Astrocytic Accumulation at the Choroid Plexus Attachment Region.

Immune cell entry into the central nervous system (CNS) under neuroinflammatory conditions is commonly discussed in relation to established barrier interfaces, including the blood-brain barrier and the choroid plexus epithelium. In addition, an alternative anatomical route at the choroid plexus attachment region has been proposed, yet its structural organization remains poorly defined. Here, we provide a detailed descriptive analysis of the choroid plexus attachment region and identify glial structures reminiscent of a glia limitans. We analyzed the morphology of the choroid plexus attachment region in control mice using immunohistochemical and ultrastructural techniques. Immune cell accumulation under neuroinflammatory conditions was examined in the CupEAE mouse model, and human post-mortem sections were analyzed for comparative insights. Our results show that the murine choroid plexus attachment region contains an astrocytic accumulation in close proximity to a continuous basal lamina facing the choroid plexus stroma. Ultrastructural analyses indicate the presence of components associated with junctional complexes. In the CupEAE model, CD3-positive immune cells were more frequently observed in the choroid plexus base than in the brain parenchyma adjacent to the attachment region. In humans, choroid plexus attachment regions also exhibit astrocytic accumulation, though with a different basal lamina pattern. These results underline the complex organization of the choroid plexus attachment region. This anatomical description might be the basis for further studies to evaluate the relevance of the choroid plexus attachment region in immune cell migration to the brain parenchyma.