"Oh, welcome back"

After the meeting at the TV station at midnight, Miyahara dragged his tired body back to the apartment house in Tokyo, and Risako Sakashita lined up the knobs and Finger food on the table and enjoyed a drink alone. There was a Whitesnake's song she liked in the room. When Miyahara heard from her, she explained that she stopped at Miyahara's apartment because the distance traveled from her place of work today was closer than when she returned to her apartment.

"Isn't it" staying "instead of stopping by? Instead of a hotel ... "

Miyahara pointed out with a bitter smile, and Risako asked without answering the question.

"If you want to get something in your stomach, I'll make it."

"No, you don't have to. I bought it at the convenience store opposite."

"You are always eating only greasy foods, you'll get fat .... Well, I'm the same."

Risako said somehow happily while looking at Miyahara's tired face. Then, while saying "May I offer you a cup of sake?", She poured the 'Joppari’ she was drinking into a new cup, she brought from the kitchen, and she offered it in front of Miyahara.

"You're not working anymore today, right?"

"Oh, I'm going to do that"

He received the cup that was presented while standing, took a sip, returned the cup to Risako, hung his coat and jacket on a hanger, and loosened his tie. He sat in a chair opposite Risako, saying "Yokorasho," and chewed a piece of smoked cheese on a plate. After saying, "Oh, by the way ...", he talked about meeting Yuji Kanzaki, who also knows Risako. "He is always getting a good look," he continued, but the story didn't stop there, and naturally extended to biologica clones.

After hearing Miyahara's words, Risako said, "What's that?" And immediately got involved in the story. And she showed her wisdom without hesitation. "Well, it's not uncommon for plants," she said. "Then it was successful with sea urchin larvae in 1891. A long time later, it succeeded in the leopard frog in 1952 and in the sheep in 1981. In the case of somatic cell clones, Xenopus laevis was produced in 1962, and Dolly, a sheep derived from the nucleus of the mammary gland, was produced in 1996." Her explanation didn't stop. "Then the cell fusion method was invented, and in 1988, cows, then using the Honolulu method (Note 4), in 1998, mice, sheep, horses, Successful cases of goats, rabbits, pigs, cats and rats have been reported. In 2005, even a dog, which was pointed out to be difficult because of its complexity, succeeded. Well, that Korean researcher was later reported a lie, but it seems that it was true when it came to dog clones. "

After talking up to that point, Risako moistened her throat with sake and continued to talk relaxedly.

"For the time being, humans haven't succeeded yet, but what about the reality?" She twisted her head. "And it seems that every animal clone has a shorter telomere length for cell division than the original, so even if someone can make it well, the clone is shorter-lived than the original clone so far. In other words, if it is for organ transplantation, it needs to be grown one after another. For example, if you grow it in a culture tank, it doesn't have enough muscle, so it must look different, "she continued.

There is a tendency that the clone and the clone source are considered to be exactly the same shape, but that is not the case. For example, fingerprints and vascular patterns are developmentally acquired. Appearance and biometric information are not always the same. It has been confirmed that the coat color of cats and the nose print of cows are different from those of the clone source. Of course, if the breeding environment changes, the appearance will change as much as possible.

"If it's a biological clone, it looks like a human. However, pointing out that the clone is for organ transplantation and therefore has no human rights may be a bit of a challenge in this era. It doesn't matter if it's good or bad ... Considering the above, is there a possibility that iPS cells (Note 5) will remain? The STAP cell (Note 6) went somewhere in no time. ES cells (Note 7) can only be obtained from embryos in the early stages of development, and their collection may damage the mother's body. In addition to that, there is also the ethical question of whether embryos that may grow to individuals can be used in experiments. In that respect, iPS cells can be safely collected from blood and skin, and there is also the endorsement of the director of the Pope's Office of Life Sciences. "

In general, plants can regenerate the whole from a section of tissue, but in animals, except for some lower species, only fertilized eggs are said to have that ability (Note 8). However, in culture, there are cells that have the ability to differentiate into all tissues. Therefore, if the necessary tissues can be differentiated and formed from these cells, the arms and organs lost due to an accident or illness can be transplanted without donation from a donor. Further, in that case, since the donor and the recipient are the same individual, the rejection reaction does not theoretically occur.

