The Cellular Compass

動物の発生で働く細胞のコンパス 平面内細胞極性

By Paul N. Adler /Jeremy Nathans P. N. アドラー /J. ネイサンズ
English 日本語 日本語
Building a body is not simple. Fish, frogs and people all start from a single cell that becomes, seemingly against many odds, a highly organized, very complicated creature. Fertilized eggs split into two cells that become four, then eight, 16 and—within a matter of weeks—tens of thousands of cells. By this point the original spherical ball has rearranged itself into an elongated shape, bulging rounder and thicker at one end, with a shallow furrow running along its length. Soon another astonishing cellular ballet begins. The furrow deepens, and the cells that make up its walls begin to lean toward one another until they touch and stick together, forming a long, hollow tube that will eventually give rise to the brain at the bulging end and the spinal cord at the other.  身体を作り上げるのは簡単な仕事ではない。魚もカエルも人間もみな,たった1個の細胞から始まり,それが不思議なことに高度に組織化された複雑な生き物になる。受精卵が2つに分裂し,4個,8個,16個と倍々ゲームで増えて,ものの数週間で数万個の細胞になる。最初は球状の塊だった胚が,この時点ではやや細長い形になり,一方の端は他方よりも丸く太く膨らんで,胚の前後(軸)方向に1本の浅い溝ができる。
To assemble so precisely, these and other cells in the embryo must sense where they are in relation to the rest of the organism. Each cell needs to know where an animal’s front, back, top and bottom are located. Each cell also must figure out which direction is closer to or farther from the rest of the body. We and other developmental biologists have spent the past few decades trying to understand how this cellular orientation system works. As part of this larger quest, we have discovered a key component that contains several proteins that function together as a miniature compass within each cell. Without this compass, the heart, lungs, skin and other organs could not develop properly. In humans, when one of these proteins is altered by mutation, serious birth defects are the result.  このように緻密に組み上がるには,胚の細胞は自分が他の組織に対してどちらを向いているかを検知しなければならない。身体の頭とおしり,そして背と腹がどちらなのか,それぞれの細胞が知る必要がある。また,どちらが身体の胴体で,どちらが手足の先端なのかもわかっている必要がある。
Although there is much that we still do not understand about how this orientation system functions, what we have discovered so far sheds new light on fundamental processes of development across the animal kingdom. So far we have learned the most about how the compass works in epithelial cells, which typically cover a tissue surface like flagstones on a sidewalk, forming layers that are just one cell in thickness. If the cotton sheet on a bed were made up of epithelial cells, the proteins that we and others have found would allow any given cell in the sheet to sense which of its sides is closer to the head or the foot of the bed.  この方位検知システムの作用の仕組みについては未知の部分がまだ多く残っているものの,これまでの発見はすべての動物に共通する発生の基礎プロセスに新たな光を投げかけている。最も研究が進んでいるのは上皮細胞における方位磁石の働きだ。上皮細胞は歩道の敷石のように器官の表面を覆っている細胞で,厚みが細胞たった1個分の薄い層をなしている。この膜をベッドにかかった木綿のシーツにたとえると,私たちが発見したタンパク質は,シーツを構成する個々の細胞がベッドの頭がどちら側で足はどちら側なのかを検知できるようにしている。
Organisms with cells that know where they are within the body benefit from a distinct evolutionary advantage: their complex tissues no longer need to be symmetrical in all directions; different parts can specialize. The hairlike cilia at one end of the cochlear duct of the ear, for example, distinguish high-frequency sounds; those at the other end detect low-frequency sounds. Scientists refer to the ensuing asymmetry of the tissue layer as planar polarity because opposing poles can be seen through the plane of the tissues.   細胞が身体内での自分の向きを検知できる場合,その生物には進化上で明らかな利点が生じる。組織はもはや全方位に対称的である必要はなくなり,複雑な形を取ることができ,身体の部分部分が特殊化できるのだ。例えば内耳の蝸牛管の一端にある不動毛(細い細胞突起)は高周波の音を識別し,他端の不動毛は低周波の音を検出している。上述したようなシーツ内で細胞が発達させる非対称性は,組織平面内全体にわたって細胞の向きがそろっていることから,「平面内細胞極性」と呼ばれている。