File:Extrinsic versus intrinsic aging mechanisms in satellite cells and hematopoietic stem cells..jpg

English: Schematic depictions of satellite cell (SC) and hematopoietic stem cell (HSC) locations in the muscle fiber and bone marrow, respectively, and observed regenerative outcomes with respect to the age of the stem cell and its microenvironment or niche. A. Young or old SCs, when placed in contact with old systemic or local niches, both in vitro and in vivo, fail to generate productive tissue repair. Conversely, when SCs from young or old animals are immersed in a young microenvironment, both in vitro and in vivo, they regenerate muscle tissue appropriately. Thus the age of the extrinsic environment most likely determines the regenerative potential of SCs. The potential effects of the aged niche on SC responses are summarized in the red box. B. HSCs from young animals, when transplanted into either young or old hosts, produce successful regenerative responses while aged HSCs fail to regenerate properly regardless of the age of the host. Colored arrows indicate the procedure and proper outcomes of heterochronic HSC transplantations. Therefore it appears that, unlike SCs, HSC aging is probably regulated by intrinsic mechanisms that are unaffected by the age of the host environment. These prospective intrinsic aging mechanisms in HSCs are summarized in the red box.