Nuclear transfer

Nuclear transfer is a technique used to make a transgenic embryo. An oocyte (also called an ovocyte) with its chromosomes removed is fused with a transgenic bovine cell. After artificially activating this single cell embryo so that it divides like a normal embryo, it is grown for 7 days till it reaches the blastocyst stage. Viable embryos are then transplanted into recipient cows.

Questions to consider Do all blastocysts develop into viable embryos? What might prevent this from happening?

DR GOETZ LAIBLE So the cell has taken up and integrated the gene construct – we call this a transgenic bovine cell – but we are using a technology called nuclear transfer or cloning. We are using ovocytes from the abattoir, so bovine ovocytes, and from these ovocytes, we remove their own genetic materials, and then we fuse this empty ovocyte with one of these transgenic cells. And by doing so, the nucleus or the genome of this 1 cell that we are fusing to this empty ovocyte enters this now 1-cell reconstructed embryo

We have to artificially activate this single cell embryo so that it starts dividing like a normal embryo, and that can be done also either chemically or by stimulating it with an electric pulse. And normally what we do is, after activation, we cultivate these embryos up to a certain stage of embryonic development – it’s called a blastocyst stage – and that takes around 7 days to develop into this ball of cells. There’s about 150 cells at that stage. We are using an in vitro process. We are not expecting that there will be 100% efficiency from the ovocytes to develop into day-7 embryos, but to be fair, the efficiency can be very high in these in vitro processes – up to 80% maybe.

The next step is then to take this in vitro developed embryo – the blastocyst – and it then gets transferred into a recipient cow to be developed in vivo like a normal embryo would be. In a natural process, not all the ovulated eggs that are fertilised will develop into a viable embryo, so there are normal check marks that have to be passed. And one of the problems is certainly the chromosomal abnormalities that can occur in some of these early embryos, and then they are getting sorted out – we recognise there are mistakes – and then they go no further, and it’s a completely normal, natural process

Acknowledgements: Carl Williams AgScan