The pia mater at the site of the entry of blood vessels into the central nervous system

Summary The entry of blood vessels into the central nervous system (CNS) has been studied at the surface of the brain stem and the spinal cord of two cats and two rabbits. The study was carried out by scanning electron microscopy (SEM). The results obtained by SEM concerning the layers of the pia mater corresponded to those obtained by other authors who used transmission electron microscopy. Between the basal lamina of the superficial glial membrane of the CNS and the cells lining the subarachnoid space, a pial connective tissue space could be clearly recognized. The blood vessles crossing the subarachnoid space were also covered by leptomeningeal lining cells. At the site of entry of these blood vessels into the pia mater their cellular coverings were reflected onto the pial surface. At this point, some vessels were partly surrounded by a small depression of the surface of the pia mater. Having crossed the pial connective tissue space the vessel enters a funnel-shaped channel that leads into the CNS. Within this channel, the vessel is surrounded by pial connective tissue. The outer wall of the channel consists of the basal lamina of the superficial glial membrane. The outer surface of the vessel is also covered by a homogeneous membrane, interpreted as being the basal lamina of the tunica media or, if the tunica media was absent, that of the tunica intima. Only in two instances could it be noted that an entering vessel was surrounded by deep recesses of the surface of the pia mater. In one of these cases, not too deep in the recess to prevent examination by SEM, it could be seen that the cellular coverings of the outer wall of the vessel were reflected onto the layer of the pial lining cells. This observation would indicate that also the deeper perivascular depressions of the pial surface probably do not conform to the classic concept of the Piatrichter (funnel-shaped perivascular depression of the subarachnoid space communicating with the perivascular space of intracerebral vessels). This mode of entry of blood vessels, commonly described in text-books, could not be substantiated by this study.