Cellular Aging The Hayflick Limit

The name Leonard Hayflick immediately comes to mind when cellular aging is discussed. In the early 1960s, Hayflick was interested in the division potential of human diploid cells, and performed a number of key experiments at The Wistar Institute in Philadelphia. In two subsequent publications in 1961 and 1965 he described the isolation and growth capacities of primary cells originating from human tissue (Hayflick 1965, Hayflick and Moorhead 1961). He noted that cells are readily recovered from human tissue slices, and can rather easily be subculti-vated at a split ratio of 1:2 twice a week. This maintenance schedule can be continued for months, until, after approximately 50 population doublings, the cultures fail to display the numerous mitotic figures characteristic of healthy growth, and cease to divide. Once the cultures arrested, Hayflick found it impossible to recover cell growth, an observation that still holds up today. Based on these results, he suggested

Jan Karlseder

The Salk Institute for Biological Studies, 10010 North Torrey Pines Rd., La Jolla, California,

United States.

e-mail: [email protected]

K.L. Rudolph (ed.), Telomeres and Telomerase in Ageing, Disease, and Cancer. © 2008 Springer-Verlag Berlin Heidelberg

Fig. 1.1 Schematic of telomere dynamics during aging. Telomeres in telomerase-negative cells shorten over time (green line). When telomerase is introduced into such cells, telomere length is stabilized and the cells become immortal without accumulating chromosomal aberrations (purple line). When telomeres become short, cells arrest in senescence (blue line). Suppression of DNA damage pathways allows further telomere shortening (turquoise line), leading to dysfunctional and critically short telomeres, genome fragmentation and crisis (red line). Upregulation of telomerase allows telomere elongation, escape from crisis, and establishment of immortal clones with unstable genomes (orange line) (See Color Plate)

Fig. 1.1 Schematic of telomere dynamics during aging. Telomeres in telomerase-negative cells shorten over time (green line). When telomerase is introduced into such cells, telomere length is stabilized and the cells become immortal without accumulating chromosomal aberrations (purple line). When telomeres become short, cells arrest in senescence (blue line). Suppression of DNA damage pathways allows further telomere shortening (turquoise line), leading to dysfunctional and critically short telomeres, genome fragmentation and crisis (red line). Upregulation of telomerase allows telomere elongation, escape from crisis, and establishment of immortal clones with unstable genomes (orange line) (See Color Plate)

that the growth characteristics of such cultures could be divided into three phases. Phase 1, or the early growth phase, is when the culture establishes itself. During Phase 2 the cells grow exponentially, maintain a diploid chromosome set, but eventually stop growing. Phase 3 is termed the degeneration phase, during which tetra-ploid cells arise occasionally, and nuclei take on various unusual appearances (Hayflick and Moorhead 1961). In more detailed experiments Hayflick demonstrated that primary human cell strains enter Phase 3 after 50 +/-10 passages, no matter when and how long they have been frozen in between passages (Hayflick 1965). Consequently his experiments demonstrate that such cells have a finite lifetime in vitro, and that a cell-intern counting mechanism exists that monitors the accumulative number of population doublings (Fig. 1.1).

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