In many species a loss of proliferative capacity of somatic cells can be observed during cellular ageing. The shortening of telomeres is discussed as one responsible factor for the replicative senescence of somatic cells, and is proposed to be one of the reasons why animals age and show an age-related increase of mortality (Wright and Shay, 2005). The telomere structure, consisting of DNA repeat sequences (5′-TTAGGG-3′), is highly conserved among vertebrates (Meyne et al., 1989) as well as among invertebrates (Traut et al., 2007). Once telomere length reaches a certain threshold cells seem to approach senescence, the so-called Hayflick limit (Bolzán and Bianchi, 2006; Hayflick, 1965).

Activation of the enzyme telomerase (Aubert and Lansdorp, 2008) or other telomere lengthening pathways (Bryan et al., 1997) can maintain telomere lengths, leading to theoretically unlimited proliferative potential. Thus, telomerase is expressed primarily in germ cells (Zalenskaya and Zalensky, 2002), stem cells (Mason, 2003) and in actively proliferating transit cells (Buchkovich and Greider, 1996). In regular human tissue telomerase is activated in early embryogenesis and whilst the first trimester, after that it is repressed in adult tissues, supposedly resulting from the relationship of active telomerase linked to cancer (Forsyth et al., 2002; Shay and Wright, 2011). 

Progressive telomere shortening is very often linked to tissue and organismal ageing (Campisi, 1996; Proctor and Kirkwood, 2002) or to stressful environments (Metcalfe and Monaghan, 2003), furthermore telomere maintenance has been shown to play a key role in organismal longevity (Haussmann et al., 2005; Joeng et al., 2004). In contrast, an increase of telomere length with age has been observed in the extreme long-lived bird (Leach‘s storm petrel) which reaches a maximum life span of 36 years (Haussmann et al., 2003). Similar results with such a positive relationship of telomere length and or telomerase activity with age are observed in long-lived trees (Pinus longaeva, (Flanary and Kletetschka, 2005)), the water python (Liasis fuscus, (Ujvari and Madsen, 2009)) and the sand lizard,  (Lacerta agilis, (Olsson et al., 2010)). Even more fascinating examples are planarian flatworms or colonial ascidians, showing significantly different patterns in telomere maintenance and telomerase activities (Sköld et al., 2011; Tan et al., 2012).

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