title: Potential of human twin embryos generated by embryo splitting in assisted reproduction and research.
abstract: Embryo splitting or twinning has been widely used in veterinary medicine over 20 years to generate monozygotic twins with desirable genetic characteristics. The first human embryo splitting, reported in 1993, triggered fierce ethical debate on human embryo cloning. Since Dolly the sheep was born in 1997, the international community has acknowledged the complexity of the moral arguments related to this research and has expressed concerns about the potential for reproductive cloning in humans. A number of countries have formulated bans either through laws, decrees or official statements. However, in general, these laws specifically define cloning as an embryo that is generated via nuclear transfer (NT) and do not mention embryo splitting. Only the UK includes under cloning both embryo splitting and NT in the same legislation. On the contrary, the Ethics Committee of the American Society for Reproductive Medicine does not have a major ethical objection to transferring two or more artificially created embryos with the same genome with the aim of producing a single pregnancy, stating that 'since embryo splitting has the potential to improve the efficacy of IVF treatments for infertility, research to investigate the technique is ethically acceptable'.
  
  
  
  Embryo splitting has been introduced successfully to the veterinary medicine several decades ago and today is a part of standard practice. We present here an overview of embryo splitting experiments in humans and non-human primates and discuss the potential of this technology in assisted reproduction and research.
  
  
  
  A comprehensive literature search was carried out using PUBMED and Google Scholar databases to identify studies on embryo splitting in humans and non-human primates. 'Embryo splitting' and 'embryo twinning' were used as the keywords, alone or in combination with other search phrases relevant to the topics of biology of preimplantation embryos.
  
  
  
  A very limited number of studies have been conducted in humans and non-human primates. The published material, especially the studies with human embryos, is controversial. Some reports suggest that twinning technology will find clinical use in reproductive medicine in the future, whereas others conclude the opposite that human twin embryos created in vitro are unsuitable not only for clinical, but also for research, purposes.
  
  
  
  The blastomere biopsy technique of embryo splitting seems to be unsuitable for either clinical or research purposes; however, embryo bisection, a preferable method of cloning in veterinary medicine, has not yet been tested on human embryos.
authors.affiliation: Division of Women's Health, Guy's Hospital, London SE1 9RT, UK.|Assisted Conception Unit, Guy's Hospital, London SE1 9RT, UK.
authors.forename: Laila
authors.initials: L
authors.lastname: Noli
authors.affiliation: Genetics Laboratories, King's College London, Guy's Hospital, London SE1 9RT, UK.
authors.forename: Caroline
authors.initials: C
authors.lastname: Ogilvie
authors.affiliation: Division of Women's Health, Guy's Hospital, London SE1 9RT, UK.|Assisted Conception Unit, Guy's Hospital, London SE1 9RT, UK.
authors.forename: Yacoub
authors.initials: Y
authors.lastname: Khalaf
authors.affiliation: Division of Women's Health, Guy's Hospital, London SE1 9RT, UK.|Assisted Conception Unit, Guy's Hospital, London SE1 9RT, UK.
authors.forename: Dusko
authors.initials: D
authors.lastname: Ilic
mesh_terms: D000818:Animals; D001755:Blastocyst; D002417:Cattle; D019976:Cloning, Organism / Q000941:ethics / Q000331:legislation & jurisprudence; D004624:Embryo Transfer / Q000379:methods*; D047108:Embryonic Development; D005260:Female; D006801:Humans; D007246:Infertility / Q000628:therapy / Q000662:veterinary; D011323:Primates; D038522:Twinning, Monozygotic; D014427:Twins
keywords: blastomere biopsy; developmental clock; embryo splitting; embryo twinning; lineage commitment