library_construction_protocol: After dissection of the cranial bone, the dura mater was first microdissected into extransinusoidal and sinusoidal compartments and then digested for 30 minutes at 37 °C in HBSS supplemented with 1:200 with murine RNAase inhibitor (New England Biolabs, M0314) and 1:2000 Actinomycin D (Sigma-Aldrich, A1410). After digestion, single cells were collected by passing the solution through a 50 µm cell strainer prior to spinning down and collected in FACS Buffer (2% BSA in PBS). CD45+ cells from the dura mater were sorted into 1.5 ml collection tubes containing 25 µl Sort Buffer (FACS Buffer supplemented with 1:200 RNAase inhibitor, New England Biolabs, M0314) using a MoFlo Astrios EQ sorter (Beckman Coulter, Brea, CA, USA). Up to 20,000 sorted cells per reaction were loaded on a Chromium controller (10x Genomics). Complementary DNA amplification and library preparation were performed according to the user guide for the Chromium Next GEM Single Cell 3′Reagent kits v.3.1 (CG000388 RevC). The libraries were sequenced on a NextSeq 1000/2000 machine (Illumina) with a sequencing depth appropriate to reach 20,000 reads per cell.