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Proembryo, or pro-embryo, is a botanical term that has been used for centuries, to denote the series of cells that are formed after fertilization within the ovule of a flowering plant, before formation of the embryo.〔(Webster's Revised Unabridged Dictionary, ) (1913).〕 More recently, this same term has been given a more extensive meaning, with reference to human egg cells (ova) that have been fertilized but not yet implanted into a uterus. This more recent meaning is synonymous with "preembryo" or "pre-embryo,"〔 and corresponds to stages with more accepted terms zygote and blastocyst. ==Preembryo in context of human development== A preembryo is a human conceptus at least between fertilization and implantation, and is known too as a proembryo.〔 A conceptus between fertilization and implantation is also frequently classified as an embryo.〔Dyson, Anthony and Harris, John. (Ethics and Biotechnology ), page 94 (Routledge 1994).〕 Even after implantation begins, a preembryo (and proembryo) may exist up until formation of the primitive streak.〔Pickering, Neil and Evans, Donald. (Creating the Child ), page 39 (Martinus Nijhoff Publishers 1996).〕 Implantation begins at about six days after fertilization, and lasts for about a week, during which time formation of the primitive streak occurs.〔Steinbock, Bonnie. ( “Life Before Birth,” ) pages 50 and 214-215 (Oxford University Press 1992).〕 Use of the terms preembryo and proembryo, in the context of human development, has drawn criticism from opponents of embryo research, and from scientists who have considered this categorization invalid or unnecessary.〔Spallone, Patricia. “Bad Conscience and Collective Unconscious: Science, Discourse and Reproductive Technology” in Rosenbeck, Bente et al. (Forplantning, køn og teknologi ), pages 131-132 (Museum Tusculanum Press 1995).〕〔Mulkay, Michael. (The Embryo Research Debate ), page 31 (Cambridge University Press 1997).〕〔Braude, Peter. (“Preimplantation Genetic Diagnosis and Embryo Research-Human Developmental Biology in Clinical Practice,” ) ''International Journal of Developmental Biology'', volume 45, page 607 (2001).〕 One rationale that has been advanced for distinguishing an early fertilized human conceptus from an embryo is that there is a potential for the conceptus to split into identical twins prior to implantation, and so (the argument goes) the conceptus cannot be regarded before implantation as a single human being. However, the conceptus before implantation exhibits self-actuated activity, which has led to the assertion that it is an embryo.〔Ashley, Benedict and O’Rourke, Kevin. (Ethics of Health Care: An Introductory Textbook ), page 127 (Georgetown University Press 2002).〕 Further, identical twinning is an instance of asexual reproduction whereby a conceptus, without ceasing to be what it is (a new human being), provides a cell or cells as a new conceptus, entirely separated or partially separated (a 'siamese' twin) from the original conceptus, but in any event self-actuated in its development from the moment that the act of asexual reproduction (twinning) is complete. By this asexual reproduction, the parents of the original conceptus in effect become grandparents to the identical twin so conceived. In the United States, a report by the National Institutes of Health stated that a conceptus could be both a pre-implantation embryo and a pre-embryo at the same time.〔(Report of the Human Embryo Research Panel of the National Institutes of Health ) (1994).〕 However, Ann Kiessling (a leader in stem cell research) has written that those categories are inaccurate.〔Kiessling, Ann. (What is an Embryo?” ) Connecticut Law Review 36 (4): 1051–93.〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Proembryo」の詳細全文を読む スポンサード リンク
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