毛球之上的细胞形成毛发，产生由分化的表皮细胞构成的同心环状区，形成内根鞘和外根鞘。
The hair is formed from cells just above the bulb, which also give rise to concentric zones of differentiated epithelial cells destined to form the inner and outer root sheaths.
毛干和内根鞘当到达体表时融合在一起：外根鞘仍然保持在原来的位置。
The hair shaft and inner root sheath move together as the hair grows toward the surface: the outer root sheath remains fixed in position.
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