Cell adhesion to hydroxyl groups of a monolayer film.

We have studied cells on chemically defined monomolecular films of the long-chain alcohol docosanol. Langmuir-Blodgett films of the alcohol were deposited on glass coverslips, previously made hydrophobic with octadecyl groups. This gives films in which the alcohol headgroups face outwards to the water. Molecular orientation and film integrity were shown by a fluorescence adsorption test. Cell contacts on the films were observed in media without proteins by interference reflection microscopy (IRM) and the mechanics of detachment were examined by hydrodynamic shearing in a flow chamber. Cell contact with docosanol was compared with that on an adjacent area of octadecyl glass without a monolayer. Dictyostelium amoebae settled and spread on both docosanol and octadecyl glass, but little or no locomotion was seen on docosanol. On octadecyl glass the amoebae moved actively, forming ultrathin cytoplasmic lamellae, which look dark under IRM, and left distinctive trails of membranous debris. Hydrodynamic shearing showed that the amoebae stuck strongly to both surfaces and could not be removed from either at the maximum attainable wall shear stress of 6Nm-2. Red blood cells also adhered to both surfaces and removal from both occurred between 1 and 3Nm-2. IRM and scanning electron microscopy (SEM) studies indicated that this force leads to a minimal measure of red cell adhesion, since removal often involved the breakage of cytoplasmic tethers. Our results show that alcoholic -OH groups, in a two-dimensional array, provide a surface that is strongly adhesive for cells. No other method has made it possible to demonstrate cell adhesion purely to -OH groups, in a known orientation and density, and in the absence of any other functional groups on the interface.

Red blood cells experience electrostatic repulsion but make molecular adhesions with glass.

We have studied the detachment of unfixed red cells from glass coverslips under unit gravity and by centrifugation in buffered isotonic solutions over a range of ionic strengths. Cell-glass contact areas and separation distances were measured by quantitative interference reflection microscopy. Detachment under unit gravity is highly dependent on ionic strength: dilution increases electrostatic repulsion and greatly reduces the proportion of adherent cells. However, even at 1.5 mM some cells stick. Over the range 3-110 mM such adherent cells are progressively removed by increasing centrifugal forces, but in a manner virtually independent of ionic strength. This fact, together with the irreversibility of pre-adherent cells as ionic strength is progressively reduced, as well as the resistance of cells to lateral shearing forces, provide evidence sufficient to reject the notion of secondary minimum adhesion for unfixed cells at any ionic strength down to 1.5 mM. We conclude that all unfixed cells that stick at ionic strengths from 157 to 1.5 mM make molecular contacts with glass. Comparison with long range force calculations suggests that to penetrate the electrostatic repulsion barrier the contact regions are unlikely to have average surface properties. A new method that compares frequency distributions of contact areas with responses to detachment forces shows that detachment forces are not linearly related to contact areas. This lack of relationship is less clearly evident for rigid glutaraldehyde-fixed cells and may therefore depend on the degree of cellular deformability.