Bennhold’s congo red staining protocol was used for staining paraffin sectioned 3D cultures with slight modification. The hydrated paraffin-sectioned 3D cultures were incubated with 1% congo red solution (Sigma-Aldrich) for 60 min at room temperature. After briefly rinsing with distilled water for three times, the sections were dipped several times into alkaline alcohol solution (30% EtOH, 0.01% (w/v) NaOH) until the background was cleared. The slides were then washed twice with DDW.
