  Cell membranes are studded with protrusions that were thoroughly analyzed
with electron microscopy. However, the nanometer-scale three-dimensional
motions generated by cell membranes to fit the topography of foreign surfaces
and initiate adhesion remain poorly understood. Here, we describe the dynamics
of surface deformations displayed by monocytic cells bumping against
fibronectin-coated surfaces. We observed membrane undulations with typically 5
nm amplitude and 5-10 second lifetime. Cell membranes behaved as independent
units of micrometer size. Cells detected the presence of foreign surfaces at 50
nm separation, resulting in time-dependent amplification of membrane
undulations. Molecular contact then ensued with apparent cell-membrane
separation of 30-40 nm, and this distance steadily decreased during the
following tens of seconds. Contact maturation was associated with in-plane
egress of bulky molecules and robust membrane fluctuations. Thus, membrane
undulations may be the major determinant of cell sensitivity to substrate
topography, outcome of interaction and initial kinetics of contact extension
