The pia-arachnoid, which is usually regarded as a passive structural integument of the nervous system, may in fact have an active role to play in the development of the brain and the maintenance of its extracellular environment. We are interested in the properties of pia-arachnoid cells in culture and their interactions with cell types that they normally come into contact with, including astrocytes and endothelial cells. An important question is how these properties may manifest themselves in vivo during brain development and also following trauma to the central nervous system, when activation of the regenerative properties of the pia-arachnoid may become a significant impediment to restoration of neural integrity. Also, it is possible that the pia-arachnoid may be an active participant in calcium signaling between brain cells and the innervation of the brain blood vessels.