Fig. 1. Nuclei of cells under shear flow do not deform. (A) Four phase-contrast micrographs of Swiss 3T3 cells subject to a shear flow (wall shear stress=9.4 dyn/cm²) taken over 23 minutes. Vertical and horizontal lines are guides to the eye. Cells were sheared for 27 minutes; arrows indicate the direction of flow. Three nucleoli are indicated. (B) Examples of relative surface areas of contact of two sheared cells with their substratum (dashed lines) and (apparent) surface areas of nuclei (solid lines) of the same sheared cells as a function of shearing time. The initial values of nucleus and cell surface areas are, respectively, 5100 µm² and 710 µm² for the cell shown in A (black lines; see movie M.2, http://jcs.biologists.org/supplemental/) and 4810 µm² and 350 µm² for the cell shown in movie M.3 (red lines). Surface areas were measured by morphometric analysis of phase-contrast micrographs of the cells under shear. Same colors correspond to the same cell. (Inset) Displacements of the centroids of sheared cells. Arrow indicates flow direction. This figure shows how cells can either move in the flow direction (black line) or counter-current (red line) (corresponding to cell shown in A). The starting point is where the two trajectories meet in the middle. (C) Typical movements of nucleoli centroids and nucleus centroid of a sheared cell (cell shown in A). In this particular case, nucleoli 1-3 and nucleus move from left to right (compare micrographs in A), and therefore move counter-current. The time lapse between symbols is 1 minute. 1-3 and 1'-3', respectively, indicate start and end points of the trajectories. (D) Time-dependent distances between nucleoli centroids and the nucleus centroid (red lines) of a cell under shear and distance between cell centroid and nucleus centroid (black line) of the same sheared cell.