How do animals regrow their limbs? And why can't humans do it? - Jessica Whited

譯者: Lilian Chiu 審譯者: Amanda Zhu

For some animals, losing a limb is a decidedly permanent affair.

對於某些動物來說，斷肢

是永久且無法改變的事。

But for salamanders, particularly axolotls,

但對於蠑螈而言， 特別是墨西哥鈍口螈，

amputation is just a temporary affliction.

斷肢只是種暫時性的麻煩。

Not only can they grow back entire limbs in as little as six weeks,

牠們不僅可以在短短六週內 就把斷掉的部分完全長回來，

they can also regenerate heart and even brain tissue.

連心臟，甚至大腦組織都可以再生。

So how does this astonishing adaptation work?

這種驚人的適應機制是如何運作的？

Regardless of regeneration,

先不談再生，

every limbed creature had to grow their arms and legs at some point.

每一種有四肢的生物 都會在某個時點長出手腳來。

And whether that process starts in the womb or the world,

無論這個過程是從子宮裡 就開始還是出世後才開始，

it almost always begins with little bumps called limb buds.

幾乎都始於一種小突起，叫做肢芽。

These buds are full of progenitor cells—

肢芽裡面滿是先驅細胞，

a cornucopia of cell types that can differentiate into various tissues,

也就是可以分化成各種組織的 多種類型的細胞，

including muscles, cartilage, ligaments, and tendons.

包括肌肉、軟骨、韌帶，和肌腱。

Some of these progenitors are stem cells,

有一些先驅細胞是幹細胞，

capable of developing into a range of specialized cells and tissues,

能夠發展成多種 專門化的細胞和組織，

while others are merely derived from stem cells.

其他的則只是由幹細胞衍生出來的。

But in either case, the progenitors differentiate and multiply rapidly

但不論是哪一種先驅細胞， 都會隨著肢芽的發展

as the limb bud develops.

而快速分化和增殖。

Nerves grow into the limb from nearby cell bodies

神經會從附近的細胞體 生長到四肢裡面，

and a network of blood vessels form which fuel the process with oxygen.

還會形成血管網絡， 為這個過程提供氧氣。

Eventually, that tiny bud grows into a full infant limb.

最終，小芽長大成為 完整的新生四肢。

Most salamanders, including axolotls, develop their limbs in the same way.

大部分的蠑螈，包括墨西哥鈍口螈，

用同樣的方式長出牠們的肢體。

But unlike other animals, they can also start this process all over again

但和其他動物不同的是，

如果有需要，牠們可以 一而再再而三啟動這個過程。

if they need to.

When salamanders lose a limb,

蠑螈失去一肢時，

surrounding skin cells quickly surge across the wound’s surface.

周圍的皮膚細胞 會迅速覆蓋傷口表面。

This new layer of skin is called the wound epidermis,

這層新的皮膚稱為傷口表皮，

and once established, it signals cells in the underlying limb stump

形成之後，它會發送訊號 給皮膚下殘肢中的細胞，

to undergo something called dedifferentiation.

進行一個叫做「去分化」的過程。

This process reverts nearby cells from fully developed limb tissues

這個過程會將附近的細胞還原， 從發展完成的肢體組織

back into earlier, less specialized progenitor cells.

回復到較早期、尚未分化成 有專門功能的先驅細胞。

At the same time, the peripheral nervous system fires up stem cells

同時，

周邊神經系統會啟動 蠑螈全身的幹細胞。

throughout the salamander’s body.

This would be impossible for most multicellular organisms,

大部分的多細胞生物 都做不到這一點，

whose stem cells typically lose their regenerative capacity with age.

牠們的幹細胞通常會 隨著年齡失去再生能力。

But when salamander stem cells near the injury get the right signal,

但當蠑螈傷口附近的幹細胞 接收到對的訊號時，

they reactivate and start multiplying.

它們會重新啟動並開始增殖。

Researchers don’t know what ratio of stem cells

研究人員還不知道再生需要

and dedifferentiated progenitor cells regeneration requires.

