Direct, dynamic assessment of cell-matrix interactions inside fibrillar collagen lattices

Cell mechanical behavior has traditionally been studied using 2-D planar elastic substrates. The goal of this study was to directly assess cell-matrix mechanical interactions inside more physiologic 3-D collagen matrices. Rabbit corneal fibroblasts transfected to express GFP-zyxin were plated at low density inside 100 μm-thick type I collagen matrices. 3-D datasets of isolated cells were acquired at 1-3-min intervals for up to 5 h using fluorescent and Nomarski DIC imaging. Unlike cells on 2-D substrates, cells inside the collagen matrices had a bipolar morphology with thin pseudopodial processes, and without lamellipodia. The organization of the collagen fibrils surrounding each cell was clearly visualized using DIC. Using time-lapse color overlays of GFP and DIC images, displacement and/or realignment of collagen fibrils by focal adhesions could be directly visualized. During pseudopodial extension, new focal adhesions often formed in a line along collagen fibrils in front of the cell, while existing adhesions moved backward. This process generated tractional forces as indicated by the pulling in of collagen fibrils in front of the cell. Meanwhile, adhesions on both the dorsal and ventral surface of the cell body generally moved forward, resulting in contractile shortening along the pseudopodia and localized extracellular matrix (ECM) compression. Cytochalasin D induced rapid disassembly of focal adhesions, cell elongation, and ECM relaxation. This experimental model allows direct, dynamic assessment of cell-matrix interactions inside a 3-D fibrillar ECM. The data suggest that adhesions organize along actin-based contractile elements that are much less complex than the network of actin filaments that mechanically links lamellar adhesions on 2-D substrates.