(A) BrdU-labeling index of germinative zone lens epithelial cells from 1- or 8-month-old rats breathing 11%, 21%, or 60% oxygen for 3 days. Dashed line: basal BrdU-labeling index at different ages. (B) Intraocular oxygen level, measured with an optical oxygen sensor, in young or older rats breathing 11%, 21%, or 60% oxygen. (C) BrdU-labeling index in the germinative zone of 8-month-old rat lens epithelia in animals breathing different levels of oxygen for 3 days. NS, not significantly different (P > 0.05). *P < 0.05 and ***P < 0.001 compared with 21% O2. All measurements are ± SEM.