The hallmarks of cellular ageing, in plain English

If you read enough cellular biology, you start to notice something strange: ageing keeps showing up in completely unrelated places. A geneticist looking at chromosome ends, a biochemist watching proteins misfold, a microbiologist studying gut bacteria, an immunologist tracking low-grade inflammation — all of them, eventually, are talking about the same problem from different windows. The question is whether they are looking at one process or twelve.

In 2013, a group of European biologists led by Carlos López-Otín tried to answer that question with a single paper. They proposed that ageing in mammals could be organised around nine “hallmarks” — biological signatures that consistently appear in old cells, plausibly drive the dysfunction we see at the tissue and organ level, and can, in principle, be measured and modulated. Ten years later, in 2023, the same authors expanded the list to twelve. That update has become the most-cited paper in modern longevity biology, and the framework it describes is now the field’s de facto map.

This article is a plain-English walk through that map. It is long because the map is detailed, and because the alternative — a glossary of disconnected jargon — is what most readers encounter when they first try to understand cellular ageing. The goal here is to leave you with a working mental model: what each hallmark is, why it matters, and how the twelve fit together.

Where this framework came from

The original paper, López-Otín et al., 2013, was published in Cell under the title “The Hallmarks of Aging.” Its authors — López-Otín, Maria Blasco, Linda Partridge, Manuel Serrano, and Guido Kroemer — were borrowing a rhetorical move from cancer biology, where Douglas Hanahan and Robert Weinberg’s “Hallmarks of Cancer” papers had organised an unruly field around a shared vocabulary. The 2013 ageing paper proposed three criteria for any hallmark: it should appear during normal ageing, experimentally aggravating it should accelerate ageing, and experimentally ameliorating it should slow ageing or extend healthy lifespan.

The 2023 update, López-Otín et al., 2023 — “Hallmarks of aging: An expanding universe,” also in Cell — kept that scaffolding but added three new entries (disabled macroautophagy, chronic inflammation, and dysbiosis) and reorganised some of the original nine. The authors also retained their original three-tier grouping, which is the most useful structure for a non-specialist:

• Primary hallmarks are the upstream sources of damage. They are unambiguously negative: more of them is always worse.
• Antagonistic hallmarks are the cell’s responses to that damage. At low intensity they protect; at chronic intensity they become part of the problem.
• Integrative hallmarks emerge when the first two tiers can no longer be compensated for. They are what we eventually see and feel as ageing.

The framework has limitations, and we will come to those at the end. But it is the closest thing the field has to a shared periodic table, and a recent review by the original authors — Tartiere, Freije & López-Otín, 2024 — argues it is most useful when treated as a scaffold rather than a final answer.

Primary hallmarks: the damage

1. Genomic instability

Every day, the DNA in each of your cells is hit by tens of thousands of damaging events — ultraviolet light, oxygen radicals from normal metabolism, errors during replication, environmental chemicals. Cells have an elaborate repair toolkit for this. But the toolkit is not perfect, and across decades the unrepaired or mis-repaired damage accumulates: point mutations, broken strands, chromosomal rearrangements, loss of nuclear architecture.

The consequence is not abstract. Some of the clearest evidence comes from human progeroid syndromes — rare genetic conditions in which a single broken DNA-repair gene produces a phenotype that resembles accelerated ageing. The authors of the 2023 hallmarks update treat this as one of the strongest causal arguments in the entire field: if you cripple the genome-maintenance machinery, you get something that looks like premature ageing. Genomic instability therefore sits at the root of the hallmarks tree.

2. Telomere attrition

The ends of your chromosomes are capped by repetitive DNA sequences called telomeres. Each time a cell divides, those caps get slightly shorter, because the DNA-copying machinery cannot fully replicate the very end of a linear molecule. When telomeres become critically short, the chromosome ends look, to the cell, like broken DNA — and the cell responds either by permanently arresting (senescence) or by dying.

This problem and its solution were worked out in the 1980s. Greider & Blackburn, 1985 identified telomerase, an enzyme that rebuilds telomeres, in the pond-dwelling ciliate Tetrahymena. Most adult human cells switch telomerase off, which is part of why our cells are not immortal. Telomere length, on average, declines with age, and short telomeres are associated with several age-related diseases — though, importantly, the relationship is statistical and noisy at the level of individual people.

3. Epigenetic alterations

Your cells all carry the same genome but switch different parts of it on or off depending on tissue and context. That switching is run by the epigenome: chemical marks on DNA (mainly methyl groups), modifications on the histone proteins DNA wraps around, and the higher-order packaging that decides which regions of the genome are accessible.

With age, that pattern drifts. Some regions lose methylation, others gain it; histone marks shift; previously silenced regions become noisy. The most striking demonstration of this is the “epigenetic clock” described by Horvath, 2013, which uses methylation levels at 353 specific sites to estimate biological age with surprising accuracy across nearly every human tissue. Whether epigenetic drift causes ageing or merely tracks it is still being argued, but the correlations are strong enough that epigenetic clocks are now standard tools in ageing research.

