Fig. 2.

Comparison between in vitro and in vivo sample analysis. a Solution-based assays rely on simple diffusion in an essentially homogenous solution. b Cell-based in vitro assays allow analysis of dynamics of intracellular processes such as (1) extracellular diffusion, (2) transcytosis and (3) the formation of signalling filopodia such as cytonemes. c In vivo (including three-dimensional collagen/matrigel cell-based) assays involve all aspects observed in cell-based in vitro assays and further parameters of molecular diffusion such as (4) tortuosity (hindrance of diffusion path by impermeable objects), (5) transient binding of molecules to the extracellular matrix (ECM) and the cell membrane, and (6) restrictive clearance for gradient formation and the additional Z-axis that introduces further dimensions for diffusion