Fig. 5. Force transfer through discrete molecular networks in living cells. Polarization optics (A,B,E,F), phase contrast (C,D) and fluorescence (G) views of cells whose integrin receptors were mechanically stressed using surface-bound glass micropipettes coated with fibronectin (A-F) or uncoated micropipettes with ECM-coated microbeads (G). (A) Cells exhibiting positively (white) and negatively (black) birefringent cytoskeletal bundles aligned horizontally and vertically, respectively, in the cytoplasm of adherent cells. (B) Birefringent cytoskeletal bundles that originally appeared white in A immediately changed to black (black arrow) as they turned 90° and realigned vertically along the axis of the applied tension field when integrins were pulled laterally (downward in this view). (C,E) An adherent cell immediately before a fibronectincoated micropipette was bound to integrin receptors on its surface and pulled laterally (downward in this view) using a micromanipulator as shown in D,F. (D) The black arrow indicates nuclear elongation and downward extension of the nuclear border along the applied tension field lines. (F) White arrows abut on white birefringent spots that indicate induction of molecular realignment within nucleoli in the center of the nucleus by applying mechanical stress to integrins microns away on the cell surface (see Maniotis et al., 1997a). (G) A cell containing EYFP-labeled mitochondria that was stressed by pulling on a surface-bound RGD-microbead using a micromanipulator. Vertical arrow, direction and extent of bead displacement; white circle, position of bead after stress application; green, position of mitochondria before stress application; red, their position approximately 3 seconds after stress was applied; Nuc, nucleus of the cell. Note that long distance transfer of mechanical force across integrins result in movement of mitochondria deep in the cytoplasm. Panel G reproduced with permission from the National Academy of Sciences (Wang et al., 2001).