ヒトの器官で最大の器官が新たに発見される 2018年3月29日（木）松岡由希子器官とは、多細胞生物の体を構成する単位で、その形態を周囲と区別でき、全体としてまとまった機能を担うものをいう。これまで、ヒトの器官で最も大きいものは、体重のおよそ16％を占める皮膚とされてきたが、このほど、皮膚を上回る大きさの新たな"器官"が見つかった。米ニューヨーク大学医学部を中心とする研究プロジェクトは、2018年3月27日、科学誌「サイエンティフィック・リポーツ」で研究論文を発表。「皮膚の下にあり、消化管や肺、泌尿器系に沿ったり、動脈や静脈、筋膜を囲んだりしている層は、従来、結合組織と考えられていたが、実は、体液を満たし、相互に連結し合う区画が、全身にネットワーク化されたものであることがわかった」とし、「これを間質という新たな器官として定義すべき」と世界で初めて提唱した。体重のおよそ20％に相当する体液で満たされた間質は、強度の高いコラーゲンと柔軟性のあるエラスチンという2種類のタンパク質による網目構造で支えられており、臓器や筋肉、血管が日常的に機能するように組織を守る"衝撃緩衝材"のような役割を担っている。また、注目すべき点として、体液の移動通路としての働きがある。この体液がリンパ系に流れ込むことで、いわば、免疫機能を支えるリンパの元となっているのだ。
かつての顕微鏡での解析方法では、観察できなかった 従来、顕微鏡での解析では、固定により生化学反応を停止させた組織が使われてきたため、間質そのものを観察することができなかった。固定によって、体液が流れ出て、体液で満たされていた区画を囲むタンパク質の網目構造が平たくつぶれてしまっていたからだ。そこで、この研究成果に大きく寄与したのが、生きた組織を顕微鏡レベルで観察できる高性能なプロープ型共焦点レーザー顕微鏡（pCLE）だ。研究論文の共著者でもあるデビッド・カーロック博士は、2015年秋、この新しい技術を用い、患者の胆管でがんの転移を調べていたところ、胆管の内面を覆う粘膜下組織レベルにおいて、これまでの解剖学とは合致しない、相互に連結する空洞を偶然見つけた。
がんの転移や炎症などの研究に新たな道をひらく さらに、ニューヨークのベス・イスラエル医療センターで膵臓の手術を受けた患者13名の胆管から生きた組織を採取して観察した結果、その画像からも同様の空洞が認められたという。間質を器官と定義づけるべきかについてはまだ議論の余地があるものの、この研究成果は、従来、結合組織にすぎないと見過ごされてきた間質の機能や役割を改めて見直し、がんの転移や炎症などの研究に向けた新たな道をひらくものとして注目されている。
The largest organ in the body may have just been discovered — and it could reshape our understanding of human anatomy Thursday, 29 March, 2018, 11:59am It might seem like we should already have identified all the structures in the human body, even if we don't know the function of every cell and organ. But that assumption might be very wrong, if the authors of a study newly published in the Nature journal Scientific Reports are correct. In between the spaces in our bodies — beneath the skin, lining the gut and lungs, surrounding blood vessels and fascia between muscles, and more — there's a fluid-filled network of tissue. The idea that there's tissue and fluid in these spaces isn't new; interstitial fluid is one of the significant types of fluid in the body, though we didn't know it was contained in these structures. But the authors of the new study say this tissue has a unified structure and function throughout the body that makes it an organ. Using that definition, it could be the largest organ in the body, taking up a bigger volume than even our skin. This organ might help protect the rest of our organs and tissue. It could also explain the spread of certain cancers, as well as how a number of diseases progress in the body. A newfound organ, the interstitium, is seen here beneath the top layer of skin, but is also in tissue layers lining the gut, lungs, and urinary systems, as well as those surrounding blood vessels and the fascia between muscles. The organ is a body-wide network of interconnected, fluid-filled compartments supported by a meshwork of strong, flexible proteins. Researchers previously thought the area between other organs and tissues in our body was largely solid, comprised of structural supportive proteins known as collagen, as well as stretchier elastin connective proteins. But an analysis using a newer kind of imaging technology (called probe-based confocal laser endomicrosopy) revealed that interconnected fluid-filled sacs run through the collagen and elastin structures. Past attempts to look at this tissue under a microscope made it impossible to see these connected fluid-filled spaces and overall structure. Slicing it up caused the fluid to drain, emptying the sacs and causing them to collapse, and just leaving the supporting proteins behind. Looking at things in a new way allowed researchers to view the structure of the tissue without causing it to collapse, significantly changing how we understand our insides, Dr. Neil Theise, a professor in the Department of Pathology and co-senior author of the study, explained in a news release. There are several ways the network of interstitial fluid and connective tissue could better explain how the human body functions. The fluid-filled sacs throughout this tissue could help interstitial fluid travel throughout the body. That's important because fluid in this network is the largest source of lymph, which is a critical part of the immune system."This finding has potential to drive dramatic advances in medicine, including the possibility that the direct sampling of interstitial fluid may become a powerful diagnostic tool," said Theise. A better understanding of this interstitial network could help us understand why skin wrinkles as we age, why limbs get stiff, and how inflammatory diseases spread. It may also explain why cancers that spread into the space between organs are more likely to show up in other parts of the body. As something that surrounds our blood vessels and organs, the interstitium might help protect them from tearing and may help absorb shocks that could otherwise damage parts of our body. It's even possible that studying these spaces could help reveal what acupuncture is or isn't doing within the body. It'll take time and research before the medical community at large decides whether or not to treat the interstitium as an organ. But the possibility that this network of fluid and tissue is an entirely new organ reveals that even in parts of ourselves we thought we understood, we still have plenty to learn.