23. Planaria and Hydra Show How Continual Renewal Can Support Negligible Senescence

Planaria and hydra reveal that certain organisms can support sustained function by renewal and robust pattern control (49, 50). Extensive turnover and regeneration, driven by abundant, continuously active stem-cell populations, can dilute, replace, or overwrite accumulated deleteriome components, producing negligible or weak senescence at the organism level. The lesson is not that they completely avoid producing damage, but that their system architectures, together with regeneration, can prevent its build-up and propagation into irreversible organismal failure. This is the strategy that mammals cannot implement due to their different biological organization (as they rely on fully differentiated irreplaceable cells), precluding organism-wide replacement of aged/dysfunctional components.