Publication date: 25th July 2016
Physical cues of extracellular matrices (ECMs) including elasticity and topography, are thought to mechanically regulate cell behaviors. Progresses have been made in understanding bulk matrix-mediated mechanotransduction, especially hydrogel matrix with flat surfaces. But little is known how local microenvironments of hydrogels with fibrous structures affect cell spreading.
Here, we prepared collagen gels with varying local microenvironment to investigate interactions between cells and different fibrillar structures. Collagen gels were polymerized at different temperatures (4, 21 and 37 ℃) to control fibrillar microenvironment independent of collagen density. Microarchitecture with bundled, short fibers, and large porosity were observed in 4 ℃-formed collagen gels. Atomic force microscopy (AFM) indentation showed that local fibers stiffness were much higher than bulk gels stiffness, the stiffness of fibers could be adjusted from 1 to 9.3 KPa with the polymerization temperature ranging from 37 to 4 ℃.
These structural and mechanical differences of local microenvironment modulated cell spreading through cell-fiber interactions. We found that cells spreading dynamics were locally regulated by cellular-mediated fiber recruitment and continuous force transmission via focal adhesions (FAs). Confocal and time lapse images showed that high fiber stiffness together with loose bundle structure could limit cellular traction forces to concentrate nearby fibers as well as FAs formation, and then significantly delayed initial spreading of cells. Sequentially, because of large porosity and short fibers, it was unable for cells to form long-range, continuous force transmission through the collagen matrix which limited the matrix remodeling and reduced the mechanical interaction over a long distance, then further suppressed cell spreading.
Therefore, cell spreading on collagen gels are regulated by cell-fiber interactions including fiber recruitment-induced ligand density and force transmission-induced contractile force. And we propose that physical cues in fibrillar microenvironment, including fiber stiffness, pore size and fiber length played a synergistic role in regulating cell spreading.
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