Microvessels play a particular role in the lung, where they optimally distribute blood around the alveoli for oxygenation. In lung diseases such as pulmonary hypertension, blood vessels are constricted and remodeled, but the cause and the mechanisms of these changes remain largely unknown. To study microvascular remodeling more closely, we developed an in vitro microfabricated platform where primary human lung cells self-assembled into long-living microvascular networks. The microvessels were perfusable, functionally responsive to a vasoconstrictor and showed physiologically relevant permeability values. In this study, functional microvessels were built with primary human lung cells only for the first time and the role of pericytes highlighted. These cells line the endothelium and stabilize the microvasculature. Most strikingly, upon exposure to phenylephrine a vascoconstrictor, microvessels surrounded by pericytes contracted. Such functional microvascular model opens up new ways to study microvascular remodeling in vitro and test pharmacological compounds in an in vivo-like setting.
Figure: chip and perfused microvascular network.
Left: Microfluidic chip with two compartments in which lung microvasculature is reproduced.
Right: Detail view of perfusable and stable microvasculature with vessel sizes ranging from 20 to 130 micrometers.
Tissue Eng Part A. 2015 Aug;21(15-16):2166-76.
Bichsel CA1,2, Hall SR3,4, Schmid RA3,4, Guenat OT1,2,3, Geiser T2,4.
1Lung Regeneration Technologies, ARTORG Center, University of Bern , Bern, Switzerland .
2Division of Pulmonary Medicine, University Hospital of Bern , Bern, Switzerland .
3Division of Thoracic Surgery, University Hospital of Bern , Bern, Switzerland .
4Department of Clinical Research, University of Bern , Bern, Switzerland.
The formation of blood vessels is a complex tissue-specific process that plays a pivotal role during developmental processes, in wound healing, cancer progression, fibrosis, and other pathologies. To study vasculogenesis and vascular remodeling in the context of the lung, we developed an in vitro microvascular model that closely mimics the human lung microvasculature in terms of three-dimensional architecture, accessibility, functionality, and cell types. Human pericytes from the distal airway were isolated and characterized using flow cytometry. To assess their role in the generation of normal microvessels, lung pericytes were mixed in fibrin gel and seeded into well-defined microcompartments together with primary endothelial cells (human umbilical cord vein endothelial cells). Patent microvessels covering an area of 3.1 mm(2) formed within 3-5 days and were stable for up to 14 days. Soluble signals from the lung pericytes were necessary to establish perfusability, and pericytes migrated toward endothelial microvessels. Cell-cell communication in the form of adherens and tight junctions, as well as secretion of basement membrane were confirmed using transmission electron microscopy and immunocytochemistry on chip. Direct coculture of pericytes with endothelial cells decreased the microvascular permeability by one order of magnitude from 17.8×10(-6) to 2.0×10(-6) cm/s and led to vessels with significantly smaller and less variable diameter. Upon phenylephrine administration, vasoconstriction was observed in microvessels lined with pericytes, but not in endothelial microvessels only. Perfusable microvessels were also generated with human lung microvascular endothelial cells and lung pericytes. Human lung pericytes were thus shown to have a prominent influence on microvascular morphology, permeability, vasoconstriction, and long-term stability in an in vitro microvascular system. This biomimetic platform opens new possibilities to test functions and interactions of patient-derived cells in a physiologically relevant microvascular setting.