In the 1980s, procedures were developed to produce genetically identical twin offspring (clones) by bisecting or splitting individual sheep, goat, swine, cattle, and horse embryos (5 8 days of age). A glass needle or a razor blade is used to bisect the embryo, and the pregnancy rates (45 70%) obtained in cattle after the transfer of one-half of a bisected embryo (''half'' embryo or demi-embryo) are similar to those obtained with intact embryos from the same donor female. Today, methods are available to bisect animal embryos with a glass microscope slide and a handheld razor blade. This low-cost microsurgery procedure is simple and relatively easy to learn.
Embryo bisection offers the potential of doubling the number of viable embryo transplant offspring produced from valuable donor females. For example, 100 good-quality cattle embryos may result in 65 transplant offspring born, whereas 100 similar-quality embryos divided into halves would yield 200 demi-embryos, which then may result in 130 demi-embryo transplant calves born (130% pregnancy rate from 100 embryos). Twin calves produced by microsurgery will result in genetically identical offspring of the same sex. This removes the concern for freemartinism, if both demi-embryos are transferred to the same recipient female.
A new cloning method called nuclear transfer (transferring individual undifferentiated embryonic cells to enucleated oocytes) emerged in the mid 1980s. Multiple nuclear transfer-derived offspring from individual blasto-meres from a single early-stage embryo have been produced in several farm animal species (e.g., sheep, cattle). Nuclear transfer-derived offspring have been produced four generations from a single cattle embryo. Unexpectedly, some of the nuclear transplant calves have extended gestation lengths, and there are reports of abnormally large term offspring (e.g., calves, lambs) that need assistance at birth. The reason for these problems is not clear, although laboratory culture conditions have been implicated as a potential cause. Even though improvements in the nuclear transfer methodology are still needed, this approach has a lot of potential for seedstock producers.
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