A new paper on senescent cells and aging brings together the current knowledge on the subject and presents it in a handy review format.
What are senescent cells?
As you age, increasing numbers of your cells enter into a state known as senescence. Senescent cells do not divide or support the tissues of which they are part; instead, they emit a range of potentially harmful chemical signals that encourage nearby healthy cells to enter the same senescent state. Their presence causes many problems: they reduce tissue repair, increase chronic inflammation, and can even eventually raise the risk of cancer and other age-related diseases.
Senescent cells normally destroy themselves via a programmed process called apoptosis, and they are also removed by the immune system; however, the immune system weakens with age, and increasing numbers of senescent cells escape this process and begin to accumulate in all the tissues of the body.
By the time people reach old age, significant numbers of these senescent cells have built up, causing chronic inflammation and damage to surrounding cells and tissue. These senescent cells are a key process in the progression of aging [1, 2].
Senescent cells only make up a small number of total cells in the body, but they secrete proinflammatory cytokines, chemokines, and extracellular matrix proteases, which, together, form the senescence-associated secretory phenotype, or SASP. The SASP is thought to significantly contribute to aging  and cancer ; thus, targeting senescent cells and removing them has been suggested as a potential solution to this problem.
The researchers of this new paper have collated the information from existing publications relating to senescent cells in the context of aging and present it here in the form of a review . There is a vast array of data concerning senescent cells, and the increased interest in them has led to a wealth of scientific publications on the topic in recent years. This review does a great job at marshaling the current knowledge on senescent cells all in one place and is well worth reading.
Senescent cells in tissues and organs are considered to be pivotal to not only the aging process but also the onset of chronic disease. Accumulating evidence from animal experiments indicates that the magnitude of senescence can vary within and between aged tissue samples from the same animal. However, whether this variation in senescence translates across to human tissue samples is unknown. To address this fundamental question, we have conducted a systematic review and meta-analysis of all available literature investigating the magnitude of senescence and its association with chronological age in human tissue samples. While senescence is higher in aged tissue samples, the magnitude of senescence varies considerably depending upon tissue type, tissue section, and marker used to detect senescence. These findings echo animal experiments demonstrating that senescence levels may vary between organs within the same animal.
The contribution of senescent cells to aging and age-related diseases is, at this stage, fairly well known; the big question now is whether or not therapies that target and remove them will help to mitigate age-related diseases in people as they have in animal studies.
One thing is clear, there is an urgent need for more accurate ways to detect the presence of senescent cells in various tissues but also for therapies that are effective at targeting and removing only those harmful cells at a safe but significant level.
 López-Otín, C., Blasco, M. A., Partridge, L., Serrano, M., & Kroemer, G. (2013). The hallmarks of aging. Cell, 153(6), 1194-1217.
 van Deursen, J. M. (2014). The role of senescent cells in ageing. Nature, 509(7501), 439-446.
 Freund, A., Orjalo, A. V., Desprez, P. Y., & Campisi, J. (2010). Inflammatory networks during cellular senescence: causes and consequences. Trends in molecular medicine, 16(5), 238-246.
 Coppé, J. P., Desprez, P. Y., Krtolica, A., & Campisi, J. (2010). The senescence-associated secretory phenotype: the dark side of tumor suppression. Annual review of pathology, 5, 99.
 Waaijer, M., Tuttle, C., Slee-Valentijn, M., Stijnen, T., Westendorp, R., & Maier, A. (2018). CELLULAR SENESCENCE AND CHRONOLOGICAL AGE IN VARIOUS HUMAN TISSUES: A SYSTEMATIC REVIEW AND META-ANALYSIS. Innovation in Aging, 2(Suppl 1), 94.