Astrocytes (which are brain glia cells) also contribute to the BBB, and they are attached with their endfeet to the per-icytes and the endothelial cells. Astrocytes are glial cells responsible for the homeostasis and the ion regulation in the brain , but their endfeet cover the blood vessels only partially. In contrast to endothelial cells and pericytes, astro-cytes are not connected to other cells by tight junctions, and they do not have a common basal membrane. Therefore, polar molecules (such as proteins) can enter the interstitial liquid and be directly transported to the pericytes and the endothelial cells .
That astrocytes are important for the induction and maintenance of the BBB properties can be deduced from the following observations in cell cultures: in the presence of astrocytes or medium conditioned by astrocytes, endothelial cells express markers important for BBB characteristics and develop tight junctions [9-11]. On the other hand, endothe-lial cells promote the development and differentiation of astrocytes. This interaction between both cell types actually occurs even when there is no contact between the two cell types, indicating that some soluble, extracellular factors are mediators of BBB development.
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