For many of us farm animals are cows, horses, sheep or pigs that we read about, see on television, or notice as we ride through the countryside. The actual economics of raising and selling farm animals, however, is big business. One of the primary goals of these industries is to produce the best animals and sell them for top dollar. Until recently animal breeders combined a certain amount of experience, a good eye, some knowledge of simple hereditary genetics and some good luck to selectively breed animals. The goal of this work was to produce animals that demonstrated all of the desirable qualities and few or no undesirable characteristics. That's the way it had been done for thousands of years; slow and sometimes unsteady progress toward our modern farm animals from their early ancestors.
This is not the way it is done today. Today, a fine specimen of a farm animal is produced by genetic engineering along with dozens of clones. Each animal is genetically identical. Each one a champion. The method for producing these clones is very easy to understand. It does require a bit of modern technology but a description of the process need not be too technical to understand. The process described below is essentially the same for all mammals (including humans). It has been applied to pigs, sheep, cows, horses, (and humans). The name for one step in this process and the generic name for the entire process is nuclear transfer. It involves many steps, one of which is the actual transfer of a nucleus from an embryo to a donor cell to form a new embryo.
Let's start at the beginning. A newly fertilized egg under the proper conditions begins to divide almost immediately. Within a few days this single cell has doubled and redoubled time and again until it is an embryo of about 16, 32, or 64 cells. These cells are called blastomeres. Using specialized tools and some chemical buffers these blastomeres are separated from each other. Each individual cell is genetically identical to its neighbor within the embryo. They all started from just that single cell, fertilized egg.
If the blastomeres are separated from the embryo or from each other they will not survive for long because they lack the biochemical support of their sister cells and at this stage they have used up most of the nutrient cytoplasm from the original egg. The nutrients for these blastomeres are provided by other cells. Using cows as an example; oocytes (eggs) are collected from some cows that have been treated with hormones that cause them to release many more oocytes than they normally would during ovulation. The nuclei are removed from these oocytes leaving intact the cytoplasm that is the normal nutrient source for a developing embryo. Since the nuclei of these oocytes are removed, the donor animal of the oocytes may be genetically distinct from the donor of the original embryo.
Individual blastomeres from the original embryo are then fused to the nutrient cytoplasm rich enucleated oocytes. Although all of the needed ingredients are present a new embryo will not form without a little push. A small electric current is passed through the now together blastomere and cytoplasm. This electricity fuses the membranes of the enucleated oocyte and resets the biochemical clock of the newly formed system so that it will begin to function as a newly formed embryo. Normal cell division resumes and the single blastomere begins to form a complete embryo.
While this is going on the new embryo must be protected. The processes of blastomere separation, oocyte enucleation, and electrofusion all take place in laboratory glassware. Developing embryos may be kept in a nutrient solution or a preferred method is to implant them into a temporary host animal while they undergo the first critical phases of development. The temporary host animal need not even be the same species of the new embryos. Sheep are used to hold bovine embryos. Rabbits, because of their small size and ease of shipping, have also been used to hold bovine embryos. Once these newly formed embryos have undergone several cell divisions, they are ready for implantation into the animal that will be their birth mother or may be used to generate more identical blastomeres to continue and magnify the process. As with the original blastomeres and the donor oocytes, the mothers-to-be of these embryos need not be related to any of the original genetic donors. The embryos then continue to develop into normal offspring.
The result of this entire process is that a single cell may be manipulated into many genetically identical offspring called clones.