Objective Lymphatic vessels (LVs) maintain fluid homeostasis by draining interstitial fluid. A failure in lymphatic drainage triggers lymphatic diseases such as lymphedema. Since lymphatic drainage is regulated by lymphatic barrier function, developing experimental models that assess lymphatic barrier function is critical for better understanding of lymphatic physiology and disease. Methods We built a lymphatic vessel‐on‐chip (LV‐on‐chip) by fabricating a microfluidic device that includes a hollow microchannel embedded in three‐dimensional (3D) hydrogel. Employing luminal flow in the microchannel, human lymphatic endothelial cells (LECs) seeded in the microchannel formed an engineered LV exhibiting 3D conduit structure. Results Lymphatic endothelial cells formed relatively permeable junctions in 3D collagen 1. However, adding fibronectin to the collagen 1 apparently tightened LEC junctions. We tested lymphatic barrier function by introducing dextran into LV lumens. While LECs in collagen 1 showed permeable barriers, LECs in fibronectin/collagen 1 showed reduced permeability, which was reversed by integrin α5 inhibition. Mechanistically, LECs expressed inactivated integrin α5 in collagen 1. However, integrin α5 is activated in fibronectin and enhances barrier function. Integrin α5 activation itself also tightened LEC junctions in the absence of fibronectin. Conclusions Lymphatic vessel‐on‐chip reveals integrin α5 as a regulator of lymphatic barrier function and provides a platform for studying lymphatic barrier function in various conditions.

The human circulatory system is divided into two complementary and different systems, the cardiovascular and the lymphatic system. The cardiovascular system is mainly concerned with providing nutrients to the body via blood and transporting wastes away from the tissues to be released from the body. The lymphatic system focuses on the transport of fluid, cells, and lipid from interstitial tissue spaces to lymph nodes and, ultimately, to the cardiovascular system, as well as helps coordinate interstitial fluid and lipid homeostasis and immune responses. In addition to having distinct structures from each other, each system also has organ-specific variations throughout the body and both systems play important roles in maintaining homeostasis. Dysfunction of either system leads to devastating and potentially fatal diseases, warranting accurate models of both blood and lymphatic vessels for better studies. As these models also require physiological flow (luminal and interstitial), extracellular matrix conditions, dimensionality, chemotactic biochemical gradient, and stiffness, to better reflect in vivo, three dimensional (3D) microfluidic (on-a-chip) devices are promising platforms to model human physiology and pathology. In this review, we discuss the heterogeneity of both blood and lymphatic vessels, as well as current in vitro models. We, then, explore the organ-specific features of each system with examples in the gut and the brain and the implications of dysfunction of either vasculature in these organs. We close the review with discussions on current in vitro models for specific diseases with an emphasis on on-chip techniques.

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Dysfunction of the aquaporin‐4 (AQP4)‐dependent glymphatic waste clearance pathway has recently been implicated in the pathogenesis of several neurodegenerative diseases. However, it is difficult to unravel the causative relationship between glymphatic dysfunction, AQP4 depolarization, protein aggregation, and inflammation in neurodegeneration using animal models alone. There is currently a clear, unmet need for in vitro models of the brain's waterscape, and the first steps towards a bona fide “glymphatics‐on‐a‐chip” are taken in the present study. It is demonstrated that chronic exposure to lipopolysaccharide (LPS), amyloid‐β(1‐42) oligomers, and an AQP4 inhibitor impairs the drainage of fluid and amyloid‐β(1‐40) tracer in a gliovascular unit (GVU)‐on‐a‐chip model containing human astrocytes and brain microvascular endothelial cells. The LPS‐induced drainage impairment is partially retained following cell lysis, indicating that neuroinflammation induces parallel changes in cell‐dependent and matrisome‐dependent fluid transport pathways in GVU‐on‐a‐chip. Additionally, AQP4 depolarization is observed following LPS treatment, suggesting that LPS‐induced drainage impairments on‐chip may be driven in part by changes in AQP4‐dependent fluid dynamics.

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