4. Loss of proteostasis

“Proteostasis” is shorthand for protein homeostasis: the cell’s ability to make proteins, fold them correctly, keep them folded, and dispose of them when they break. This requires a small army of molecular chaperones and degradation systems, surveyed comprehensively by Hartl, Bracher & Hayer-Hartl, 2011.

The system declines with age. Chaperone levels fall, degradation slows, and misfolded proteins begin to clump together. The most visible consequence is the aggregate-driven neurodegenerative diseases — Alzheimer’s, Parkinson’s, Huntington’s — but loss of proteostasis is not just a brain problem. It is one of the reasons old cells, broadly, work less well than young ones.

5. Disabled macroautophagy

This is the first of the three hallmarks added in 2023. Autophagy — literally “self-eating” — is the cell’s recycling system. The most studied form, macroautophagy, wraps damaged organelles or protein aggregates in a double membrane and ships them to the lysosome for breakdown into reusable parts. The full molecular machinery, including the ATG genes that won Yoshinori Ohsumi the 2016 Nobel Prize, is described in detail by Mizushima, 2018 and later reviews.

In ageing cells, autophagy slows down. Damaged mitochondria stop being cleared; aggregated proteins linger; the cell carries more and more rubbish. The 2023 hallmarks paper promoted macroautophagy to its own category because the experimental evidence had become hard to ignore: in multiple model organisms, boosting autophagy lengthens healthy lifespan, and disabling it shortens it.

Antagonistic hallmarks: the responses that turn against us

6. Deregulated nutrient sensing

Cells decide whether to grow, divide, store energy, or recycle based on chemical signals about food. Four pathways do most of this work: insulin/IGF-1, mTOR, AMPK, and the sirtuins. The first two say “we have plenty, build things”; the second two say “we are running low, conserve and recycle.”

One of the most robust findings in ageing biology is that turning down the “build things” signals — through caloric restriction, intermittent fasting, or pharmacological inhibition — extends lifespan across yeast, worms, flies, and rodents. The mTOR pathway is the best-studied case; Johnson, Rabinovitch & Kaeberlein, 2013 reviewed mTOR as a central modulator of ageing and age-related disease. With age, these sensors drift toward the constitutively “fed” position even when they shouldn’t, which the hallmarks framework treats as a maladaptive cellular response that compounds over decades.

7. Mitochondrial dysfunction

Mitochondria are the cellular power plants, and they have their own small genome — about 16,500 base pairs of mtDNA, descended from a bacterium that became a permanent guest in our ancestors’ cells. mtDNA sits next to the chemistry that produces oxygen radicals, lacks the elaborate repair machinery of nuclear DNA, and accumulates mutations across a lifetime.

The classical “mitochondrial free-radical theory of ageing” said this damage drove ageing directly. The picture is now more complicated: as Pinto & Moraes, 2024 review, mtDNA mutations matter, but the dominant story is broader — old mitochondria are smaller, leakier, worse at producing ATP, and worse at signalling to the rest of the cell. They also produce abnormal molecules that act as inflammatory signals, linking this hallmark to inflammation downstream.

8. Cellular senescence

In 1961, Leonard Hayflick showed that human fibroblasts in a dish divide a limited number of times — roughly forty to sixty — before stopping permanently while remaining metabolically active. That paper, Hayflick & Moorhead, 1961, established cellular senescence as a real biological state, distinct from death and from quiescence.

For decades, senescence was treated as a tumour-suppressor mechanism: damaged cells stopped dividing rather than risk becoming cancerous. It still is. But senescent cells do not just sit quietly. They secrete a complex cocktail of cytokines, chemokines, growth factors, and proteases known as the senescence-associated secretory phenotype, or SASP — the phenomenon characterised in detail by Judith Campisi and colleagues and reviewed by Gorgoulis et al., 2024. The SASP recruits the immune system to clear damaged cells, which is useful when the load is small. When senescent cells accumulate with age and are not cleared, the same SASP becomes a chronic inflammatory drip that damages nearby tissue. Senescence is the textbook antagonistic hallmark: protective in the short term, corrosive in the long term.

Integrative hallmarks: the system-level failure

9. Stem cell exhaustion

Most adult tissues are maintained by small populations of stem cells that replace cells lost to damage or normal turnover. With age, those reservoirs shrink and the cells that remain become worse at their job — slower to activate, more prone to differentiation errors, more likely to enter senescence. The result is a steady decline in regenerative capacity: skin heals more slowly, blood production becomes less balanced, muscle rebuilds less efficiently after injury.

Stem cell exhaustion is the integrative consequence of the earlier hallmarks acting on a specific cell population. As reviewed by Goodell and colleagues, 2025, the same forces — DNA damage, epigenetic drift, proteostasis collapse, mitochondrial decline — degrade stem cells, but the downstream consequences are amplified because each failed stem cell means many missing daughter cells over a lifetime.

