Fig. 7

SLYM and the ABC layer can be differentiated by their distinctive immunomarker expression. (A) The unstained SLYM in this claudin-11 stained section located at a middle coronal position of the circle of Willis, at the suprachiasmatic level, indicated by arrows, seems to fuse with the pia along both medial and lateral borders of the cisterna caroticus (CiC) -blue rectangle- which is shown in higher magnification in (B–I). Note the positive Cld-11 reactivity in the oligodendrocytes of the cerebral peduncle (Cp) in contrast to the negative appearance of SLYM in (B). (C) E-cadherin staining also provided a negative response in SLYM, pia and adventitia of the internal carotid artery (ACI). (D–I) show SLYM covered by an inner and outer layer of different types of arachnoid leptomeningeal fibroblasts. The PDPN-stained section (D) demonstrates a strongly reactive layer (arrowheads) facing the outer SAS (oSAS), a weaker stained SLYM in the middle and yet another thin leptomeningeal layer facing the inner SAS (iSAS) with a positively reacting pia. The adventitia (ADV) surrounding the internal carotid artery (ACI in B) is also positively stained. (E) ER-TR7 immunoreactivity demonstrates collagen type VI in particular on the inside of SLYM but is also associated with adventitia of ACI and pia. (F) As expected by its “lymphatic-like” nature, SLYM acts as an immunological niche. LYVE1 antibody highlights perivascular macrophages (arrowheads) located either on the inner, middle or outer part of SLYM. (G) Retinaldehyde dehydrogenase (RALDH2) is particularly strongly stained along the inner SLYM layer. (H) Plectin, a possible key cytoskeleton interlinking molecule, is also present in SLYM meningeal lamina, in particular facing the outer subarachnoid space, but also staining the pia. (I) Prox1-stained nuclei (arrows) identify SLYM as the middle layer in this triple-layered structure where nuclei in inner and outer layers are Prox1-negative (arrowheads). 3v: third ventricle, ACI: internal carotid artery, ADV: adventitia, CiC: cisterna caroticus, Cp: cerebral peduncle. Magnification is indicated by labelled bars on the individual figures