Figure 2: Rat lung. Bouin fixation. PDE5 immunohistochemistry carried out via the amplified ABC method on microwave treated sections. Haematoxylin counterstaining. (a) E15.5, (b) E17.5, (c) P3, (d) P7, (e) P14. s: saccule; a: alveolus; c: canalicular portion of the respiratory space; TB terminal bronchiole; LB larger bronchiole. ((a), (b)) Bar 20 m; ((c), (d), (e)) Bar 50 m. ((a), (b)) positivity is restricted to the cytoplasm of several cells, the majority of which are scattered in the stroma (arrowheads), while some are closely apposed to the respiratory tubule (arrows). The tubular epithelium is unlabelled. ((c), (d), (e)) labelling is restricted to the distal parenchyma, including sacculi, alveoli, and canalicular portions of respiratory spaces. The epithelium of terminal bronchioles is unstained.