Fig. 4.

Positive reinforcement of tissue robustness in youth and its reversal in old age. In youth, poorly functioning or potentially malignant cells are eliminated from tissues, preserving tissue health and competitiveness. In old age, reduced tissue health impairs cell competition and the elimination of cells with altered functions, in a feedforward mechanism that further reduces tissue health. These mechanisms reflect investments in the soma through reproductive years and the waning of this investment in later years. [Colour figure can be viewed at wileyonlinelibrary.com]