Human HSPC dynamics in vivo. Schematic representation of the role of primitive and committed HSPC subpopulations during physiological aging and after transplantation. HSC pool increases in size during childhood, reaching the final adult reservoir that allows maintenance of hematopoietic production during life. As results of continuous challenging from external stimuli (such as infections, environmental pollution, and radiations), the HSC pool progressively loses its clonal complexity. Although previous works described increased frequency of HSC pool in the elderly,^58,59  a comprehensive assessment of the maintenance, increase, or decrease of HSC number during aging remains to be elucidated. After transplantation, hematopoietic output is sustained by different HSPC subsets over time.⁴⁹  In the initial engraftment, myeloid production is sustained by short-living myeloid progenitors (CMP/GMP). Short-term HSC/MPPs activated in the early phases provided the first hematopoietic reconstitution. Around 1 to 2 years after transplant, the HSPC compartment stabilized and LT HSCs maintained steady-state hematopoiesis.