Expression of ALDH1 was absent in regions with poorly differentiated tumor cell morphology while adjacent differentiated regions were highly stained. [A] Numerous cells expressed ALDH1 in differentiated tumor regions (D) with micro-papillary architecture. [B] Representative section with adjacent areas showing strikingly different ALDH1 expression in differentiated and poorly differentiated regions. [C] Reduced or absent ALDH1 expressing cells in poorly differentiated regions (P) with solid tumor cell nests. Sections were counterstained with hematoxylin. (Original magnifications: ×200, ×100 and ×200 respectively).