1. Histological structure of the human SLIL
Analysis of the human SLIL using HE staining confirmed that this structure was histologically heterogeneous, with several histological differences found at the different zones analyzed in this work (Figure 2). When the dorsal region of the SLIL was analyzed, we first found that the D1 zone consisted of a dense ECM with abundant fibers oriented in different spatial directions and a scarce population of elongated or spindle-shaped cells dispersed across the ECM. Then, D2 showed an ECM containing abundant well aligned and oriented fibers, and an abundant population of spindle-shaped cells displaying similarities with the cells in D1. At the membranous region, we found that M1 contained a dense ECM with no identifiable fibers and rounded cells scattered within the tissue ECM. Interestingly, these cells tended to form groups resembling the human hyaline cartilage isogenic groups, with clusters of 2 to 4 cells surrounded by a clear ECM with a peripheral capsule that was very similar to the structure found in normal human cartilage. In turn, the M2 zone showed a very dense ECM with fibers apparently oriented in layers with different spatial directions. Cells found in M2 were not abundant and tended to form rows that partially resembled human fibrochondrocytes and were also surrounded by a pericellular capsule. Finally, the palmar region of the SLIL contained abundant fibers and scattered spindle-shaped cells. For P1, we found abundant cells within a dense ECM with properly oriented fibers. However, analysis of the P2 zone revealed the presence of fewer elongated or rounded cells, and the fibers were oriented to different spatial directions.