"... But it's still a difficult task to start with iPS cells and create transplantable kidneys and pancreas."

The human body is composed of about 60 trillion cells, but if we go back to the past, we will reach only one fertilized egg. The one fertilized egg repeats proliferation and differentiation, and eventually a human shape is completed.

In general, the ability of fertilized eggs to differentiate into these tissues is called totipotency. Both ES cells, iPS cells, and STAP cells, if any, are totipotent, but ethical issues pose a research shackle for ES cells. On the other hand, iPS cells (and STAP cells) do not have such a problem. However, in the case of iPS cells, although a method for avoiding canceration of growing cells, which was a concern at the earliest stage, was found, the efficiency of cell production is low. At this time, it does not reach the previous method using oncogenes.

Research on totipotent all-purpose cells, including patent rights, is being pushed forward while competing with each other around the world. However, the current situation is that there are many unclear points regarding the ability to complement the tissue when it is actually transplanted. Therefore, it must be said that the road to regenerative medicine, which aims to regenerate various tissues and organs, is still steep.

"Somehow, you're completely familiar with that direction, but did you undertake such a job?"

Miyahara said to her impressed by Risako who was speaking of expertise.

"That's not the case. The other day, on a coverage trip with you, the misery of reality stuck in my head. That's why I looked it up. Unlike you, I graduated from one of the best universities in Japan, which boasts outstanding memorization ability. Besides, I have a body that can't give birth to children. "

Risako explained to Miyahara the reason for her knowledge disclosure.

"But it's hard to imagine an organ of human origin other than yourself that has no rejection. At least in the last few decades of research. Perhaps it is nearing completion at a state-of-the-art research institute in a secret nation, but even so, I don't think that such information will be leaked. If so, is the closest xenotransplantation pig? It may be a mini pig that knocked out that hyperacute rejection-related gene (Note 9). "

"But in the case of mini pigs, the only target for organ transplantation into humans would be the pancreas. And even if the hyperacute rejection is suppressed, there is no guarantee that the virus integrated into the porcine chromosome during the production process will not be harmful at the transplant destination. "

"If so, we would like to dream, but I must say that donating organs to humans without rejection is just a stage of possibility."

Note 4

A method of cloning without cell fusion. Put somatic cells directly into the egg without the nucleus.

Note 5

Induced pluripotent stem cells.

Note 6

Stimulus-Triggered Acquisition of Pluripotency cells

Note 7

Embryonic stem cells.

Note 8

It was STAP cells that overturned that common sense.

Note 9

Gene knockout is a genetic engineering technique that introduces a dysfunctional gene into an individual organism. The word knockout in this case means "unusable / malfunctioned". When a foreign substance is mixed in the human body, rejection occurs in the order of hyperacute, acute, and chronic. However, in the case of mini pigs whose organs are used, the alpha galactose gene, which is said to cause hyperacute rejection that does not exist in humans, has been removed, and MCP (Monocyte Chemoattractant Protein) gene, which is said to have an action of suppressing hyperacute and acute rejection, is em.bedded in it.

「あら、お帰りなさい」

　深夜のテレビ局での打ち合わせが終わり、宮原が疲れた身体を引き摺るようにして都内の共同住宅に帰宅すると坂下理紗子がテーブルにつまみを並べ、独り晩酌を愉しんでいる。部屋には彼女の好きなホワイトスネークの曲がかかっている。話を聞くと本日の仕事先からの移動距離が自分のアパートへ帰るよりも近かったので宮原のところへ寄ったと説明する。

