We examined whether the presence of the cell cortex might explain, in part, why previous studies using atomic force microscopy (AFM) to measure cell modulus (E) gave higher values with sharp tips than for larger spherical tips. We confirmed these AFM findings in human umbilical vein endothelial cells (HUVEC) and Schlemm's canal (SC) endothelial cells with AFM indentation ≤ 400 nm, two cell types with prominent cortices (312 ± 65 nm in HUVEC and 371 ± 91 nm in SC cells). With spherical tips, E (kPa) was 0.71 ± 0.16 in HUVEC and 0.94 ± 0.06 in SC cells. Much higher values of E were measured using sharp tips: 3.23 ± 0.54 in HUVEC and 6.67 ± 1.07 in SC cells. Previous explanations for this difference such as strain hardening or a substrate effect were shown to be inconsistent with our measurements. Finite element modeling studies showed that a stiff cell cortex could explain the results. In both cell types, Latrunculin-A greatly reduced E for sharp and rounded tips, and also reduced the ratio of the values measured with a sharp tip as compared to a rounded tip. Our results suggest that the cell cortex increases the apparent endothelial cell modulus considerably when measured using a sharp AFM tip.
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