The process of biological aging or the state of an organism being biologically old.


senescence /se·nes·cence/ (sĕ-nes´ens): the process of growing old, especially the condition resulting from the transitions and accumulations of the deleterious aging processes.

Dorland's Medical Dictionary for Health Consumers. © 2007 by Saunders, an imprint of Elsevier, Inc. All rights reserved.


Aging is an inevitable consequence of living. As organisms live they are constantly damaged, both at the cellular level and at the level of the whole organism. Maintenance is required to keep cells replicating, or to keep a heart functioning for example, but aging, or senescence, cannot be avoided as damage builds up and the organism passes its prime.

Cellular senescence can be caused by a range of sometimes inescapable damaging agents, including: ultra-violet radiation from exposure to sunlight, pathogens such as bacteria, chemicals such as alcohol or bisphenol A, or even by reactive-oxygen species - a by-product of respiration. Senescence is even proposed to have a causative role in promoting aging in a self-perpetuating manner [1].

Somatic cells of the body (i.e. not stem cells) also have a predefined replicative lifespan, or Hayflick limit - the maximum number of times a cell can divide. This is determined by telomeres - lengths of DNA attached to the ends of chromosomes to keep them intact during cell division.

Not all cells have a replicative lifespan - they can express the enzyme telomerase and replace their telomeres as they get run out. Embryonic stem cells have potentially unlimited lifespans, were it not for the inevitable build-up of damage that would occur through many rounds of cell division.

Bacteria also undergo aging, and can become uncultrable as the population enters a senescent state [2].


Questions can relate to any aspect of biological aging. Be it human aging and age-related diseases such as cardiovascular disease or dementia, or senescence at the cellular level, including the Hayflick limit, damaging agents that can trigger senescence, and the role of senescence in promoting aging.

Organismal and cellular aging are intrinsically linked, incredibly complex, and not yet fully understood. Theories around the evolution of cellular senescence are mixed, due to the simultaneous benefits and costs that come with a replicative lifespan [3].


  1. Nelson G., et al (2012) A senescent cell bystander effect: senescence-induced senescence. Aging Cell. 2012 Apr;11(2):345-9. doi: 10.1111/j.1474-9726.2012.00795.x
  2. Nyström T (2007) A Bacterial Kind of Aging. PLoS Genet. 3(12): e224. doi:10.1371/journal.pgen.0030224
  3. Rodier F, Campisi J (2011) Four faces of cellular senescence. J Cell Biol. 192(4): 547–556. doi:10.1083/jcb.201009094