「寄ったんじゃなくて、泊まり、だろう？　ホテル代わりに……」

　苦笑しながら宮原が指摘すると、その質問には答えず理沙子が問う。

「何かお腹に入れたいのなら作るわよ」

「いや、いいよ。向かいのコンビニで買って来たから」

「脂っぽいものばっかり食べていると太るわよ。……ってまあ、あたしもだけどさ」

　宮原の疲れた顔をつくづくと眺めながら、どこか愉快そうに理沙子が言う。ついで、「まあ一献」と台所から持ってきた新しいコップに自分が飲んでいたじょっぱりを注ぐと宮原の前についと差し出す。

「今日は、もう仕事しないんでしょ？」

「ああ、そのつもりだ」

　立ったまま差し出されたコップを受け取り、一口呑み、そのコップを理沙子に返すとコートと上着をハンガーに掛け、ネクタイを緩める。「よっこらしょ」と口にしながら理紗子の向かいの椅子に腰掛け、皿に並んだスモークチーズを一欠け頬張る。ついで、「ああ、そういえば……」と理紗子も顔見知りの神崎雄二に会った話をし、「あいつも、いい顔付きになってきたよ」と続けるが、話はそれには留まらず、当然のように生体クローンに及ぶ。

「なによ、それ？」と宮原の言葉を請け、理沙子が早速話に絡む。臆することなく自身の博識振りを披露する。「まあ、植物だったら珍しくもないんだけどね」と前置きし、「一八九一年にウニの幼生、それからずっと下って一九五二年にヒョウガエル、さらに下って一九八一年にヒツジで成功してるわ。で、体細胞クローンだと一九六二年にアフリカツメガエル、一九九六年には乳腺細胞核由来のヒツジのドリーが作られている」と止まらない。「それから細胞融合法が発明されて一九九八年にウシ、それからホノルル法（註４）を使って一九九八年にマウス、同じ方法でヒツジ、ウマ、ヤギ、ウサギ、ブタ、ネコ、ラットの成功例が報告されているわね。で、二〇〇五年に――いろいろと複雑だから難しいといわれていた――イヌでも成功。まあ、あの韓国の研究者には後で土が付いたけど、イヌのクローンに関しては本当だったみたいね」

　そこまでを一気に喋ってから理紗子は酒で咽を潤し、その先をまったりと続ける。

「一応、ヒトではまだ成功していない……ってことになってるけど、実際のところはどうなんだろう？」と首を捻り、「それにどの動物クローンでも細胞分裂に必要なテロメアの長さが元より短いらしいから、たとえ上手く作ることができたにしても今のところクローンはクローン元より短命なのよね。……ということは臓器移植用なら次々と育てる必要があるってことで例えば培養槽の中で育てる場合は筋肉だって付かないから、見た目もきっと元と異なるわ」と続ける。

　世間一般にはクローンとクローン元の姿形がまったく同一だと捉えられている節があるが、それは違う。例えば指紋や血管パタンは発生生物学的に後天的なものだ。見かけや生体情報が必ずしも瓜二つになるとは限らない。ネコの場合は毛色が、ウシの場合は鼻紋が、クローン元と異なることが確認されている。もちろん育成環境が変われば外観はいくらでも変わる。

「だけどさ、生体クローンだとすると見かけは人間なのに臓器移植用だから人権ナシっていうのは、この時代じゃ、ちょっと難しいかもしれないわね。それが良いか悪いかは別にして。……とすると残るのは、やっぱりｉＰＳ細胞（註５）かな？　ＳＴＡＰ細胞（註６）はあっという間に何処かへ行っちゃったし……。ＥＳ細胞（註７）だと発生初期の胚からしか得られないし、その採取で母体を傷付ける可能性があるし、それに個体まで育つかもしれない胚を実験で使っちゃっていいのかっていう倫理的な問題も付きまとうしね。その点ｉＰＳ細胞だったら、血液や皮膚から安全に採取できる――ってことになってる――し、ローマ法王庁・生命科学アカデミー所長のお墨付きもありますからね」

