Cell–Cell Interaction Proteins (Gap Junctions, Tight Junctions, and Desmosomes) and Water Transporter Aquaporin 4 in Meningothelial Cells of the Human Optic Nerve

Intercellular junctions mediate adhesion and communication between adjoining endothelial and epithelial cells. In the endothelium, junctional complexes comprise tight junctions, adherens junctions, and gap junctions. The expression and organization of these complexes depend on the type of vessels and the permeability requirements of perfused organs. Gap junctions are communication structures, which allow the passage of small molecular weight solutes between neighboring cells. Tight junctions serve the major functional purpose of providing a "barrier" and a "fence" within the membrane, by regulating paracellular permeability and maintaining cell polarity. Adherens junctions play an important role in contact inhibition of endothelial cell growth, paracellular permeability to circulating leukocytes and solutes. In addition, they are required for a correct organization of new vessels in angiogenesis. Extensive research in the past decade has identified several molecular components of the tight and adherens junctions, including integral membrane and intracellular proteins. These proteins interact both among themselves and with other molecules. Here, we review the individual molecules of junctions and their complex network of interactions. We also emphasize how the molecular architectures and interactions may represent a mechanistic basis for the function and regulation of junctions, focusing on junction assembly and permeability regulation. Finally, we analyze in vivo studies and highlight information that specifically relates to the role of junctions in vascular endothelial cells.

To describe the anatomy and the arrangement of the arachnoid trabeculae, pillars, and septa in the subarachnoid space of the human optic nerve and to consider their possible clinical relevance for cerebrospinal fluid dynamics and fluid pressure in the subarachnoid space of the human optic nerve. Postmortem study with a total of 12 optic nerves harvested from nine subjects without ocular disease. All optic nerves used in this study were obtained no later than 7 hours after death, following qualified consent for necropsy. The study was performed with transmission (TEM) and scanning electron microscopy (SEM). The subarachnoid space of the human optic nerve contains a variety of trabeculae, septa, and stout pillars that are arranged between the arachnoid and the pia layers of the meninges of the nerve. They display a considerable numeric and structural variability depending on their location within the different portions of the optic nerve. In the bulbar segment (ampulla), adjacent to the globe, a dense and highly ramified meshwork of delicate trabeculae is arranged in a reticular fashion. Between the arachnoid trabeculae, interconnecting velum-like processes are observed. In the mid-orbital segment of the orbital portion, the subarachnoid space is subdivided, and can appear even loosely chambered by broad trabeculae and velum-like septa at some locations. In the intracanalicular segment additionally, few stout pillars and single round trabeculae are observed. The subarachnoid space of the human optic nerve is not a homogeneous and anatomically empty chamber filled with cerebrospinal fluid, but it contains a complex system of arachnoid trabeculae and septa that divide the subarachnoid space. The trabeculae, septa, and pillars, as well as their arrangement described in this study, may have a role in the cerebrospinal fluid dynamics between the subarachnoid space of the optic nerve and the chiasmal cistern and may contribute to the understanding of the pathophysiology of asymmetric and unilateral papilloedema. All the structures described are of such delicate character that they can not even be visualised with high resolution magnetic resonance imaging (MRI).

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.