Every cell division clips a little DNA off the chromosome ends (the end-replication problem); telomeres buffer this loss until they run out, tripping the Hayflick limit. Senescent cells don't die β they secrete an inflammatory cocktail (SASP) that stresses their neighbours, spreading dysfunction through the tissue.
L(n) = L0 β n Β· Ξtel β β« k_ox dt β SASP_bystander
Senescence triggers when L(n) β€ L_crit (the Hayflick limit)
SASP_bystander β (fraction of neighbours already senescent)
- Telomere attrition / division β DNA lost from chromosome ends each time a cell divides; higher values hit the Hayflick limit sooner.
- Oxidative stress β continuous ROS-driven damage independent of division count (mitochondrial dysfunction, genomic instability).
- Proliferation rate β how often cells attempt division; faster turnover burns through telomere reserve faster.
- Rapamycin/CR β mTOR inhibition / caloric restriction cuts both attrition and oxidative damage rates, extending replicative lifespan, as seen in real mouse trials.
- Senolytic clearance β selectively removes senescent cells (as dasatinib+quercetin or navitoclax do in trials), instantly clearing SASP and letting the tissue replace them with fresh cells.
Real-world relevance: this is the same logic behind first-in-human senolytic trials (idiopathic pulmonary fibrosis) and the TAME/rapamycin studies that treat ageing rate itself as a modifiable target.