Although ma ny different kinds of multipotent stem cells have been identified, adult stem ce lls for all cell and tissue types have not yet been found in the adult human.

• 例如，我们 还没有在人体中定位成年心脏干细胞或成年胰岛干细胞。其次，成年干细胞通常仅以微小的 数量出现，很难分离和纯化它们，它们的数量也会随年龄增长而降低。