札幌医科大学医学部
分子医学研究部門 近況と話題
2001年７月31日ｃ The meaning of life.
ES細胞に関してのディスカッションです。従って、ここではLifeは生命。人生ではありません。人生をしっかり考えることの方が、私たち人間にとっては大切なことですが、このホームページでは、再生医療、再生医学に関連して、ヒトの生命の方を中心に考えてゆきたいと思います。
ところで、言葉の使い方についてコメント。ヒトと綴るか人と綴るかでも意味が違うのです。芭蕉が池で見たのは蛙または「かえる」です。大学の教養学部で解剖したのはカエルです。実験動物はイヌ、ネコ、ヤギ。子供の頃からの仲良しは犬、猫、山羊です。解説します。つまり、生物学や医学の文献で生き物を表現するときはラテン語の学名はイタリックになります。ところが、日本語をイタリックに書いて見分けることは難しいので、便宜上、生き物はカタカナで書いて区別するのです。皆さん、ご存じだったでしょうか。原則は簡単。でも、よく考えて使ってください。遺伝子治療の基礎研究では、ヒトの治療法を研究します。しかし、臨床の場面では、生物としてのヒトだけでなく、社会の中の人としても対応してゆきます。国語の記載法に関しても意識して使い分けてください。
それはさておき、NatureのLifeに関する見解を以下に引用いたします。
'Pro-life' groups define the beginning of human life as the union of sperm and egg, and equate the harvesting of human embryonic stem cells to homicide. But our biological understanding l...
Nature 412, 255 (19 July 2001)
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＜以下引用＞
The meaning of life
'Pro-life' groups define the beginning of human life as the union of sperm and egg, and equate the harvesting of human embryonic stem cells to homicide. But our biological understanding lends little support to these views.
Last week, a private clinic in Virginia further inflamed the debate over embryonic stem (ES) cell research. Researchers at the Jones Institute for Reproductive Medicine in Norfolk announced that they had created human embryos by in vitro fertilization for the sole purpose of harvesting ES cells. 'Pro-life' groups denounced the clinic. "It's still killing a human being," a representative of the Virginia Society for Human Life told The Virginian-Pilot newspaper.
Central to this argument is the view that human life begins at fertilization. ES cells show promise in the field of regenerative medicine, which seeks to grow tissues to replace those lost to disease or injury. But they are harvested by destroying an embryo comprising a hollow ball of cells called a blastocyst. The idea that this represents the destruction of a human life has an appealing moral simplicity.
Biology, however, is not that simple. Recent advances in reproductive medicine have emphasized that mammalian life need not start with the union of sperm and egg. Fertilization is not required to create embryos by nuclear-transfer cloning, the technique used to produce Dolly the sheep.
And earlier this month at a meeting in Lausanne, Switzerland, an Australian team described experiments in which mouse eggs were 'fertilized' with cells taken from adult mice. The resulting embryos contained an extra set of chromosomes, but could be induced to expel them and begin to develop as normal.
Those who oppose the extraction of ES cells from human blastocysts also tend to oppose such experimental manipulations of reproductive biology. But the natural phenomenon of identical twins similarly creates problems for the simple view that human life starts at fertilization. An embryo can split to form two or more viable embryos at any stage up to 'gastrulation', when its cells begin to migrate into distinct layers that form the basis of the adult body plan.
Because of twinning, many bioethicists take the view that it is only after gastrulation that an embryo can begin to be considered as an individual human being. This definition can be applied whether an embryo is created by conventional fertilization or by any other procedure. It does not deny that pre-gastrulation embryos are alive ? but then so too, in some senses, are sperm and egg cells.
Framing the discussion over ES-cell research in the context of when biological individuality arises, rather than when life begins, could lead to a more meaningful debate. That is why it is disappointing to see one of the companies working on ES cells apparently buying into the 'life begins at fertilization' argument.
Advanced Cell Technology (ACT) of Worcester, Massachusetts, says it is trying to generate human embryos by cloning, and then harvest ES cells from them. The company hopes to sidestep moral objections, as fertilization is not involved. Indeed, the chair of ACT's ethical advisory board argues that an embryo created in this way is not a bona fide embryo, and suggests the term 'ovumsum'.
The procedure that ACT is experimenting with, known as therapeutic cloning, might one day prove useful in generating ES cells that are genetically matched to patients requiring tissue grafts. But to suggest that it does not involve the creation of embryos is misleading.
In 1990, the British parliament was persuaded in favour of allowing a limited range of research projects on pre-gastrulation embryos by arguments that such embryos have not progressed to the point that they can be considered as individuals. Exactly the same logic can be applied to the current ES-cell debate.
＜以上、引用終わり＞
NatureのOpinionですから、権威あるコメントと思ってよいのですが、一層Lifeについてわからなくなりますね。この手の論議は、刑法や民法などの法律の観点、さらにキリスト教などの宗教的バックグラウンド、などが微妙に絡まった議論となり、最終的にはプラクティカルなプラグマティズムに落ち着くようです。この、Natureのオピニオンでは、ヒトの個人（つまり人）とは考えられない細胞や細胞集団に関しては、限られた範囲で研究を認めてもよい、というイギリス議会の判断を引用してあります。Framing the discussion over ES-cell research in the context of when biological individuality arises, rather than when life begins, could lead to a more meaningful debate. この種のディスカッションの場合には、Lifeという言葉よりも、「biological individuality」という言葉をキーワードにした方がよい。その通りですね。
日本の多くの研究者の皆さんには余りピンとこない話になってしまいますので、わたしのホームページでは、再生医学に関しては、できるだけサイエンスの観点から、人ではなく、ヒトの話を展開してゆくように努めます。
濱田洋文
