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

Background: Mouse brains can contain specific polyglucosan aggregates known as Periodic Acid-Schiff (PAS)-granules. Generated in astrocytes, these granules increase with age and exhibit neo-epitopes of carbohydrate nature that are recognized by natural IgM antibodies (IgMs). The existence of neoepitopes on PAS granules suggests the presence of neoepitopes in other brain structures, and this is investigated here. To this end, brain sections from SAMP8 and ICR-CD1 mice were examined at different ages.

Results: We have identified two novel structures that, apart from PAS granules, are recognized by natural IgMs. On one side, IgM reactive (IgM⁺) granular structures which are placed in the longitudinal fissure, the quadrigeminal cistern, and a region that extends from the quadrigeminal cistern to the interpeduncular cistern. This last region, located between the telencephalon and both the mesencephalon and diencephalon, is designated henceforth as the fissura magna, as it is indeed a fissure and the largest in the brain. As all these regions are extraparenchymal (EP), the IgM⁺ granules found in these zones have been named EP granules. These EP granules are mainly associated with fibroblasts and are not stained with PAS. On the other side, some IgM⁺ astrocytes have been found in the glia limitans, near the above-mentioned fissures. Remarkably, EP granules are more prevalent at younger ages, while the number of IgM⁺ astrocytes increases with age, similarly to the already described evolution of PAS granules.

Conclusions: 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.