A model for transport studies of the blood-brain barrier.

The blood-brain barrier (BBB) is the cellular structure between the blood flowing through the brain and the parenchymal tissues of the brain. This physiological barrier is formed by the endothelial cells of the capillary walls. It exquisitely regulates the passage of substances into and out of the brain. Astrocytes (astroglial cells) signal the endothelial cells to adopt BBB characteristics. An in vitro BBB model can be very useful for the study of the nutrition, physiology, and pharmacology of the brain. We took advantage of numerous advances made by previous researchers in this field to develop a co-culture BBB model. Capillary endothelial cells and astrocytes are isolated from the brains of miniature swine and grown on permeable membranes suspended between two chambers of media: analogous to the capillary lumen and the interstitium of the brain, respectively. The endothelial cell isolation procedure includes mechanical and enzymatic digestion of the brain tissue followed by separation of the capillary fragments, based on size and density, from other brain cells. Astrocytes are purified from these "other" cells. The endothelial cells of the capillary fragments proliferate in culture flasks and are then seeded onto the upper surface of a polycarbonate semi-permeable membrane suspended between two chambers of fluid. Astrocytes are seeded on the underside of the membranes. Their close proximity enables the astrocytes to communicate with the endothelial cells and encourage their expression of BBB characteristics without disrupting the endothelial cell monolayer. Transport studies across the monolayer can be conducted by introducing test compounds into the media on one side and observing its appearance on the other side. Mechanisms of transport can also be studied.