Quantitative analysis of fibrillar collagen organization in the immediate proximity of embedded fibroblasts in 3D collagen hydrogels.

Fibroblasts embedded in a 3D matrix microenvironment can remodel the matrix to regulate cell adhesion and function. Collagen hydrogels are a useful system to study cell-matrix interactions in a 3D microenvironment. While major matrix reorganizations are easily recognizable, subtle changes in response to environmental or biochemical cues are challenging to discern in 3D hydrogels. Three-dimensional collagen gels at 1.0 mg/ml vs 1.5 mg/ml were labelled with DQ-collagen and imaged by confocal reflectance microscopy to evaluate these small changes. An image analysis pipeline was developed, hydrogel area and number of crosssections analysed were optimized, and fibrillar collagen properties (number of branches, number of junctions, and average branch length) were quantified. While no significant changes were seen in fibrillar collagen organization between 1.0 mg/ml and 1.5 mg/ml collagen hydrogels, embedded mouse fibroblasts caused a significant increase in collagen branching and organization. Using the phalloidin-labelled cells, this change was quantitated in immediate proximity of the cell. A distinct increase in branch and junction numbers was observed, significantly altered by small changes in collagen concentration (1.0 mg/ml vs 1.5 mg/ml). Together, this analysis gives a quantitative evaluation of how cells respond to and modify their immediate microenvironment in a 3D collagen hydrogel.