Inside your body are cells that have stopped functioning normally. They no longer divide. They no longer fulfil their tissue role. But they don't die either. They persist indefinitely, secreting a cocktail of inflammatory molecules that disrupt the healthy cells around them.
Scientists call them senescent cells. In popular science writing, they've been nicknamed "zombie cells". And although the term is informal, it captures something essential: these cells are neither truly alive in a functional sense, nor eliminated. They take up space, consume resources, and progressively poison their environment.
Cellular senescence and the low-grade chronic inflammation it generates — inflammaging — are among the most actively studied aging mechanisms in geroscience today.
What is cellular senescence?
DNA damage is the most classic trigger. When a cell accumulates irreparable genomic lesions — double-strand breaks, critically short telomeres, chromosomal instability — specific signalling pathways (p53/p21, p16/Rb) activate a permanent cell-cycle arrest. It's a protective response: rather than dividing with a damaged genome, the cell "chooses" to stop.
Chronic oxidative stress, notably that generated by dysfunctional mitochondria, can also induce senescence through the accumulation of oxidative damage to DNA and membranes. Oncogenic activation likewise triggers senescence as a tumour-suppression mechanism: a cell whose oncogene has been activated enters senescence to avoid malignant transformation. Finally, the SASP of neighbouring cells can spread senescence in a paracrine manner — partly explaining the contagious nature of tissue aging.
The SASP: when senescent cells turn toxic
The most important feature of senescent cells isn't their proliferative arrest — it's their active secretion. Senescent cells develop a secretory phenotype formalised as the SASP (Senescence-Associated Secretory Phenotype), described by Judith Campisi starting in the 2000s. The SASP is a complex cocktail of pro-inflammatory molecules: interleukins (IL-6, IL-8, IL-1β), growth factors (HGF, EGF), matrix proteases (MMP-3, MMP-9) and chemokines.
In small, transient amounts, the SASP serves useful functions: it recruits immune cells through a process called immune surveillance, and contributes to tissue healing. The problem arises with age. As senescent cells accumulate and immune surveillance loses efficiency, the SASP becomes chronic and systemic. It degrades the extracellular matrix, disrupts stem-cell function, induces senescence in neighbouring cells, and fuels inflammaging at a systemic scale.
Inflammaging: the chronic inflammation that ages the organism
The term inflammaging was introduced by Claudio Franceschi in 2000. It describes the state of low-grade, sterile, chronic inflammation that progressively sets in with age — chronic, low-grade, sterile and systemic. Its sources are multiple: dysfunctional mitochondria release mitochondrial DNA fragments into the cytoplasm, activating innate-immunity receptors (cGAS-STING) and triggering an inflammatory response; gut dysbiosis increases intestinal barrier permeability, allowing the systemic passage of bacterial fragments (LPS) that chronically activate the innate immune system.
CD38 and declining NAD+ form a well-documented inflammatory loop: pro-inflammatory cytokines activate CD38, which massively degrades NAD+. The drop in NAD+ reduces the activity of anti-inflammatory sirtuins — which worsens inflammation, further activates CD38, and reduces NAD+ still more. A biochemical vicious cycle with systemic consequences.
Inflammation isn't the consequence of chronic age-related diseases. It's the ground on which they grow.
The biomarkers of inflammaging
Interleukin-6 (IL-6) is the most widely used marker. Its levels rise exponentially after age 60. High levels are associated with increased risk of cardiovascular disease, cognitive decline, sarcopenia and all-cause mortality. C-reactive protein (CRP), produced by the liver in response to interleukins, is used in its ultra-sensitive form (hsCRP) in longevity studies as an indicator of inflammaging level. TNF-α, for its part, contributes to insulin resistance, sarcopenia and immunosenescence.
Furman et al. (Nature Medicine, 2019) showed that these markers allow the identification of two subgroups among older individuals: those whose inflammation stays low despite chronological age — associated with better functional longevity — and those with high inflammation — associated with accelerated decline.
Senolytics and senomorphics: ongoing clinical research
Senolytics are molecules capable of selectively eliminating senescent cells. First-generation compounds (dasatinib, quercetin, navitoclax) have shown remarkable effects in mouse models: extended lifespan, improved physical and cognitive function. Human clinical trials are underway. Senomorphics, or senostatics, don't seek to eliminate senescent cells but to suppress their SASP — certain natural actives including quercetin, resveratrol and NF-κB inhibitors are being studied in this context.
Why this mechanism is central to cellular longevity
Cellular senescence is a paradigmatic example of an antagonistic mechanism under the Hallmarks of Aging framework: initially protective, it becomes harmful once it accumulates chronically with age. Virtually every chronic age-related disease — cardiovascular disease, type 2 diabetes, neurodegenerative disease, cancer — emerges in a context of chronic inflammaging.
In conclusion
Zombie cells aren't a metaphor. They are a measurable biological reality, whose gradual accumulation constitutes one of the most important mechanisms of systemic aging.
Understanding cellular senescence and inflammaging means understanding that aging isn't simply the mechanical wear of an organism — it's an active biological dynamic, in which certain cells progressively disrupt the functioning of the whole.
This article is published for informational and educational purposes. It does not constitute medical advice and does not replace consultation with a healthcare professional.
