Proteoglycan deposition and Collagen Orientation in Tissue Engineered Cartilage constructs using Digital Densitometry (DD) Imaging and Polarized Light Microscopy (PLM) Techniques

Background: Articular cartilage is an essential connective tissue providing structural integrity and low-friction movement in joints. Due to its avascular nature, its ability to self-repair is limited, necessitating advancements in tissue engineering for cartilage regeneration. The extracellular matrix (ECM) components, primarily proteoglycans and collagen fibers, play a crucial role in determining the biomechanical properties of engineered cartilage. Understanding proteoglycan deposition and collagen orientation is vital for optimizing tissue engineering strategies. Objective: This study aims to quantify proteoglycan content in histological sections of tissue-engineered cartilage constructs using digital densitometry (DD) imaging and analyze collagen fiber orientation using polarized light microscopy (PLM). Constructs with different cell densities (2M, 5M, and 10M) were evaluated at four time points (Day 1, 7, 14, 21) to assess ECM maturation. Methods: Tissue-engineered cartilage constructs were cultured using gelatin methacrylic anhydride (GELMA), gellan gum (GG) hydrogel, and scaffold-free (SF) approaches. Histological sections were stained with Safranin-O, and DD imaging was performed using a calibrated Nikon microscope to quantify proteoglycan content. Collagen fiber orientation was analyzed using a Leitz Ortholux II POL microscope at 21 orientations, employing Michelson contrast and anisotropy index calculations for alignment assessment. Results: Proteoglycan Deposition (DD Imaging): Proteoglycan content increased significantly from Day 14 to Day 21, with scaffold-free constructs exhibiting the highest deposition. Constructs with higher cell densities (5M and 10M) demonstrated greater proteoglycan accumulation compared to lower-density constructs. Collagen Fiber Orientation (PLM Analysis): Polarized light microscopy revealed that constructs with higher cell densities (5M and 10M) had more organized collagen networks. Scaffold-free constructs exhibited superior collagen alignment compared to hydrogel-based scaffolds. The anisotropy index confirmed increased fiber organization over time, particularly in the 5M and 10M SF groups. Conclusion: The combination of DD imaging and PLM provided a comprehensive assessment of ECM maturation in tissue-engineered cartilage. The study demonstrated that scaffold-free constructs, particularly those with higher cell densities, exhibited enhanced proteoglycan deposition and superior collagen alignment. These findings support scaffold-free tissue engineering approaches for developing functional cartilage replacements in regenerative medicine. Future Directions: Further investigations will explore the molecular mechanisms driving ECM organization and optimize scaffold compositions to enhance cartilage regeneration potential for clinical applications.