2.4.3. Extracellular Matrix Remodeling

The analysis of the expression of genes involved in ECM remodeling revealed a decrease of *COLI* expression in end-stage OA compared to ACL IFP (*p* = 0.0156) (Figure 4a), while no differences were detected in both *COLIII* (Figure 4c) and *COLVI* (Figure S2, Supplementary Materials) gene expression (*p* = 0.1289 and *p* = 0.598, respectively). COLI and COLIII were evaluated also by Sirius red (Figure 4) showing a decrease of both collagen proteins in OA (*n* = 14) compared to ACL IFP (*n* = 18) (*p* = 0.001, *p* < 0.0001, respectively).

**Figure 4.** Extracellular matrix remodeling gene expression in ACL IFP (*n* = 8) and end-stage OA IFP (*n* = 8) and Sirius red evaluation. (**a**) *COLI* gene expression was higher in ACL IFP compared to end-stage OA. (**b**) COLI was higher in ACL IFP compared to end-stage OA evaluated by Sirius red. (**c**) *COLIII* gene expression was higher in ACL IFP compared to end-stage OA. (**d**) COLIII was higher in ACL IFP compared to end-stage OA evaluated by Sirius red. (**e**) Sirius red staining of ACL IFP (COLI (thick fibers) = yellow-red birefringence; COLIII (thin fibers = green birefringence). (**f**) Sirius red staining of end-stage OA IFP (COLI (thick fibers) = yellow-red birefringence; COLIII (Thin fibers = green birefringence). Scale bars = 50 μm. COLI = collagen type I; COLIII = collagen type 3; ACL = anterior cruciate ligament; IFP = infrapatellar fat pad; OA = osteoarthritis.