10. Altered intercellular communication

Cells talk to each other constantly, through hormones, cytokines, neurotransmitters, extracellular vesicles, and direct physical contacts. With age, the chemistry of those conversations shifts. Hormonal signalling flattens. Pro-inflammatory signals rise. The composition of circulating molecules changes enough that, in parabiosis experiments where an old and a young animal share a blood supply, the young animal ages faster and the old animal regenerates better — implicating circulating factors as carriers of “age.”

This hallmark is, in a sense, the glue between the others. Senescent cells alter the local signalling environment via the SASP. Mitochondrial dysfunction releases molecules that look to the immune system like bacterial debris. Stem cell niches lose their instructive cues. The hallmarks framework treats altered intercellular communication as the level at which cellular ageing becomes tissue-level ageing.

11. Chronic inflammation

The second of the three additions in 2023. The concept, “inflammageing,” was named by Franceschi et al., 2000 in a paper that proposed chronic low-grade inflammation as a near-universal feature of ageing biology. Two decades of follow-up have hardened the case: in older humans, basal levels of inflammatory cytokines like IL-6 are elevated; the immune system shifts toward a low-grade activated state even when no obvious infection is present; and these markers predict mortality and the onset of multiple age-related diseases.

Inflammageing is not caused by one thing. It is the integrated downstream consequence of senescent cell accumulation, mitochondrial debris leaking inflammatory signals, gut barrier deterioration, and an immune system that itself ages. The 2023 update separated it from “altered intercellular communication” because the evidence had grown to the point where it deserved its own line in the framework rather than being treated as a sub-category.

12. Dysbiosis

The newest hallmark, and the one that would have looked strange on the 2013 list. The microbial community in the human gut — roughly as many cells as our own body contains — shifts with age toward lower diversity, lower abundance of beneficial taxa, and a thinner intestinal mucus barrier. The result is more bacterial product leaking into circulation, where it interacts with the immune system and feeds inflammation. As reviewed by Ratto et al., 2025, faecal microbiome transplantation from young to old animals can partially reverse some age-related phenotypes, which is part of why the hallmarks authors finally included it.

Dysbiosis sits at the boundary between the cellular and the ecological. It is the clearest sign that “cellular ageing” cannot be understood entirely from inside a cell.

What is still debated

The hallmarks framework is useful precisely because it is a framework — not a theory of ageing. It says what changes; it does not, on its own, say why or in what order.

The most serious critique is that the twelve hallmarks are not independent. Mitochondrial dysfunction feeds inflammation. Senescent cells damage stem cell niches. Epigenetic drift influences proteostasis. The list is a useful organising scheme, but it is not a clean causal diagram, and an intervention that targets one hallmark almost always affects several. Tartiere, Freije and López-Otín themselves acknowledge in their 2024 conceptual review that the hallmarks have “relatively limited explanatory value regarding the general aetiologies of ageing” and do not cleanly distinguish primary from secondary causes.

A second debate is about completeness. Candidate hallmarks under active discussion include altered RNA processing, changes in extracellular matrix dynamics, and shifts in lipid metabolism. Each has support; none has yet cleared the original three-part bar of being present in ageing, accelerating ageing when worsened, and slowing ageing when ameliorated. A more pointed critique by Gems & de Magalhães, 2022 argues that the framework’s apparent neatness can obscure the fact that for many of the twelve, the evidence that intervening on them in mammals actually slows ageing is still preliminary.

A third issue is taxonomic. Most of the underlying experimental work has been done in yeast, worms, flies, and rodents. The translation to humans is plausible but not yet rigorous for every hallmark, and the variation between individuals is enormous. Two seventy-year-olds with the same chronological age can have radically different hallmark profiles.

None of these critiques have unseated the framework. They have refined it. The hallmarks remain the field’s working language because no proposed alternative has done a better job of organising the available evidence.

Where the field is going

The trajectory of the next decade is fairly clear, even if the specifics are not. Three trends are worth noting.

First, the hallmarks are increasingly being treated as measurable rather than just descriptive. Epigenetic clocks, senescence panels, inflammatory marker batteries, and microbiome diversity scores are beginning to be combined into composite biological-age estimates. These are still early, often inconsistent across cohorts, and not yet validated as endpoints for interventional studies — but the direction is unmistakable.

Second, the field is moving from single-hallmark to multi-hallmark thinking. Because the twelve are interconnected, the most interesting recent work asks how interventions cascade across the network rather than which single mechanism they target.

Third, and most importantly, the framework is being stress-tested in humans. The translation gap from model organisms to people is the central unsolved problem in geroscience, and the hallmarks are how that translation is being structured. Whether the eventual answer is twelve hallmarks, fifteen, or eight, the López-Otín framework will have done its job: it gave a sprawling field a shared map, and made it possible to argue productively about what the map is missing.

For a reader new to cellular longevity, that is the right takeaway. The hallmarks are not a list of “things that go wrong with age” — they are a hypothesis about how the wrongness is organised. Holding that hypothesis lightly, and using it as a way to read everything else, is the most useful thing a non-specialist can do with it.

Sources

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