Herranz Gonzalo
Department of Medical Humanities,School of Medicine,University of Navarre,Pamplona,Spain.
Zygote. 2015 Feb;23(1):27-40. doi: 10.1017/S0967199413000257. Epub 2013 Jun 5.
In the dominant model, monozygotic (MZ) twinning is universally accepted as a post-fertilization event resulting from splitting of the embryo along its first 2 weeks of development. The stage at which splitting occurs determines chorionicity and amnionicity. A short history on how the model was built is presented, stressing the role played by some embryologists, in particular George Corner, in its completion and final success. Strikingly, for more than 60 years no deep criticisms have been raised against the model, which, in virtue of its rational and plausible character, enjoys the status of undisputed truth. At close examination, the embryological support of the model shows some important weak points, particularly when dealing with late splitting. In the author's view, the model not only has contributed to 'suspend' our knowledge on the timing of MZ twinning, but seems indefensible and claims to be substituted. That factor could imply relevant consequences for embryology and bioethics. As an alternative to the model, a new theory to explain the timing of MZ twinning is proposed. It is based on two premises. First, MZ twinning would be a fertilization event. In that case, due to an alteration of the zygote-blastomere transition, the first zygotic division, instead of producing two blastomeres, generates twin zygotes. Second, monochorionicity and monoamnionicity would not depend on embryo splitting, but on fusion of membranes. Some support for this theory can be found in recent embryological advances and also in some explanations of old.
在主流模型中，单卵双胎（MZ）普遍被认为是受精后的事件，由胚胎在发育的前两周沿其纵轴分裂所致。分裂发生的阶段决定了绒毛膜性和羊膜性。本文介绍了该模型的建立简史，强调了一些胚胎学家，特别是乔治·科纳在其完善和最终确立过程中所起的作用。引人注目的是，60多年来，该模型未受到深入批评，因其合理且看似可信，享有无可争议的真理地位。仔细审视会发现，该模型的胚胎学依据存在一些重要弱点，尤其是在处理晚期分裂时。作者认为，该模型不仅导致我们对MZ双胎发生时间的认识停滞不前，而且似乎站不住脚，应被替代。这一因素可能对胚胎学和生物伦理学产生相关影响。作为该模型的替代方案，本文提出了一种新的理论来解释MZ双胎的发生时间。它基于两个前提。首先，MZ双胎是受精事件。在这种情况下，由于合子 - 卵裂球转变的改变，第一次合子分裂不是产生两个卵裂球，而是产生双合子。其次，单绒毛膜性和单羊膜性不依赖于胚胎分裂，而是依赖于膜的融合。这一理论在近期的胚胎学进展以及一些以往的解释中都能找到支持。
