INTRODUCTION
Since the discovery of multipotent stem cells by Till and McCulloch in 1961,1 further elucidation of stem cells’ functions have been identified as both facilitating development of new cells and maintaining homeostasis of current normal cells. The activity of stem cells is stimulated by the start of tissue dysfunction. Several applications using these functions have been implemented in medicine already: reestablishing the hematopoietic lineage via bone marrow transplantation,2 development of stem-cell based therapy for type 1 diabetes3,4 and retinitis pigmentosa,5 and using stem cells to advance the cure for spinal cord injury.6 One important application of stem cell biology is how these cells can be used in the context of aging and age-related dysfunctions. During aging, DNA accumulates damage, impairing protein homeostasis, cell function and communication, as well as normal organ physiology.7 Another key hallmark to aging is the exhaustion or dysregulation of the endogenous stem cell population, which aids in maintaining tissue homeostasis and repair of injured tissues. Because aging is so intimately tied to stem cell integrity, one of the major goals of stem cell biology and regenerative medicine is how one can use these cells to reverse aging and the associated dysfunctions that come with it. Stem cells are undifferentiated or partially differentiated cells that are capable of dividing and generating differentiated and proliferative cells (Fig. 1). Stem cells range from pluripotent cells that are found in the inner cell mass of preimplantation blastocysts or isolated from other sources to unipotent progenitors such as fetal tissues, birth-associated tissues, or adult tissues. Several advances have been made to apply the unique traits of this variety of stem cell types.
