Mouse brain contains age-dependent extraparenchymal granular structures and astrocytes, both reactive to natural IgM antibodies, linked to the fissura magna

The present work reports the presence of two brain-related structures that, apart from PAS granules, contain neo-epitopes of carbohydrate nature, namely EP granules and IgM⁺ astrocytes. We suggest that EP granules, associated to fibroblasts, may be part of a physiological function in brain clearance or brain-CSF immune surveillance, while both PAS granules and IgM⁺ astrocytes may be related to the increasing accumulation of harmful materials that occurs with age and linked to brain protective mechanisms. Moreover, the specific localisation of these EP granules and IgM⁺ astrocytes suggest the importance of the fissura magna in these brain-related cleaning and immune functions. The overall results reinforce the possible link between the fissura magna and the functioning of the glymphatic system.