时差N小时 头发的生长（在线收听）

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.

不论你是长着满头金发，还是几乎谢顶的黑发，头发的生长始于头皮上的皮肤。在头的表面，有大约10至20层厚的外层细胞——即表皮。实际上其中一些表皮细胞通过复杂的过程最后变成了毛发细胞。

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.

为了观察这个过程，设想一下：拿些厨房保鲜膜，把它们盖在一桶奶油奶酪上。如果你用手指戳保鲜膜，在不穿透保鲜膜的条件下碰到奶油奶酪，这就是在模拟头发生长初期的情形。为了长出一缕头发，细胞必须潜入表皮下面一点。这样就形成了一种冠状结构，这种结构与手指抽离奶油奶酪后留下的痕迹一样。

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.

那些组成这个管状结构的细胞统称为毛囊。头发的形成就是毛囊细胞向管状空间注入一层又一层的蛋白质的结果。当这些蛋白质层从管状空间长出后就形成了一缕缕头发。

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.

随着发丝的生长，成百的黑色素细胞向发丝注入色素，为头发上色。但黑色素细胞只能在遗传决定的年限内生产色素。

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.

一旦黑色素细胞失去生产色素的能力，虽然毛囊继续生产头发，但不是棕发或者红发，而是白发。从学术含义上来讲，就是没有颜色的头发。