Combination of hypoxia and hyperosmolarity reduces in vitro chondrocyte de-differentiation

Articular cartilage (AC) is an avascular tissue with a highly specialized extracellular matrix (ECM) microenvironment characterized by low oxygen tension and high osmolarity. Standard in vitro culture conditions fail to replicate these features and may promote chondrocyte de-differentiation. This study investigated the combined effects of hypoxia and hyperosmolarity on chondrocyte phenotype and function.

Human articular chondrocytes were cultured under standard conditions (20% O₂, 280 mOsm/L) or under cartilage-mimicking conditions (5% O₂, 380-480 mOsm/L). Cells were maintained in two-dimensional (2D) monolayers and three-dimensional (3D) alginate bead cultures. Gene expression of differentiation markers (COLL2A1, ACAN, SOX9), de-differentiation markers (COLL1A1, RUNX2), and adaptive markers (HIF-1a, BGT1) was assessed. Glycosaminoglycan (GAG) production was quantified across passages.

Hypoxia and hyperosmolarity synergistically reduced de-differentiation marker expression and enhanced COLL2A1 expression, particularly during early passages. This effect was more pronounced in 3D cultures. Hyperosmolarity increased GAG production across passages, while its combination with hypoxia showed a synergistic effect in early 2D cultures and consistently in 3D systems. HIF-1a expression was upregulated under combined conditions. However, these protective effects decreased at later passages as de-differentiation progressed.

Mimicking the native cartilage microenvironment modulates chondrocyte phenotype and function, particularly in early de-differentiation stages. Although responsiveness declines over time, key phenotypic markers remain influenced. These findings support the importance of physiologically relevant culture conditions to improve chondrocyte quality for cartilage repair strategies, including autologous chondrocyte implantation (ACI) and matrix-assisted chondrocyte implantation (MACI).