ECs are an essential component of all blood vessels. They line the entire luminal surface of these vessels and function as permeability barriers between the vessel wall and the blood (Fig.1). Therefore, they are responsible for vascular homeostasis.

ECs play an essential role in regulating the vascular function in response to mechanical stimuli in both healthy and diseased states. One of their main tasks is to maintain the barrier function of the endothelium to regulate the exchange of blood with the underlying tissue by controlling, for example, the vascular permeability, leukocyte transmigration, and the vessel's mechanical stability.

In the living organism, ECs are permanently subjected to the mechanical stimulus of shear stress (Fig 1B). The circulation of blood generates this shear stress which has a crucial impact on the cells.

Fig. 1. Schematic depiction of a blood vessel lined with endothelial cells (A) and blood-fluid derived wall shear stress (B).

ECs are exposed to different types and degrees of shear stress, depending on the location and type of vessel or tissue. For example, venous cells are subjected to relatively low blood pressure and shear stress, whereas ECs in arteries endure high pressure and higher shear stress (Fig. 2).

In the straight sections of the arteries, blood flow is primarily laminar and unidirectional (small arteries), or laminar pulsatile (in large arterial vessels), whereas in branches or curvatures, the flow pattern can be nonuniform and disturbed. Disturbed or turbulent flow is often associated with a higher risk of the formation of atherosclerotic plaques.

It seems evident that these forces caused by fluid flow have a fundamental impact on the biology of ECs. In the following, we elaborate on why ECs should be cultivated under physiological shear stress rather than under static conditions.

Fig. 2. Shear stress values in different human vessel types. Values derived from studies by BJ Ballerman et al., Kidney Internatl, 1998 and AM Malek et al., JAMA, 1999.

Shear stress plays a crucial role in proper endothelial cell function by activating signaling cascades and gene expression involved in important biological processes (Fig. 3).

Fig. 3: Shear stress induced cellular changes. Adapted from PF Davies, Physiological Reviews, 1995.

The mechanical stimulus generated on endothelial cells by the fluid flow is sensed by mechanosensors located in the cell wall, such ion channels, the cytoskeleton, tyrosine kinase receptors, caveolae, G proteins, and cell-matrix (Integrins) cell-cell junction molecules. These mechanical stimuli trigger signaling cascades which are then transduced to the nucleus to activate transcription factors and gene expression. Subsequently, gene expression leads to protein expression and the activation of cellular functions such as remodeling of the endothelial cell structure or cell migration (Fig. 4).

Fig. 4 Schematic diagram (adapted from Chien et al. 2007) showing the downstream effects of shear-stress induced mechanotransduction.

The morphological impact of shear stress exposure becomes visually apparent even after a short period of cultivating endothelial cells under flow. In contrast to static cell culture, cells start to orient in the direction of the flow, and a rearrangement of the cytoskeleton takes place with actin fibers aligning in the direction of the flow (Figure 5). In addition, stress fibers and the mechanical stiffness of the cells increase.

Fig. 5: Fluorescence microscopy of Human Umbilical Vein Endothelial Cells (HUVECs). Cells were cultivated for 72h with the ibidi Pump system under static (left) and flow conditions at 10 dyn/cm² (right). Afterwards, the cells were fixed and stained. Green: Actin, red = VE-Cadherin, blue = DAPI.