Co-culture of OSC1 cells and fibroblasts suggested binding of stromal HA to the tumour cell surface via CD44. OSC1 cells were cultured alone and with fibroblasts to gain mechanistic insight into the origin of OSC1-bound HA in shHAS3 transduced OSC1 cells. (A) In monoculture OSC1 cells showed a strong staining for HA (green) and a weak signal for CD44 (red). (B) Transduction with shHAS3 strongly decreased HA deposition (green) and increased CD44 expression (red, arrows). (C) In co-cultures OSC1 cells exhibited similar pericellular HA as compared to controls in A. (D) In contrast shHAS3 transduced OSC1 cell still exhibited a strong pericellular HA signal (green, arrows) in the presence of fibroblasts that co-localized with increased CD44 (red, arrows). Scale bars, 10 μm. Shown are representative images of n = 3.