Epithelial Cells under Mechanical Strain: an Investigation

Abstract

Cells are the minimal living unit of the body. Each cell plays a vital role that must withstand various stresses, from chemical environments to mechanical strain and temperature variations. One of these vital cell types in the mammalian body is the epithelial cell, which lines the inner and outer surfaces of organs and cavities. These form tightly connected cell monolayers that produce a barrier that allows for directed transport in the body.In this cell resolved study, confluent epithelial cells (MDCK II) were investigated under uniaxial strain. Since little is known about the combined effect of temperature and strain on epithelial cell layers, measurements were performed at 25 • C, 37 • C, and 39 • C under strain. For this purpose, a uniaxial cell stretcher with elastic polydimethylsiloxane membranes was designed, built, characterized, and used for studies at axial strains up to 45 %. The stretcher allowed for both geometric characterization with optical microscopy and mechanical characterization with atomic force microscopy of single cells in a confluent layer, before, during the 30 minute strain application, and directly after strain release.With strain application, the cell geometry changed with an increase in cell area and shift of the cell orientation in the direction of strain. These changes were temperature and strain-dependent, with smaller geometric changes at 25 • C and low strain. After strain release, the cells returned largely to their before strain values. Atomic force microscopy indentation experiments at 37 • C showed that during strain, the pre-stress T 0 and area compressibility modulus K A0 increased while the cell fluidity β remained constant. After strain release all parameters except tension returned to the before strain values. This increase in stiffness at constant fluidity can also be observed in the treatment of cells with methyl-beta-cyclodextrin, which reduced the excess membrane area of the cells.Therefore the changes in mechanical properties can be largely attributed to a reduction in excess membrane area. When cooling the cells to 25 • C at low strains, a shift in the scaling of stiffness and fluidity can be observed, which disappears at high strains. This change might be attributed to a lipid membrane phase transition at 27 • C which restricts the recruitment of excess membrane area at low strains. When the stain is increased, a shear-induced phase transition might occur, resulting in similar properties as at 37 • C.Overall, these results point to a passive reaction of the cells to external stress, namely recruitment of the excess surface area in response to strain.