In
Figure 4, the aspect ratio of the RPE cell was evaluated, much as the RPE cell area in
Figures 2 and
3. The aspect ratio is defined as the ratio of the minor axis length to the major axis length, calculated as the lengths of the major (minor) axis of an ellipse that has the same normalized second central moments as the cell region. The major and minor axes lengths were the output of CellProfiler.
31 When the aspect ratio is equal to 1, the cell shape is a circle. The smaller the aspect ratio, the more elongated the cell. We found RPE cell aspect ratios in WT had values of approximately 0.74, with no significant change with age (
Fig. 4A), suggesting that cell shape change was minimal in WT eyes over time.
Figure 4B illustrates the close similarity in aspect ratio of young and old WT mice in the density distributions. In the rd10 mouse (
Fig. 4C), the median aspect ratio at P30 started at the same value, 0.74, but it decreased with age to 0.60 from P180 to P330 and slightly increased to approximately 0.65 at 2 years of age, suggesting an elongation during disease progression in the rd10 mouse. Note that this median aspect ratio over the whole eye did not take into account the obvious spatial/regional inhomogeneity of distortion in rd10 mouse, as is evident in
Figures 1A and
1C. The density curves showed a more substantial difference, with a significant shift to the smaller values for old rd10 mice. When FPCA was conducted using aspect ratio, there was a limited distinction of young from old WT (
Fig. 5A) but a clear distinction of young from old rd10 (
Fig. 5B) mice.
Figure 5C combines data from aspect ratios of both genotypes and ages. Again, symbols correspond to the ages (
Fig. 5B, solid symbols for the young and open symbols for the old), and colors correspond to genotypes (
Fig. 5, black for WT and red for rd10). But we were unable to completely separate the four groups (WT young, WT old, rd10 young, rd10 old) based solely on aspect ratio (
Fig. 5C). However, we did separate WT young from rd10 young, which is a goal of this work.