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Junctions in the meninges and marginal glia
Abstract:
The meninges of various mammals were prepared for examination with the electronmicroscope by thin sectioning or freeze-fracturing. Particular attention was given to the distribution of tight junctions in order to determine the basis for the meningeal barrier between the blood circulating in dural vessels and the cerebrospinal fluid in the subarachnoid space. While some dural blood vessels are fenestrated, those in the subarachnoid space are not and their component endothelial cells are joined by an extensive system of tight junctions. An extensive and continuous system of tight junctions was also found in a layer of specialized cells at the border of the arachnoid with the dura. This arachnoid barrier layer is apparently the only basis of the meningeal barrier because often cellular layers in the dura and arachnoid lack tight junctions although they are linked by gap junctions and desmosomes. In particular, tight junctions are lacking at the border of the "subdural space" which is actually a fascial plane within the dura. Tight junctions are also lacking between astrocytes at the surface of the brain but these cells are linked by gap junctions and a new type of intercellular junction. The distribution of these junctions, as well as assemblies of intramembranous particles at the astrocytic border, raises the question whether this layer might have a role in the exchange of certain substances between the brain and cerebrospinal fluid.
Insights
The arachnoid barrier layer, featuring extensive tight junctions, forms the meningeal barrier between blood and cerebrospinal fluid. Other meningeal layers and brain astrocytes lack these junctions, suggesting specialized roles in substance exchange.
Area of Science:
- Neuroscience
- Cell Biology
- Anatomy
Background:
- The meninges form a protective barrier around the central nervous system.
- Understanding the meningeal barrier's structure is crucial for studying cerebrospinal fluid dynamics and drug delivery.
Purpose of the Study:
- To investigate the distribution of tight junctions within the meninges.
- To identify the cellular basis of the meningeal barrier between dural blood vessels and cerebrospinal fluid.
Main Methods:
- Electron microscopy (thin sectioning and freeze-fracturing) was used to examine meninges from various mammals.
- Focus was placed on identifying and characterizing intercellular junctions, particularly tight junctions.
Main Results:
- Endothelial cells of subarachnoid space blood vessels possess extensive tight junctions.
- A specialized arachnoid barrier layer exhibits a continuous system of tight junctions, identified as the primary meningeal barrier.
- Dural layers and astrocytes at the brain surface generally lack tight junctions but possess gap junctions and desmosomes.
Conclusions:
- The arachnoid barrier layer is the principal component responsible for the meningeal barrier function.
- The absence of tight junctions in other meningeal layers and between astrocytes suggests alternative mechanisms for intercellular communication and potential roles in substance exchange.