Flow control in our vessels: vascular valves make sure there is no way back

Abstract

The efficient transport of blood and lymph relies on competent intraluminal valves that ensure unidirectional fluid flow through the vessels. In the lymphatic vessels, lack of luminal valves causes reflux of lymph and can lead to lymphedema, while dysfunction of venous valves is associated with venous hypertension, varicose veins, and thrombosis that can lead to edema and ulcerations. Despite their clinical importance, the mechanisms that regulate valve formation are poorly understood and have only recently begun to be characterized. Here, we discuss new findings regarding the development of venous and lymphatic valves that indicate the involvement of common molecular mechanisms in regulating valve formation in different vascular beds.

Fig. 1

Fluid dynamics of a valve. Distinct phases of opening and closing of valve leaflets (blue): opening, equilibrium, closed, and closing, modified from [12]. Fluid pressure drop across the vessel drives direction of flow (black hollow arrow), while forces on either side of valve leaflets (blue, P _open, P _close) determine the leaflets’ position inside the lumen and the size of the valve orifice. Black arrows point at the distinct flow patterns as well as the fluid velocities at different parts of the valve pocket (relatively scaled to demonstrate magnitude differences), such as axial flow in the middle of the vessel and detached streamlines at the free ends of the open leaflets developing into vortices in the sinus. Gray line shows the level of the vessel distension during the opening phase of the valve

Fig. 2

Similarities and differences between valve formation process in veins and lymphatic vessels. a First drawing of a valve, by Salomon Alberti in 1585 ([28], reproduced from [27], Copyright (1927), Royal Society Medicine Press, UK), showing the outside and inside of part of a leg vein (indicated by A, B) with a tributary vein (C). D and E indicate the two cusps of a bicuspid valve. b Transmission electron microscopy of the tip of a lymphatic valve leaflet L lumen, S sinus. Matrix core of the leaflet is highlighted in pink and flow direction is indicated by an arrow. Scale bar 2 μm. c Schematic of the developmental process of valve formation in veins (top row, blue) and lymphatic vessels (bottom row, green). Direction of blood/lymph flow and color codes representing different tissues are shown below. Developmental time-points, as determined for the valves in the proximal femoral vein and mesenteric lymphatic vessels in mice, are indicated below each stage; E embryonic, P postnatal. Note the presence of uniform smooth muscle coating (brown) in veins prior to valve initiation, while sparse coverage of SMCs is acquired to lymphatic vessels only after valve formation and concomitant remodeling of a primitive vascular plexus to mature collecting vessels. Valve initiation in both veins and lymphatic vessels is characterized by emergence of clusters of cells expressing high levels of Prox1 and Foxc2 transcription factors (dark green nuclei), predominantly near vessel branch points, and followed by formation of leaflets with two layers of endothelial cells expressing Integrin-α9 (light green) attached to Laminin-α5 positive matrix core (red). Bottom confocal micrograph of a dermal collecting lymphatic vessel stained for Laminin-α5 to visualize the extracellular matrix core of the valve leaflet (green) and αSMA to highlight smooth muscle cells around the vessel (red). Scale bar 50 μm

Fig. 3

Model of valve morphogenesis. a Schematic representation of the development of a bicuspid venous valve by Kampmeier in 1927 ([28], reproduced from [27], Copyright (1927), Royal Society Medicine Press, UK). Endothelial layer is shown in white and mesenchymal layer in black. Flow direction is indicated by arrows; t tributary. b Schematic model of lymphatic valve morphogenesis. Some of the key regulators of different stages of valve formation are shown. Green text indicates expression in lymphatic endothelial cells, dark brown smooth muscle cells, and red extracellular matrix components. The initiation of valve formation coincides with the initiation of lymph flow in mesenteric lymphatic vessels. Clusters of endothelial cells expressing high levels of Prox1 and Foxc2 transcription factors (dark green nuclei) emerge at the sites of developing valves, which is followed by deposition of matrix molecules, such as Laminin-α5 (red) and establishment of valve territory via Calcineurin and Connexin signaling. Valve leaflet formation is initiated by the formation of an endothelial cell ring-like constriction and depends on Integrin-α9-mediated assembly of FN-EIIIA matrix. Repulsive signaling between Sema3A and NRP1 maintains valve areas free of smooth muscle cells (SMC, brown), while Calcineurin and Ephrin-B2 signaling regulate the maintenance of valve leaflets

Fig. 4

The molecular identity and morphology of valve endothelial cells. a, b Confocal micrographs of a mesenteric lymphatic vessel (a) and a femoral vein (b) stained for Integrin-α9 and Prox1 to visualize valve endothelial cells (red) and their nuclei (green), respectively. Image (b) reproduced from [32]), Copyright (2011), The Journal of Clinical Investigation, USA). Asterisks in (b) indicate a Prox1+ lymphatic vessel running in parallel of the vein. c–f Scanning electron microscope micrographs of a venous valve, showing different endothelial cell phenotypes (regarding cell shape and alignment) in different parts of the valve. Cells on the leaflet show rounded morphology (d) while cells on the inflow side (upstream) of the valve show elongated morphology and align in flow direction (e). The free edges of the leaflets are composed of cells that show transverse orientation and are highly elongated (f, arrows). I inflow, O outflow, L leaflet. Scale bars (a, b) 50 μm, (c, f) 10 μm, (d, e) 5 μm