PURPOSE: Fibronectin adhesion assay progenitors (FAA-CPs) and migratory assay progenitors (MCPs), subsets of mesenchymal-like stromal cells (MSCs), exhibit superior in-vitro chondrogenic potential compared to bone marrow (BM)-MSCs and chondrocytes. To assess this potential, differentiation studies followed by confirmatory staining for collagen deposition are utilized. Polarized light microscopy (PLM), based on birefringence principles, is a valuable tool for visualizing organized collagen fibers. Its use as a predictive tool for assessing chondrogenesis and osteogenesis has not been reported.

METHODS: This study involved FAA-CPs, MCPs, chondrocytes, and BM-MSCs derived from osteoarthritic knee joints (n = 3). After phenotypic characterization, the cells underwent chondrogenic and osteogenic differentiation, followed by Picrosirius red staining and PLM analysis, including immunohistochemical analysis for collagen types I, II, and X.

RESULTS: Birefringence assessment revealed greater collagen fibril alignment and significant remodeling in the BM-MSC group, which exhibited an arcade-like pattern. The MCP group displayed well-organized collagen fibrils in pericellular zones and as a peripheral band, while chondrocytes and FAA-CPs exhibited lower intensity birefringence, indicating random alignment. Areas with higher collagen type II deposition corresponded to reduced collagen type I and the absence of collagen type X, highlighting the unique fibrillar network seen with PLM was indicative of collagen type II.

CONCLUSION: While its application for osteogenesis was limited, probably due to the non-fibrillar nature of collagen type X, its value for chondrogenesis is notable. Although not directly reflecting chondrogenesis, PLM can serve as a valuable tool for gaining insights into collagen remodeling, particularly concerning collagen type II during chondrogenic differentiation.