Practice question

Stem cells are defined functionally by sustained ability to self-renew over extended time, frequently spanning whole organismal lifespan, contrasting sharply with transient amplifying progenitors that perform limited rounds of rapid division before terminal differentiation and exhaustion. Unlimited self-renewal is supported by high telomerase activity, avoidance of cellular senescence via Bmi1 polycomb repression of Ink4a locus, and asymmetric segregation of damaged proteins and old mitochondria. Multipotency alone does not guarantee stemness, as multipotent progenitors can exhaust quickly. Hence long-term repopulating capacity and serial transplantation remain functional gold standards distinguishing genuine stem cells from short-lived progenitors in experimental biology.