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Stem cells are unspecialized cells that can differentiate to generate more specialized cell types responsible for tissue-specific function. During development, the differentiation of pluripotent embryonic stem cells leads to the production of specialized somatic cells that are ultimately responsible for the structure and function of all adult tissues and organs. “Naturally” pluripotent cells exist only at the earliest stages of embryonic development. However, less differentiated stem and progenitor cells are also present in adult tissues. In contrast to pluripotent embryonic stem cells, adult stem cells generally only form a limited number of cell types corresponding to their tissues of origin. These multipotent or unipotent adult stem cells exist in many, though probably not all, adult tissues including bone marrow (BM), brain, spinal cord, dental pulp, blood vessels, skeletal muscle, epithelia of the skin and digestive system, cornea, and retina. “Multipotent” stem cells can generate several types of cells within a given tissue (e.g., hematopoietic/blood forming stem cells), whereas “unipotent” stem cells give rise to only one differentiated cell type (e.g., skeletal muscle stem cells). Though adult stem cells exist at a very low frequency in adult tissues, they serve a critical function in maintaining tissue homeostasis and generating replacement cells to repair tissues after injury. Here, we review the current literature on adult stem cells and specifically highlight the regulation of multipotent hematopoietic stems cells in contrast with that of unipotent skeletal muscle stem cells, as well as discuss their respective roles in tissue homeostasis and repair. |
