干细胞是具有自我更新和多向分化潜能的原始细胞群，是形成各种组织器官的起源细胞。当机体成熟细胞因衰老或受伤死亡，干细胞随时产生他们的替代品来维持各种细胞更新和组织修复。可惜的是，随着年龄的增长，人体的干细胞族群增值和分化的能力会严重虚弱。从这个意义上讲，              。
填入横线最恰当的一句话是（    ）。

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