Effects of the mechanical properties of collagen gel on the in vitro formation of microvessel networks by endothelial cells

Vascularization by endothelial cells (ECs) is an essential element in tissue-engineering of organoids. Morphogenesis of these cells is regulated not only by the biochemical properties of the extracellular matrix (ECM) but also by its mechanical properties. Here, we investigated the effect of substrate mechanical properties on the formation of capillary-like networks by ECs; in particular, we examined the three-dimensional (3D) configurations of the resulting networks. Bovine pulmonary microvascular ECs (BPMECs) were cultured on a series of collagen gels of different stiffness but the same collagen concentration. Imaging techniques revealed that cells cultured in rigid and flexible gels formed 3D networks via different processes; cells formed dense, thin networks in the flexible gel, whereas thicker and deeper networks were formed in the rigid gel. Cross-sections of the networks revealed that those formed within the rigid gel had large lumens composed of multiple cells, whereas those formed within the flexible gel had small, intracellular vacuoles. The expression of vinculin, a focal adhesion protein, appeared to change with the mechanical properties of collagen gel. Our results indicate that the mechanical properties of adhesion substrates play an important role in regulating 3D network formation.