Figure 3. Spontaneous cell death of human RPE cells from primary culture at early (P5) and late passage (P21). LDH assays demonstrated the passage-dependent spontaneous cell death of RPE cells. Cell death rates at P21 and P5 were 22.2% and 7.4%, respectively (mean±SD). The maximum LDH activity released from the cells at the same passage in response to Triton X-100 represented 100% cell death correspondingly. The difference between P5 and P21 was statistically significant (p<0.05, shown with asterisks).