Embryo Splitting

The term embryo splitting (or "embryo twinning") refers to the formation of twins or multiples through artificial microsurgical splitting of an embryo at the cleavage or blastocyst stage. At the earliest stage of embryogenesis, the cleavage stage, the single cells of the embryo (blastomeres) are still totipotent. This property is presupposed in the blastomere biopsy procedure, where single blastomeres are removed from the embryo, which under suitable conditions continue to develop like undivided embryos. The technique of blastocyst bisection involves the separation of the embryo in the later blastocyst stage into two equal halves. After separation, the genetically identical embryos can continue to develop.

In view of the long-term aim of research cloning – the production of autologous stem cells for therapeutic purposes (cf. this "In Focus", paragraph "What is the purpose of research cloning or therapeutic cloning?"), – the techniques of embryo splitting currently play a minor role. Embryo splitting allows it to obtain ideally several embryos with identical genetic features from one embryo. Regarding  the derivation of autologous stem cells for therapeutic purposes, it is however not the number of embryos, but the creation of embryos that share identical genetic features with potential patients that matters . It is from these embryos that autologous stem cells can be derived in the next step.

In the context of assisted reproduction treatments, the procedures of embryo splitting potentially provide the opportunity to gain a higher number of embryos by dividing one embryo, which increases the chance of successful nidation and therefore the overall likelihood of a pregnancy. Whereas these procedures have commonly and successfully been applied in livestock breeding, they have not been applied in human medicine as a result of legal restrictions and the more complex development processes in human cells. With regard to the (potentially) successful application of the procedure on primates and humans, research groups come to different conclusions, especially in connection with the at times reduced development capacity of 'divided' embryos.

In the context of basic research the procedures of embryo splitting facilitate standardised studies because the embryos produced this way are genetically completely identical. Studies in this context are currently being conducted in veterinary contexts. If and to what extent these procedures and objectives can be realised in human medicine is presently the subject of discussion. 

For a presentation of the procedures of embryo splitting and an evaluation of potentials and limitations in human medicine see e.g.: 

Noli, Laila / Ogilvie, Caroline / Khalaf, Yacoub / Ilic, Dusko (2017): Potential of human twin embryos generated by embryo splitting in assisted reproduction and research. In: Human Reproduction Update 23(2), 156-165. doi: 10.1093/humupd/dmw041 Online Version

Illmensee, Karl / Levanduski, Mike (2010): Embryo splitting. In: Middle East Fertility Society Journal 15(2), 57-63. doi: 10.1016/j.mefs.2010.05.001 Online Version 

Schramm, Ralph Dee / Paprocki, Ann Marie (2004): Strategies for the production of genetically identical monkeys by embryo splitting. In: Reproductive Biology and Endocrinology 2, 38. doi: 10.1186/1477-7827-2-38 Online Version 

National Advisory Board on Ethics in Reproduction (1994): Report on Human Cloning through Embryo Splitting: An Amber Light. In: Kennedy Institute of Ethics Journal 4(3), 251-281. doi:10.1353/ken.0.0067 Online Version 

Heinemann, Thomas (2005): Klonieren beim Menschen: Analyse des Methodenspektrums und internationaler Vergleich der ethischen Bewertungskriterien (Studien zu Wissenschaft und Ethik, 1), Berlin [u.a.]: De Gruyter, 200-203. (German)

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