　一般に、植物は組織の切片からその全体を再生することができるが、動物では――一部の下等な種を除き――受精卵以外、その能力を持たないとされる（註８）。だが培養下においてはすべての組織に分化し得る能力を持つ細胞が存在する。よって、それらの細胞から必要な組織を分化形成することができれば事故や病気で失った腕や臓器をドナーからの提供なしに移植することが可能になる。またその場合はドナーとレシピエントが同じ個体であるため、理論的に拒絶反応も生じない。

「……とはいってもｉＰＳ細胞から始めて移植可能な腎臓や膵臓を作り出すのは、まだ至難の業だよね」

　ヒトの身体は約六十兆個の細胞から構成されているが、過去に戻ればたった一個の受精卵に辿り着く。その一個の受精卵が増殖分化を繰り返し、やがて人の形が出来上がる。

　一般に受精卵だけが持つこの組織への分化能力を全能性と呼んでいる。

　ＥＳ細胞もｉＰＳ細胞も――実際にあるとすればＳＴＡＰ細胞も――全能性を有しているが、ＥＳ細胞では倫理的な問題が研究上の枷となる。一方ｉＰＳ細胞（及びＳＴＡＰ細胞）にはそのような問題はないが、ｉＰＳ細胞の場合は最初期に懸念された成長細胞の癌化を回避する方法は発見されたものの細胞作出の効率が悪く、現時点でも癌遺伝子を用いた以前の方法に至らない。

　全能性を持つ万能細胞の研究は――特許権も含めて――世界各国でしのぎを削り合いながら押し進められているが、実際に移植された場合の組織補完力などについては不明な点が数多いのが現状だ。よって種々の組織や各種臓器の再生を目論む再生医療への道程は、まだまだ険しいと言わざるを得ない。

「なんか、すっかりそっち方面の物知りになってるけど、そんな仕事でも引き請けたわけ？」

　専門知識をスラスラと口にする理紗子に感心し、宮原が水を向けると、

「別にそういうわけじゃないのよ。この前一緒に行ったあのときの取材で悲惨さが頭にこびりついちゃってね。それで調べたわけ。ホラ、わたし、あなたと違って抜群の暗記力を誇る日本最高学府の一つを出てますからね。……それにあたし子供が出来ない身体だし」

　理紗子は自分の知識開陳の理由をそんな言葉で宮原に説明する。

「だけどさ、拒絶反応がまったくない自分以外のヒト由来の臓器なんて考え難いわよ。少なくとも、ここ数十年の研究スパンではね。もしかしたらどこかの秘密国家の最先端研究所では完成間近なのかもしれないけど、仮にそうだとしてもそんな情報が漏れてくるとも思えない。とすると一番近いのが異種移植でブタかしら？　例の超急性拒絶反応関連遺伝子をノックアウトしたミニブタ（註９）……」

「でもミニブタの場合はとりあえずヒトでの臓器対象は膵臓だろう？　それに超急性拒絶反応が抑えられたにせよ、作出過程で豚の染色体に組み込まれたウィルスが移植先で悪さをしないという保証もないし」

「……となると夢は見たいけども、やっぱりまだ拒絶反応なしのヒトへの臓器提供は単なる可能性の段階だと言わざるを得ないんじゃないかしら？」

　註４　細胞融合を行わずにクローンを作る方法。核を除いた卵子に体細胞を直接入れる。

　註５　人工多能性幹細胞／誘導多能性幹細胞。

　註６　刺激惹起性多能性獲得細胞。

　註７　胚性幹細胞。

　註８　その常識を覆したのがＳＴＡＰ細胞だったのだが……。

　註９　遺伝子ノックアウトとは、ある生物個体に機能不全の遺伝子を導入する遺伝子工学上の技法で、この場合のノックアウトとは『使えなくした／機能不全にした』という意味。人体では異物が混入すれば、超急性、急性、慢性の順で拒絶反応が生じるが、対象となるミニブタでは超急性拒絶反応を起こすとされる人間にはないアルファガラクトース遺伝子が除去され、かつ超急性および急性拒絶反応を抑制する作用を持つといわれるＭＣＰ（Monocyte Chemoattractant Protein）遺伝子が導入されている。