprogramed. The methylation is erased and there's practically no instructions that seem to be retained from sperm DNA.
Kat - Completely wipe that clean. Wipe it all down. It’s like eww! Wipe it all down!
Courtney - So there was probably only a handful of exceptions we can find. But for the most part, it seems that that’s the case whereas the maternal DNA seems to keep the modifications a bit longer. It’s slower to erase the marks and some marks don’t get erased at all.
But we’ve only been able to look at 3 so far in the community, that’s all that’s been published. And so, we only have sort of the tip of the iceberg of really what is being transmitted.
Kat - What can you do with this kind of information? What does this start to tell us about how development works?
Courtney - I suppose it can really reveal two things. Development is a really unique time where we have a lot of dynamics in epigenetics. So it’s extremely valuable for understanding how the instructions are laid down and how we use certain components of the genome.
In adult tissues, the epigenetic profile is fairly static. And so, we really don’t get the changes and the programming that helps us figure out what instructs what. And then the other aspect is understanding how we get different lineages.
So we start from one cell that contains the instructions for every single cell of the human body or the mouse, or whatever organism you're studying. But we don’t know how at certain points in development, these now split into two groups and this one will make the placenta and these group of cells will make the embryo.
And so, I think we know that epigenetics is key to it, but we really don’t understand how in these group of cells, it would be regulated one way and then these group of cells another.
Kat - Given that these egg cells seem to be managing to control their gene regulation without the kind of patterns of methylations that we might expect then why do we need them in normal cells? What's going on?
Courtney - Yeah. There has been some discussion of the idea of, during these stages of development where we need very dramatic reprograming and reorganisation of the genome to facilitate the transition from an oocyte to an embryo, that maybe transcription has become uncoupled from epigenetics.
And so, it’s an interesting idea to think about maybe some of the regulators that have linked these two processes are downregulated so that we can change the epigenome as much as the cell needs without affecting transcription. And this could be the case.
So when we looked in growing oocytes through the different stages of oogenesis, we actually find there are very few transcriptional changes. So it’s possible they setup the programme early when the processes are still coupled and then by uncoupling the process then the epigenome can regulate entirely independently of gene expression.
Kat - So you just remember what genes you need to switch on to go from oocyte to egg. Just keep doing that and then everything else can kind of go to hell around you.
Courtney - Yup, exactly.
Kat - Courtney Hanna, from the Babraham Institute.