Understanding of the blood-brain barrier (BBB) and the impact of neuropathology on its function, is greatly facilitated by the use of in vitro models. Systems that model this highly specialised tissue with great accuracy have so far proved elusive.
Endothelial cells of the brain microvasculature interface with the systemic circulation and perform the primary barrier and transport functions. These cells are adapted for this role by the presence of tight junction forming complexes, which preclude para-cellular transport, and by a particular influx and efflux transporter expression profile. For the aforementioned reasons, it was initially assumed that an in vitro model consisting of endothelial cells in culture, could sufficiently model the BBB’s barrier and transport functions. When cultured, these cells dedifferentiate due to the absence of paracrine signals present within the neurovascular unit (NVU). Models involving the co-culture of endothelial cells with other NVU comprising cells have been developed to ameliorate this problem of dedifferentiation. Furthermore the absence of shear forces generated by the systemic circulation contributes to this loss of BBB-specific phenotypes, such as cell polarisation in the direction of flow. Regardless of these disadvantages, resultant 2 dimensional static endothelial cell culture systems have successfully been used to model transport across the BBB.
Schematic of 3-D static endothelial monolayer blood-brain model. Taken from Cho et al., 2015.
The quest for more physiologically relevant in vitro models continues and recently there have been exciting innovations borne through the application of microfluidics to experimental BBB modelling. Cho et al., 2015 reported the development and validation of a static 3-D model of the BBB on a microfluidic platform. In this model, the BBB is simplified to a tube of single layered endothelial cells with ‘vascular’ and ‘neural’ compartments, with the vascular compartment designed to be geometrically similar to brain micro-vessels. Although still plagued by weaknesses similar to those of 2-D static models comprised of endothelial monolayers, it allows modelling of gradients across the BBB and experimentation involving manipulation of either or both compartments. A more ‘true to nature’ BBB model was developed by Brown et al., 2015. This model included all the various cell-types comprising the NVU in a 3-D culture, with perfusion mimicking physiological blood flow on a microfluidic platform. Based on the results of the various validation tests reported in the paper, the model captures to a great degree the BBB’s salient phenotypes. In addition to being more accurate, this model can be used as a self-contained system or potentially connected to another organ-on-a-chip to model the interplay between these tissues/organs and the BBB.
Brown, J. A., Pensabene, V., Markov, D. A., Allwardt, V., Neely, M. D., Shi, M., Brit, C. M., Hoilet, O. S., Yang, Q., Brewer, B. M., Samson, P. C., McCawley L. J., May, J. M., Webb, D. J., Deyu, L., Bowman A. B., Reiserer, R. S., Wikswo, J. P. (2015). Recreating blood-brain barrier physiology and structure on chip: A novel neurovascular microfluidic bioreactor. Biomicrofluidics, 9(5), 054124. http://doi.org/10.1063/1.4934713
Cho, H., Seo, J. H., Wong, K. H. K., Terasaki, Y., Park, J., Bong, K., Arai, Ken., Lo, Eng H., Irimia, D. (2015). Three-Dimensional Blood-Brain Barrier Model for in vitro Studies of Neurovascular Pathology. Scientific Reports, 5, 15222. http://doi.org/10.1038/srep15222