Expression of cell adhesion molecule E-cadherin in human arachnoid villi

✓ Calcium-dependent epithelial cell adhesion molecules designated as E-cadherin (also known as uvomorulin or L-CAM) were identified in human arachnoid villi by immunoblotting and immunocytochemical analyses using a monoclonal antibody HECD-1 raised against human mammary carcinoma MCF-7 cells. Immunoblot analysis showed that HECD-1 recognizes E-cadherin with a molecular weight of 124 kD. In all arachnoid cells of an arachnoid villus, E-cadherin was detected by immunolight microscopy within the cytoplasm rather than the cellular boundaries as seen in the control group. Furthermore, the extent of expression by immunolight microscopy varied from portion to portion. The expression was usually weak in the syncytial cluster which was ultrastructurally composed of tightly juxtaposed cells characterized by few extracellular cisterns and numerous cell junctions, while it was intense in the reticular cluster and the surface layer which were ultrastructurally characterized by abundant extracellular cisterns and smaller numbers of cell junctions. The cells of the reticular cluster and the surface layer contained more free ribosomes than those of the syncytial cluster. Immunoelectron microscopy showed that E-cadherin was localized not only to the opposing plasma membranes and the cytoplasm around the free ribosomes or the rough endoplasmic reticulum but also to the extracellular cisterns. As the expression of E-cadherin was closely related to the arachnoid cells adjacent to the cerebrospinal fluid pathway, it is suggested that, instead of the cell junctions, E-cadherin may play an important role in the flexible adhesion of arachnoid cells even in the presence of the cerebrospinal fluid.

Arachnoid cysts of the Sylvian fissure

✓ Morphological and enzyme ultracytochemical evidence is presented to support the contention that the walls of arachnoid cysts secrete fluid. Clinical evidence has already suggested this phenomenon, including intracranial pressure elevation and expansion in some cases, and the observation that arachnoid cysts constitute closed compartments with a fluid content that cannot be derived from other cerebrospinal fluid-containing spaces. Ultrastructurally, the cyst lining showed a similarity to subdural neurothelium and the neurothelial lining of arachnoid granulations in such morphological features as intercellular clefts with sinusoid dilatations, desmosomal intercellular junctions (upon which tonofilaments may be abutting), pinocytotic vesicles, multi-vesicular bodies, lysosomal structures, and the presence of a basal lamina. Some of these features, together with the presence of microvilli on the luminal surface, are consistent with fluid secretion. Moreover, enzyme cytochemistry demonstrated (Na+ + K+)-ATPase in the plasma membranes lining the cavity, either directly (the apical membranes), or via the intercellular clefts (the basolateral membranes), and, with alkaline phosphatase occupying the opposite plasma membranes, this structural organization indicates fluid transport toward the lumen. It may be surmised that arachnoid cysts derive from subdural neurothelium differentiating towards arachnoid villus mesothelium.

The role of the arachnoid villus in the removal of red blood cells from the subarachnoid space

✓ The electron microscope examination of arachnoid villi from dogs after the subarachnoid injection of blood reveals that erythrocytes accumulate and degenerate in the villi. A continuous layer of endothelium separates the villus from the lumen of the sagittal sinus. No evidence for pores through the endothelium or for the passage of intact erythrocytes from CSF to blood through arachnoid villi has been found.