多少比例的幹細胞 和去分化先驅細胞。

But we do know these cells come together

但我們確實知道

這些細胞結合起來形成 這個過程中最重要的部分：

to form the most important part of the process: the blastema.

胚基。

This structure is almost identical to a limb bud—

它的結構和肢芽幾乎一模一樣——

the primary difference is that it’s made of recycled, repurposed cells,

主要的差別在於胚基 是回收後再利用的細胞所構成，

and potentially reserved cells, rather than completely new ones.

以及可能被儲備下來的細胞， 而不是全新的細胞。

Beyond that, blastemas and limb buds have the same mission:

除了這個差異，胚基 和肢芽都具有相同的任務：

to make thousands of new cells and organize them

製造數千個新細胞，並將它們組織

into the muscle, bone, skin, and nerve tissue

成為有功能的四肢需要的肌肉、 骨頭、皮膚，及神經組織。

required for a functional limb.

As this process unfolds, nerves and blood vessels spanning the injury site

隨著這個過程發展，

分布在受傷部位的神經和血管

transmit nutrition and oxygen.

會遞送營養和氧氣。

Over several weeks, the stump will steadily grow

在幾週內，殘肢會穩定地 長出小型肢體，

a miniature limb with translucent skin.

帶有透明的皮膚。

And when the process is complete,

此過程完成後，

not only will the limb match the rest of the salamander,

重新長出來的腳 不僅能匹配蠑螈的其他部位，

there won't even be a scar.

還不會留下疤痕。

The relationship between scarring and regeneration

形成疤痕和再生之間的關係

is just one of this processes’ many mysteries.

只是這個過程的許多奧秘之一。

Scientists are still tracking salamander cells on the molecular level

科學家仍在從分子層面 追踪蠑螈的細胞，

to determine how they revert from a mature stage into a regenerative one.

以確定這些細胞如何 從成熟階段回復到再生階段。

And research into transplanting blastema cells investigates

移植胚基細胞的研究所探究的是

how other animals might replicate this reconstructive wizardry.

其他動物要如何複製這種重建奇蹟。

We also don’t understand how salamanders’ bodies know

我們也不明白蠑螈的身體怎麼會知道

what part of the limb has been lost or how much needs to be regrown.

斷掉的是腳的哪個部分 或者需要重新生長多少。

［左臂：施工中］

One theory is that blastema cells have a form of positional memory,

有種理論認為胚基細胞 具有某種形式的位置記憶，

allowing them to determine how much to grow in relation to one another.

讓它們能夠根據彼此的比例， 確定該生長多少。

And it’s equally important to understand how these limbs know when to stop growing

同等重要的是：要了解 這些肢體怎麼知道何時該停止生長，

to prevent overdevelopment, like in cancerous tumors.

以預防過度生長， 如癌症的腫瘤細胞那樣。

But one of regenerations essential ingredients doesn’t belong solely

但再生的要件——

胚基——並非蠑螈獨有。

to salamanders: the blastema.

Deer antlers use a similar healing tissue to regenerate each year,

鹿角就會使用類似的 癒合組織每年再生，

even though their skin scars like ours.

不過鹿的皮膚和我們的 一樣會留下疤痕。

Spiny mice can also restore skin, hair, and some other appendages scar-free.

棘鼠也可以讓皮膚、毛髮，及其他 一些凸出於軀幹外的部位復元，

且不留下疤痕。

And even humans can regenerate the tips of our fingers and toes

就連人類的

指尖和趾尖也可以 用非常類似的方式再生，

in a surprisingly similar manner.

We still don’t know whether this ability is tied

我們還不知道這種能力是來自於

to our shared ancestry with salamanders

我們和蠑螈的共同祖先，

or fueled by distinct biological mechanisms.

或者是由不同的生物機制所驅動。

But with time and research,

但隨時間和研究，

who knows what evolutionary knowledge we might grow back.

誰又知道我們可能會 長回哪些進化知識。