Whether you have a full head of curly blond hair or a nearly bald head with dark hair, the growth of your hair begins in the skin on your scalp. On the surface of your head, the outer layer of cells—called the epidermis—is between ten and twenty layers thick. Some of these epidermal cells actually become hair cells by a complex process.
不論你是長著滿頭金發，還是幾乎謝頂的黑發，頭發的生長始于頭皮上的皮膚。在頭的表面，有大約10至20層厚的外層細胞——即表皮。實際上其中一些表皮細胞通過復雜的過程最后變成了毛發細胞。
To visualize this process, think about taking some plastic kitchen wrap and covering a tub of cream cheese with it. If you poked your finger into the wrap, and down into the cream cheese without breaking the wrap, you would be simulating the beginning of hair development. In order for a strand of hair to grow, skin cells must dive down to just below the epidermis. This creates a tube-like structure, much like the one left over if you pulled your finger out of the cream cheese.
為了觀察這個過程，設想一下：拿些廚房保鮮膜，把它們蓋在一桶奶油奶酪上。如果你用手指戳保鮮膜，在不穿透保鮮膜的條件下碰到奶油奶酪，這就是在模擬頭發生長初期的情形。為了長出一縷頭發，細胞必須潛入表皮下面一點。這樣就形成了一種冠狀結構，這種結構與手指抽離奶油奶酪后留下的痕跡一樣。
The cells that line this tubular structure are collectively referred to as the hair follicle. Hair production is the result of the cells of the hair follicle depositing layer after layer of protein into this tubular space. These layers of protein grow out of the tubular space as a strand of hair.
那些組成這個管狀結構的細胞統稱為毛囊。頭發的形成就是毛囊細胞向管狀空間注入一層又一層的蛋白質的結果。當這些蛋白質層從管狀空間長出后就形成了一縷縷頭發。
As the hair strand develops, hundreds of pigment-producing cells called melanocytes inject it with pigment, giving a strand of hair its color. Melanocytes are only able to produce pigment for a genetically-determined number of years.
隨著發絲的生長，成百的黑色素細胞向發絲注入色素，為頭發上色。但黑色素細胞只能在遺傳決定的年限內生產色素。
Once the melanocytes lose their ability to produce pigment, hair follicles continue to produce hair, but instead of brown or red hair, they produce gray hair, which, technically speaking, is hair with no color at all.
一旦黑色素細胞失去生產色素的能力，雖然毛囊繼續生產頭發，但不是棕發或者紅發，而是白發。從學術含義上來講，就是沒有顏色的頭發。
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