Replicative Senescence in vivo

The early observation that young cultured fibroblasts have a higher growth potential than those derived from adults led to the proposal that senescent cells may play a causal role in organismal ageing (Hayflick, 1961). For this to be the case, senescent cells need not only to be shown to be present in living tissue but also to persist for long periods. However, cellular senescence at the time of Hayflick’s proposal was thought by many to be a tissue culture artefact, with no relevance to normal human ageing. Since the growth conditions of cultured cells are dissimilar to that found in vivo, it was thought that these differences resulted in the formation of senescent cells in culture but not in vivo. It has also been argued that if senescent cells are present within tissues, the fraction would be so small that they are unlikely to have any impact on the surrounding tissue and ageing in general.

At the time of Hayflicks proposal there was no way of detecting senescent cells in vivo. It wasn’t until over 30 years later that a marker was used to demonstrate the presence of senescent cells in human dermis in vivo (Dimri et al, 1995). This detection system is based on a modified beta-galactosidase assay and is termed senescence-associated beta-galactosidase (SA-β-Gal). Skin samples were taken from 20 human donors aged 20-90 years, sectioned and stained for SA-β-Gal. Results showed an age-dependent increase in SA-β-Gal staining in dermal fibroblasts and epidermal keratinocytes. None of the young donors (<40yr)>69 yr) donor did display positive staining. About half of the young donors showed some epidermal staining whereas positive staining was always observed in the epidermis of old donors. Using the same technique to identify senescence cells, another group looked at senescence of the retinal pigment epithelium (RPE) of Rhesus monkeys (Hjelmeland et al, 1999). Results also showed an accumulation of SA-β-Gal positive cells in the eyes of older monkeys. Another study also found little or no SA-β-Gal staining in HCECs of corneas from young donors but was easily detected in corneas from older donors (Mimura and Joyce, 2006).
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More recent studies have used other markers to detect cellular senescence in mitotic tissues (Herbig et al, 2006, Jeyapalan et al, 2007). These studies investigated cellular senescence in the tissues of ageing primates. They used markers of senescence such as telomere damage, active checkpoint kinase ATM, high levels of heterochromatin proteins and elevated levels of p16 in skin biopsies from baboons with advancing age. The number of dermal fibroblasts containing damaged telomeres reached a value of over 15% of total fibroblasts in very old animals (26-30 years) compared to young (5-6 years) where DNA damage were rarely detected. However, in skeletal muscle, a postmitotic tissue, only a small percentage of damage to telomeres was detected regardless of age.