Cocultures of meningeal and astrocytic cells—A model for the formation of the glial‐limiting membrane

The glial‐limiting membrane at the border of the central nervous system (CNS) consists of glial endfeet covered by a basal lamina. The formation of the glia limitans seems to be controlled by adjacent meninges but only little is known about this interaction. In the present study astrocytes and meningeal cells were investigated in vitro to see if cocultures of these cells can serve as a suitable model for the differentiation of the glial‐limiting membrane and can be used to define the conditions under which the glial‐limiting membrane develops. The following observations were made in cocultures of meningeal and astrocytic cells of two‐day‐old rats: (i) epithelioid astrocytes were transformed into stellate cells; (ii) single colonies of proliferating epithelioid astrocytes were generated; (iii) the area around these colonies becomes devoid of meningeal cells, which seem to form a circular border around the astroglial islands; (iv) from the glial colonies long thin glial processes grow towards the surrounding meningeal cells, terminating at the site of contact; (v) in the contact zone between meningeal cells and astrocytes irregular shaped deposits of electron dense material resembling a basal lamina were seen. These observations indicate that indeed a structure resembling a glial‐limiting membrane develops in cocultures of meningeal and astrocytic cells. Its formation depends on the balance of growth promoting effects of meningeal cells on astrocytes and growth inhibiting effects of astrocytes on meningeal cells. Both activities can be enriched from conditioned media of pure astrocytic or meningeal cell culture. The proposed model of meningo‐astrocytic cocultures may be a helpful instrument for further investigations on the formation of the glia limitans.