Nuclear Transfer from Cell Lines

Abstract

Abstract The production of cloned animals following nuclear transfer, using somatic cells grown in culture, represents a remarkable feat of developmental biology. It demonstrates the potential of a differentiated nucleus to be reprogrammed back to an embryonic state when exposed to a suitable cytoplasmic environment, such as that of an oocyte. It involves fundamental changes to the patterns of DNA ( deoxyribonucleic acid ) methylation and chromatin modification imposed on a specialized nucleus to enable the precise temporal‐spatial sequence of gene expression necessary for normal embryogenesis. However, reprogramming is often incomplete with the development going astray, resulting in a continuum of embryo, fetal and post‐natal mortality. The majority of the clones that do survive to adulthood, and their sexually derived progeny, do, however, appear normal. This provides encouragement for the practical applications of nuclear cloning in the fields of agriculture, animal conservation and biomedicine. Key concepts: The cytoplasm of mature oocytes and young zygotes has the potential to reprogramme the epigenetic state and pattern of gene expression of somatic donor cells back to that of an embryo, to enable the creation of a cloned animal. Undifferentiated embryonic blastomeres result in greater animal cloning efficiency but practically all somatic cell types are equally difficult to reprogramme. Faulty reprogramming compromises subsequent development and is associated with serious animal welfare concerns affecting the acceptability of the current procedures in food‐producing species. In cattle, approximately 50% of nuclear transfer embryos derived from somatic cells that are transferred to surrogate females may initiate pregnancies, but only 15% produce calves with ultimately 10% resulting in healthy adult clones. The cloning efficiencies in most other species are less than this. Any epigenetic errors that may persist in surviving clones appear to be reset through gametogenesis and are not transmitted to progeny following sexual reproduction. Cloning can be used in agriculture to multiply high‐value breeding animals to effectively disseminate genetic gain. Somatic cell cloning is a valuable reproductive tool to resurrect genetically superior livestock after slaughter, accidental death, injury or disease to avoid the lost of valuable genotypes. The milk and meat from surviving cloned livestock has similar composition to that produced from conventional livestock and is safe to consume. Nuclear transfer is a valuable method to produce transgenic livestock from genetically modified cells for specific agricultural and biomedical applications. The controversy over the use of human somatic cell nuclear transfer to generate patient‐specific embryonic stem cells for regenerative medicine has abated after the revelation that somatic cells can be directly reprogrammed into induced pluripotent stem cells in vitro , signalling the latest revolution in this field.