Acute lung injury

Diffuse alveolar damage (DAD)/Acute respiratory distress syndrome

Type I cells are eroded from inflamed, edematous alveolar walls with scattered PMNs. Proteinaceous exudate (the hyaline membrane) lines respiratory bronchioles, alveolar ducts (arrow), and some alveoli. Type II cell hyperplasia indicates early repair.

Organizing diffuse alveolar damage

Loose, pink granulation tissue (arrow) replaces hyaline membranes. Occurs about 7 days following injury. Acute interstitial pneumonia (Hamman-Rich syndrome) is organizing DAD with no known cause.

Cryptogenic organizing pneumonia (bronchiolitis obliterans organizing pneumonia)

Granulation tissue plugs bronchiolar lumens (upper arrow) and adjacent alveoli (lower arrow) while more peripheral alveoli are normal. Lesions are all the same age.