Table 1.

Different models of the blood brain barrier; their features, advantages, and disadvantages.

Model type	Typical components	Advantages	Limitations	Representative of BBB phenotype	References
Single-cell transwell systems (non-co-culture)	A monolayer of HBMECs cultured in the apical compartment of the transwell insert.	Very easy to set up. Minimal cost. Low labor intensity. Useful if wanting to study endothelial cells alone.	TEER is typically low.	Cobblestone appearance of HBMECs, barrier formation. Little information on the impact of additional cell types.	Borges et al., 1994; Hartz et al., 2010
Co-culture /multicellular transwell systems	HBMECs cultured on the apical side of the transwell insert and astrocytes and/or pericytes cultured on the underside of the transwell insert.	Time and cost effective. Higher TEER. Greater barrier stability.	Some models are not fully in contact.	Closer representation of the BBB with the addition of important cell types. Able to study interactions between cell types and how they influence BBB phenotype.	Hind, 2014; Wang et al., 2015; Appelt-Menzel et al., 2017
Spheroid	3D organization of cells typically using matrigel. Typically consists of HBMECs and astrocytes and/or pericytes with some models containing neuronal cell types.	3D Cell model. No scaffold. Reduced de-differentiation.	Cannot measure permeability with this model. Expensive and greater skill required.	Microvessels wrap around endothelial cells and provide structural support. Helps to induce tight junction proteins. Closely represents the in vivo set up with cells in direct contact with each other. Applications include: cancer drug and neurotoxicity screening.	Cho et al., 2017; Nzou et al., 2018
Microfluidic systems/3D chip-style models	3D organization of cells with the added benefit of a “flow” system to mimic cerebral blood flow. Typically consists of HBMECs and astrocytes and/or pericytes with some models containing neuronal cell types.	Advantage of mimicking sheer stress which is essential for HBMECs optimum phenotype.	Difficult to set up and maintain adequate flow unless linked to a computer system.	Useful to assess the impact of blood flow on cell development and optimum phenotype. Also useful in studying cell migration and metastatic progression.	Yeon et al., 2012; Wang et al., 2017

HBMECs = human brain microvascular endothelial cells, TGFβ = transforming growth factor beta, TEER = transepithelial resistance, BBB = blood brain barrier.