  In solid tumors, cells constantly interact with the surrounding extracellular
matrix. In particular cancer-associated fibroblasts modulate the architecture
of the matrix by exerting forces and contracting collagen fibres, creating
paths that facilitate cancer cell migration. The characterization of the
collagen fibre network and its space and time-dependent remodelling is
therefore key to investigating the interactions between cells and the matrix,
and to understanding tumor growth. The structural complexity and multiscale
nature of the collagen network rule out classical image analysis algorithms,
and call for specific methods. We propose an approach based on the mathematical
modelling of the collagen network, and on the identification of the model
parameters from the correlation functions and histograms of grey-tone images.
The specific model considered accounts for both the small-scale fibrillar
structure of the network and for the presence of large-scale aggregates. When
applied to time-resolved images of cancer-associated fibroblasts actively
invading a collagen matrix, the method reveals two different densification
mechanisms for the matrix in direct contact or far from the cells. The very
observation of two distinct phenomenologies hints at diverse mechanisms, which
presumably involve both biochemical and mechanical effects.
