 The in vivo experiment involved harvested rat astrocytes that were placed into the anterior chamber of a chick-eye or on the chorioallantois.Permeable blood vessels from either the iris or chorioallantois became impermeable to blue-albumin once they had entered the transplanted bolus of astrocytes.In the in vitro experiment, endothelial cells were first cultured alone and the tight junctions were observed in freeze-fracture replicas to be discontinuous and riddled with gap junctions.