Telomerase depletion leads to a time- and tissue-dependent degeneration.

<p>A) Representative images of tissue sections of <i>tert^−/−</i> Zebrafish and <i>tert^+/+</i> siblings, stained with hematoxilin-eosin. <i>tert^−/−</i> zebrafish show progressive tissue deterioration. Severe histological abnormalities are first evident in proliferative tissues (testes, gut and head kidney marrow) and later in non-proliferative (muscle). <i>tert^−/−</i> Zebrafish show reduced sperm in testes lumen (L) (Ab and d; p<0.001). The head kidney shows progressive defects in the marrow area (white asterisks) (Ag and i, which correlates with a decrease in total blood (p = 0.0228) cells when compared to <i>tert^+/+</i> siblings from an early age (3 months, N≥5) (Aj). Mesonephric tubules in the head kidney also degenerate in <i>tert^−/−</i> (Ag and i dashed outlines). Gut atrophy in <i>tert^−/−</i>, reflected as decreased villi length (Al, n, o), becomes significant from the age of 6 months (p<0.001). Muscle fibres are significantly thinner (p<0.001) at terminal time-points (c.12 months) (Aq, s, t, dashed outline); N≥5. B) <i>tert^−/−</i> display progressive thickening of gut lamina propria, indicative of inflammation (Bb, c, yellow bar and arrow, N≥4). Data are represented as mean +/− SEM. Scale bar = 50 µm.