Cortex periodic structure and estimation of membrane-cortex compaction. (A–D) An example of periodic structure, probably representing tightly packed folds of the PM (red) and F-actin (green) couple, is visible in the cortex of a rounded nonoscillating cell (n = 8). (A) Folds at lower magnification. (B) Folds at high magnification from box in A. (C and D) Line scan of F-actin and PM mean fluorescence intensity along a line (5.5 × 0.2 µm) indicated (bottom) from box in A. (E–I) Estimation of degree of membrane-cortex compaction based on F-actin fluorescence intensity and membrane movement relative to obvious fiduciary marks. (E and G) Merged DIC and F-actin fluorescent images at two time points. (F and H) F-actin fluorescent images at high magnification from box in E and H. The yellow band indicates where the mean fluorescence intensity and perimeter length were determined. At t = 0, the segment of the perimeter, which later became a protrusion, was 18.2 µm long with a mean intensity of 1,864 ± 7,02 arbitrary units (a.u.). After 16 s, this densely folded region became a protrusion bounded by a perimeter segment of 32.0 µm with a mean intensity of 828 ± 476 a.u. (I) Plot of normalized changes of the perimeter length of protrusion and corresponding changes in mean fluorescence intensity for n = 17 protrusions from 12 examined cells.