Cellular senescence is a physiological mechanism whereby a proliferating cell undergoes a stable cell cycle arrest upon damage or stress and elicits a secretory phenotype. This highly dynamic and regulated cellular state plays beneficial roles in physiology, such as during embryonic development and wound healing, but it can also result in antagonistic effects in age-related pathologies, degenerative disorders, ageing and cancer. In an effort to better identify this complex state, and given that a universal marker has yet to be identified, a general set of hallmarks describing senescence has been established. However, as the senescent programme becomes more defined, further complexities, including phenotype heterogeneity, have emerged. This significantly complicates the recognition and evaluation of cellular senescence, especially within complex tissues and living organisms. To address these challenges, substantial efforts are currently being made towards the discovery of novel and more specific biomarkers, optimized combinatorial strategies and the development of emerging detection techniques. Here, we compile such advances and present a multifactorial guide to identify and assess cellular senescence in cell cultures, tissues and living organisms. The reliable assessment and identification of senescence is not only crucial for better understanding its underlying biology, but also imperative for the development of diagnostic and therapeutic strategies aimed at targeting senescence in the clinic.

Organismal ageing is a complex process driving progressive impairment of functionality and regenerative potential of tissues. Cellular senescence is a state of stable cell cycle arrest occurring in response to damage and stress and is considered a hallmark of ageing. Senescent cells accumulate in multiple organs during ageing, contribute to tissue dysfunction and give rise to pathological manifestations. Senescence is therefore a defining feature of a variety of human age‐related disorders, including cancer, and targeted elimination of these cells has recently emerged as a promising therapeutic approach to ameliorate tissue damage and promote repair and regeneration. In addition, in vivo identification of senescent cells has significant potential for early diagnosis of multiple pathologies. Here, we review existing senolytics, small molecules and drug delivery tools used in preclinical therapeutic strategies involving cellular senescence, as well as probes to trace senescent cells. We also review the clinical research landscape in senescence and discuss how identifying and targeting cellular senescence might positively affect pathological and ageing processes.

Data availability The RNA sequencing datasets (GSE117930) and the single cell RNA sequencing datasets (GSE131508) are deposited in the Gene Expression Omnibus (GEO, NCBI) repository. The proteomic datasets are deposited in PRoteomics IDEntifications (PRIDE) repository (PXD010597). Author Contribution L.O. designed and performed most of the experiments, analysed and interpreted the data and contributed to the manuscript preparation. E.N. assisted with data collection, performed all the 3D-scaffold co-culture experiments, the in vivo Wisp1 experiments, the scRNA sequencing, interpreted and analysed the data and contributed to the manuscript preparation. I.K. performed the qPCR analysis, some of the tissue IF staining and analysed the data. A.M. and J.H.L. performed some of the tissue IF staining, all the lung organoid experiments, interpreted and analysed the data. V.B. performed some of the tissue IF staining. P.C. and S. H. performed bioinformatics analysis. I.H., J.K. and A.O. performed the proteomic and analysed the data. E.G.G. helped with the collection of Ly6G + cells for proteomics. G.M. performed the 3D-scaffold co-culture to analyse CD104 + cells. A.W. and L.C. performed the electron microscopy experiments. E.H. and V.S. provided human samples. L.O., E.N., I.K., V.B. and J.H.L., critically reviewed the manuscript. J.H.L., supervised the lung organoid experiments. I.M. designed and supervised the study, interpreted the data and wrote the manuscript.