17
similar to catalase convert hydrogen peroxide into
oxygen and water. However, the number of these
enzymes decreases with age causing a build-up
of hydrogen peroxide and a reduction of melanin
[1]
. The same enzymes also enhance tyrosinase
format i on, a d i f ferent t ype of enz yme that
produces melanin in hair follicles. Since melanin
is responsible for giving hair its pigmentation,
reduction i n the number of melanocy tes or
disruption in the delivery of melanin can turn hair
grey or white. The question is, how does stress tie
into this?
An experiment involving mice and human
scalp yielded some interesting results. Researchers
first studied how mice melanocyte stem cells
react to ‘stresses’ in the forms of a cut (injury)
and exposure to UV radiation. They found that
melanocyte stem cells residing in hair follicles
near the area of injury migrated toward the skin
to produce melanocytes. Some hair follicles in the
vicinity of the wound ended up completely void
of melanocyte stem cells and new hairs growing
out of these follicles were white. Melanocyte stem
cells did not completely migrate after mice were
exposed to UV and new hairs were still pigmented.
In human scalp, researchers removed melanocytes
in the samples to see if hair follicle melanocytes
would move into the area and a similar effect
seemed to occur
[2]
.
Several points should be noted. If any link
between physical stress and hair turning white is
indicated in this study, it is weak at best. For one,
it seems that the movement of melanocyte stem
cells from hair follicles to injured skin is dictated by
many factors and not injury alone. Second, what
people typically associate with hair greying has
more to do with mental stress than physical stress
such as injuries, as described in this experiment.
Thus, it is safe to conclude that to date, no solid
evidence exists that links mental stress with hair
turning grey.
The question of whether Marie Antoinette’s
hair was capable of turning white overnight is
perhaps a lot more straightforward to answer. Hair
is essentially dead tissue, and does not possess
the ability “turn” white; rather, any white hair that
appears is grown from the follicle. Since hair takes
several weeks to grow, it is quite safe to conclude
that the story remains an old wives’ tale – nothing
more and nothing less!
傳
說法國皇后瑪麗•安托瓦內特被送上斷頭台的前
一晚，頭髮一夜變白。從此以後，瑪麗的名字就被用來描
述頭髮在短時間內變白的現象。儘管一般相信頭髮花白
與壓力有關，許多所謂“證據”其實是經過渲染的傳聞。
不過壓力是否真能讓頭髮變白？瑪麗•安托瓦內特的頭髮
突然變色又是否反映一定的事實？
髮絲變白是由許多因素引起，其中最重要的，可能是
髮絲細胞產生的過氧化氫過量積累，阻礙黑色素的形成。
黑色素決定髪色、膚色和眼虹膜的顏色。在正常情況下，
類似過氧化氫酶的酶會將過氧化氫轉變成氧氣和水。但
隨著年齡的增長，這種酶漸漸減少，令到過氧化氫積聚，
黑色素減少
[1]
。這種酶也會增加酪氨酸酶的形成，酪氨酸
酶能讓毛囊產生黑色素。由於頭髮的顏色是來自黑色素，
黑色素細胞的數量減少或黑色素不能正常輸送，頭髮就
會變白。那麼壓力又是如何影響這種種因素呢？
從小鼠和人類頭皮實驗得出了有趣的結果。科學家首
先研究損傷和紫外線輻射等壓力，究竟對小鼠的黑色素
幹細胞有什麽影響。他們發現，位於傷口附近的毛囊裏的
黑色素幹細胞，會移動到皮膚產生黑色素細胞。結果是傷
口附近有些毛囊裏沒有黑色素幹細胞，從這些毛囊裏長
出來的是白毛。小鼠被紫外線照射後，黑色素幹細胞卻沒
有完全移走，新長出來的毛仍然帶有色素。在人類頭皮實
驗，研究人員將樣本的黑色素細胞移除，以了解毛囊的黑
色素幹細胞是否會遷移，結果發現相似的現象
[2]
。
值得留意的是，即使實驗顯示生理上的壓力與頭髮變
白有關連，證據依然薄弱。首先，損傷之外，還有很多因
素促使黑色素幹細胞由毛囊移動到受傷的皮膚。其次，我
們通常所理解的壓力是精神上的壓力，而不是實驗裏的
生理壓力。所以至今，沒有任何具體證據證明精神壓力可
以令頭髮變白。
瑪麗•安托瓦內特的頭髮是否真的如傳聞所說的一夜
變白？這問題不難回答。頭髮可以看作是死組織，沒有能
力變白。白髮是由毛囊裏直接長出來的。毛髮生長需要數
周，所以結論很簡單，一夜變白髮是不可能的！
References
[1] Trüeb, R. M. (2009). Oxidative Stress in Ageing of Hair.
International Journal of Trichology doi: 10.4103/0974-7753.51923
[2] Chou, W. C., Takeo, M., Rabbani, P., Hu, H., Lee, W., Chung, Y. R.,
Ito, M. (2013). Direct migration of follicular melanocyte stem cells
to the epidermis after wounding or UVB irradiation is dependent
on Mc1r signaling. Nature Medicine doi: doi:10.1038/nm.3194
