Symmetry in the Body Holds Clues to Illnesses
身體對稱隱藏疾病線索
By Carl Zimmer

One day in 1788, students at the Hunterian School of Medicine in London were opening a cadaver when they discovered something startling. The dead man’s anatomy was a mirror image of normal. His liver was on his left side instead of the right. His heart had beaten on his right side, not his left.
1788年某日，倫敦韓特醫學院的學生解剖大體時，看到一個驚人的現象。這名男性死者的內臟位置與正常狀態完全相反：肝臟位於左側而非右側，心臟則長在右側而非左側。

The report by their teacher the Scottish physician Matthew Baillie, was the first detailed description of the condition, which came to be known as situs inversus and is thought to occur in about 1 in 20,000 people. Baillie argued that the condition might help doctors understood how our bodies normally tell the right side from the left.
他們的教師是蘇格蘭籍內科醫師貝里。貝里撰寫了詳細描述這種狀況的第一份報告。醫學界稱這種狀況「內臟逆位」，大約每兩萬人就會出現一例。貝里認為，它或許有助於醫學界瞭解正常情況下人體如何辨別左右。

Mutations that cause situs inversus can lead to a number of serious disorders, including congenital heart defects. Deciphering the effects of mutated genes could lead to diagnoses and treatments for those conditions.
造成內臟逆位的突變可能引起多種嚴重疾病，包括先天性的心臟缺陷。醫學界若能解讀突變基因可能引起的後果，或許有助於這些狀況的診斷與治療。

“Understanding how you put this axis together has a lot of implications for understanding congenital heart disease,” said Rebecca Burdine, a molecular biologist at Princeton University in New Jersey.
美國新澤西州普林斯頓大學分子生物學家蕾貝卡‧柏汀說：「弄清楚你如何結合這些軸線，對深入瞭解先天性心臟疾病大有幫助。」

Dominic P. Norris, a developmental biologist at the Medical Research Council in Harwell, England, and other scientists are beginning to solve that puzzle. They have pinpointed some of the steps by which embryos’ organs develop on the left or right.
英格蘭哈威爾醫學研究協會的發展生物學家諾里斯與其他科學家已經開始破解這個謎團。他們準確找出胚胎的器官在左側或右側成長的步驟。

“I know what it is, you know what it is, but how does the embryo learn what it is?” Dr. Norris said.
諾里斯說：「我知道它是什麼。你也知道。問題是，胚胎如何知道？」

Our bodies start out symmetrical, the left side a perfect reflection of the right. Asymmetry in the human body begins to show after six weeks.
人體最初對稱成長，左右完全對稱。六個星期後，人體的不對稱開始顯現。

The heart comes first. Starting out as a simple tube, it loops to the left, later growing different chambers and vessels on each side.
心臟是第一個。它一開始只是條簡單的管子，向左繞成環。兩側此後各長出不同的心室與血管。

But those visible changes arise long after the embryo has developed differences on its left and right. Experiments have revealed that the early embryo produces different proteins on each side while it still looks symmetrical.
然而早在這些可見的變化出現的一段時間之前，胚胎的左右兩側即已出現了差異。實驗結果顯示，初期的胚胎看似對稱時，兩側就會製造不同的蛋白質。

Biologists have pinpointed a single spot where this symmetry breaking starts: a tiny pit called the node, on the embryo’s midline. The interior of the node is lined with hundreds of tiny hairs, called cilia, which spin at a rate of 10 times a second.
生物學家已經找到這種對稱開始瓦解的位置。它是位於胚胎中線的一個名為結的小凹處。這個結的內部布滿數百條細微的纖毛，每秒旋轉10次。

“It’s like a blender,” Dr. Norris said. “It just goes round and round.” Tilted, the cilia all push the fluid surrounding the embryo in one direction, from right to left. When scientists reversed that flow in mouse embryos, it resulted in reversed organs.
諾里斯表示：「它有如攪拌器。它會不斷旋轉。」這些傾斜的纖毛會把圍繞胚胎的液體從右往左推。當科學家在老鼠胚胎中逆轉這個方向時，最後出現器官逆位。

