During normal ageing, stem cells accumulate damage and subsequent stress-dependent changes (for example, de-repression of the CDKN2a (INK4a/ARF) locus or telomere shortening). This leads to the increasing abundance of senescent cells (large blue cells) within differentiated tissues. Incipient tumours, arising directly from stem cells or from more committed cells, undergo rapid proliferation (small red cells). These pre-malignant tumour cells rapidly accumulate damage, in part owing to the presence of oncogenes, leading to a higher proportion of tumour cells becoming senescent (small blue cells). Tumour progression to full malignancy is favoured when tumour cells acquire mutations that impair the senescence program (for example, mutations in Trp53 or CDKN2a).
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