Pericytes augment the capillary barrier in in vitro cocultures

Most in vitro studies of capillary permeability focus on endothelial cell (MVEC) monolayers and ignore the second cell that forms the capillary wall: the microvascular pericyte (PC). We describe a model to study the permeability characteristics of MVEC, PC, and MVEC:PC cocultures. Methods. Semipermeable culture inserts were coated with collagen and then plated with early passage bovine pulmonary MVEC. On Day 3, bovine pulmonary PC were added at concentrations to approximate MVEC:PC ratios of 1:1, 5:1, and 10:1. Electrical resistance was measured on subsequent days and fluorescently labeled (FITC) albumin was used in a permeability assay to calculate an albumin clearance for each culture. Results. The results for electrical resistance measurements and albumin assays showed a similar pattern. Resistance for endothelial cell monolayers was significantly higher and albumin permeability was significantly lower than that of controls. Addition of pericytes at a 10:1 and 5:1 ratios increased the permeability barrier compared to endothelial cells alone, although these cultures were not significantly different from one another. Cocultures at a 1:1 ratio had the best barrier, significantly better than all other cultures. Conclusions. Endothelial cell monolayers are an inadequate model of the microcirculation. As PC form a key component of the capillary wall in vivo and as addition of PC to MVEC monolayers in vitro, optimally at a 1:1 ratio, increase their barrier effect to large and small molecules, we believe it is necessary to include both cells in future in vitro studies.