library_construction_protocol: Fresh spinoids (n = 2) at day 7 were washed in cold PBS and then transferred in plastic mold pre-filled with ice-cooled OCT cryosolution (TissueTek, Leica), snap-frozen on dry ice and stored at −80 °C until sectioning using a cryotome. Sections of 20 µm were collected in the wells of a hard-shell PCR low-profile, semi-skirted 96-well plate (Bio-rad, HSL9601) that were prefilled with mineral oil (Sigma, M8410-1L) and CEL-seq2 primers. For each well, a unique, barcoded CEL-seq2 primer was used, which allowed us to pool the content of the wells after second-strand synthesis. To sequence the mRNA content of the wells, SORT-seq (robotized CEL-seq2-based scRNA-seq) was performed using a Nanodrop II liquid handling platform (GC biotech). DNA libraries were prepared using Cel-Seq2 protocol and Illumina TruSeq small RNA adapters (RPI series). Illumina TruSeq small RNA