白了少年头

Graying Mice

传说绝代艳后上断头台的前夕一夜白头。压力对发色的影响真有如此之大吗?一项关于老鼠的新研究表明确实如此,动物发色流失和神经过度紧张相关,而且我们人类很可能也一样。哈佛大学干细胞研究所的研究员对于压力和发色之间的关系很感兴趣,所以他们决定深入研究那些黑素细胞干细胞。黑素细胞会将色素带到每一根毛发的毛囊之中。因此干细胞成了研究的首要目标。

干细胞数量的改变会导致发色改变,这一点很明显,也很好发现。徐亚洁是这项研究的第一作者。起初,她和同事专门给老鼠施加了一些压力,比如倾倒它们的笼子,打湿它们的床铺,或者彻夜开灯。我们发现了什么?我们发现压力确实会导致老鼠毛发过早变白,但是,我们花了很久时间才真正找到这一现象产生的原因。

首先,研究人员以为免疫系统攻击了黑素细胞干细胞。然而那些缺乏免疫细胞的老鼠受压之后毛发仍然会过早变白,于是,他们又认为主要原因可能是皮质醇,皮质醇是典型的压力激素。但是当他们将老鼠的肾上腺切除,让老鼠无法生成皮质类激素之后,老鼠受压毛发依然会变白。

终于,研究人员将注意力转到交感神经系统之上,交感神经系统控制了身体对于压力的整体反应,包括经典的战斗逃跑反应。交感神经不仅会伸到我们的肌肉里、器官里,还会伸到我们的毛发里。神经末梢就像丝带一样包裹着每一根毛发的毛囊。后来,徐教授和研究团队将这些神经末梢连接切断,同时保持干细胞完好,这些老鼠在轻微压力之下,毛发依然乌黑鲜亮。这项研究发表在自然杂志上。

人老了就会有白头发,是不是也和交感神经相关,这一点尚不明确,但是上述研究确实给了我们一线希望,总有一天,我们能永葆发色,省得每个月都去染发。

原文

They say that Marie Antoinette’s hair turned white the night before she lost her head to the guillotine. But can stress really have such a dramatic effect on hair color? A new study in mice concludes that it can and credits overactive nerves with stripping the color from the animals’ locks—and possibly ours. Researchers at Harvard’s Stem Cell Institute were interested in the stress and hair color issue. So they decided to take a closer look at those stem cells that give rise to melanocytes—the cells that pump pigments into each hair follicle. The stem cells were an obvious target.

“Because changes in the stem cell population translate to changes in hair color, which are very visible and easy to identify.” Ya-Chieh Hsu, the study’s senior author. To start, she and her colleagues subjected mice to some rodent-sized stressors—like having their cage tilted, their bedding dampened or their lights left on all night. “So what did we find? We found that stress indeed leads to premature hair graying in mice. But it took a long time for us to actually narrow down how it occurs.”

First, they thought it could be the immune system attacking the melanocyte stem cell population. “However, mice lacking immune cells still show premature hair graying under stress.” Then they thought the key factor could be cortisol, the quintessential stress hormone. “But when we removed the adrenal glands from the mice so they cannot produce cortisol-like hormones, their hair still turned gray under stress.”

That’s when they turned their attention to the sympathetic nervous system, which orchestrates the body’s overall reaction to stress, including the classic fight-or-flight response. Those nerves reach out to our muscles, organs and, yes, even our hair. “The nerve terminals wrap around each hair follicle like a ribbon.” And when Hsu and her team cut those connections, the stem cells were spared, and the animals kept their shiny black coat even in the face of minor discomfort. The findings appear in the journal Nature.

It’s unclear whether the same sympathetic nerves make us gray as we age. But the results provide hope that we may someday be able to fight to hold onto our natural hair color—and avoid that monthly flight to the hairdresser.

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