Figure 1.6: Generation of transgenic mice
1. Mouse development begins when the egg is fertilised by a sperm, creating a single cell which has the potential to form an entire organism. The single cell rapidly divides into two, each of which in turn divides into two and so forth, producing a mass of identical cells, each of which has the potential to form a foetus.
2. Eventually the mass of dividing cells begins to differentiate and forms a blastocyst, a hollow sphere of cells containing an inner cell mass. The outer layer goes on to form the placenta and other supporting tissue, while the inner cell mass goes on to form every type of body cell. These inner cells are termed embryonic stem cells.
3. Embryonic stem (ES) cells have the ability to replicate and divide indefinitely in vitro. Thus ES cell lines can be established by removing cells from the blastocyst using a micro-pipette and transferring them to suitable culture medium.
4. To generate transgenic mice, gene sequences are introduced into the genome of the ES cell in a process known as transformation. The gene can be introduced at a particular location by using various specialised techniques. Not all cells will contain the new sequence or will contain it at the right place, so those cells containing the gene at the correct location are selected and grown and other cells removed. Specific gene mutations can also be introduced into ES cells in this way.
5. Transformed ES cells containing the new gene sequences are transferred back into a blastocyst which is implanted into a surrogate mouse.
6. Progeny from the surrogate mouse are tested for the presence of the new gene or mutation, and mice heterozygous for the new gene are mated to generate homozygous mice.
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