library_construction_protocol: To harvest cells for single-cell RNAseq the monolayer oligodendrocytes were incubated in accutase solution for 5min at 37°C. Cells were collected into centrifuges 400×g for 5 min. The pellet was resuspended in Oligo Base medium for counting. The cell suspension was placed on ice when possible. The cortical brain organoids were dissociated using the Worthington Papain dissociation system, with extended sphere incubation in papain/DNase to 45min. Ten organoids were pulled together per sample. Cells from chimeric animal tissue were collected as previously described in (Neumann et al., 2019), with noted variations. In brief, animals were perfused using ice-cold HBSS+/+ (14025092, Thermo Fisher Scientific) and a brain matrix was used to excise the cc into HBSS-/- (12549069, Thermo Fisher Scientific) supplemented with 0.4 Units of RNase inhibitors (N808011, Life Technologies), where the tissue was mechanically dissociated with a scalpel. From this point onwards, 0.2 Units of RNase inhibitors were used at each step. The tissue was washed twice with HBSS-/-, centrifuged for 5min at 1,500rpm, resuspended in 0.62 units Research Grade Liberase DH (LIBDH-RO, Roche) and incubated for 40min at 37°C with gentle rocking. 2ml of MEM (12492013, Thermo Fisher Scientific) containing 0.5% BSA, 500 Units bovine pancreas DNase (DN25, Sigma-Aldrich) & 0.5ml FBS were added, and the tissue was mechanically triturated with a P1000 pipette and strained through a 70μm mesh. Tubes containing tissue were filled to full volume with MEM containing 0.5% BSA, centrifuged for 10min at 1,500rpm and resuspended into 0.2% BSA. Two cc were pulled together per sample. Live cells were sorted with LSRFortessa analyser (BIO-RAD) at the FACS facility at the Centre for Regenerative Medicine by Fiona Rossi and mixed human and mouse cells were centrifuged and resuspended in 100μl of 0.2% BSA. Cell suspensions were processed by the Chromium Controller (10X Genomics) and barcoded libraries constructed using the Chromium Next GEM Single Cell 3' v3.1 kit (PN-1000121) following manufacturer's instructions. Samples were sequenced with Illumina NovaSeq6000.