Once the fluid starts flowing, it takes only three or four hours for the left and right sides to be determined.
一旦液體開始流動，只要3、4小時左右二側即可確定。

The rim of the node is ringed by cilia that respond to the flow without spinning. “We don’t know the nitty-gritty,” Dr. Norris said. “We don’t know the actual mechanics in these cells of what is happening.”
結的邊緣有纖毛圍繞。這些纖毛對液體的流動有反應，但不會旋轉。諾里斯說：「我們不知道其中的基本道理，也不知道這些細胞的力學原理與現象之間的關係。」

Those cilia may then release calcium atoms that spread to surrounding cells. Those cells respond by spewing out a protein called Nodal, which spreads through the left side of the embryo, leaving the left side loaded with Nodal and the right with almost none.
接著，這些纖毛可能釋出鈣原子，再擴散到周圍的細胞。這些細胞會釋出名為「結」的蛋白質。它會在胚胎左側擴散，充滿左側，右側則幾乎全無。

“Nodal seems to be directly telling the cells on the left side to move faster than the ones on the right,” Dr. Burdine said.
柏汀說：「結蛋白質似乎會直接指示左側的細胞移動得比右側細胞快。」

The fast-moving cells on the left side drag the entire heart clockwise. From that initial twist, the heart then develops its distinctive left and right sides.
快速移動的左側細胞會以順時針方向拖曳整個心臟。從此一最初的旋轉開始，心臟發展出明顯有別的左右兩側。

Some studies suggest that these early signals also influence brain development. Scientists have long known that the two sides of the human brain have some important differences. The right hemisphere, for example, plays a big role in understanding the mental lives of other people; the left hemisphere is important for focusing attention.
部分研究顯示，這些初期的信號也會影響大腦的發育。科學家已知，人類大腦的兩側有一些重要的差異。例如，右腦職司理解他人的心理狀態，左腦則攸關注意力集中。

As they look at these biological signals, scientists are also studying disorders that may be tied to their disruption.
留意這些生物信號時，科學家不忘研究可能與它們受到干擾有關的病變。

Situs inversus, the complete flip of the organs that Baillie described in 1788, may be the most dramatic of these disorders, but it is also one of the most harmless. The reversal is relatively safe because all the organs line up. The real danger is in incomplete reversals – “when you get a confusion, when you get things not quite meeting,” Dr. Norris said.
貝里1788年描述的器官完全逆位可能是最具有戲劇性的異常狀況，然而也最無害。這種異位相對安全，因為器官全部排列整齊。不完全逆位才危險。諾里斯說，「這是混亂的狀態，各部組件並未完全會合」。

Most worrisome are cases affected heart. “If you put the heart in the wrong place, and everything else is correct,” Dr. Burdine said, “that’s almost always fatal.”
最值得擔心的是影響心臟的狀況。柏汀說：「如果心臟位置不對，其他器官位置正確，幾乎一定致命。」

She hopes that research on left-right disorders will lead to genetic tests that can predict the risk of these hidden heart defects. She even sees an application to attempts to rebuild damaged hearts with stem cells.
她希望藉由有關左右異位的研究，能夠研發出預測隱藏性心臟缺陷風險的基因檢測法。她甚至認為，醫學界未來一定可以應用這方面的研究成果，以幹細胞重建受損的心臟。

“It’s going to be more than just making the right cells,” she said, adding that they would need to be placed in the proper three-dimensional structure and given the correct signals on where to go.
她說：「這不只涉及培育正確的細胞。」她又說，還得把這些細胞置於適當的三度空間結構下，並給他們正確信號以指引前進方向。

“And one of those signals,” she said, “is the left-right signal.”
她說：「其中的一個信號是左右信號。」

原文參照：
http://www.nytimes.com/2013/06/04/science/growing-left-growing-right-how-a-body-breaks-symmetry.html

Graphic：
http://www.nytimes.com/interactive/2013/06/04/science/0604-leftright.html

紐時中文版翻譯：
http://cn.nytimes.com/health/20130606/c06leftright/zh-hant/

2013-06-18聯合報/G5版/UNITEDDAILYNEWS陳世欽譯 原文參見紐時週報十版上