Acute interstitial pneumonia

High power photomicrograph of acute interstitial pneumonia (Hamman-Rich disease) shows marked alveolar septal thickening resulting from proliferating spindle cells. There is only a minor infiltrate of mononuclear inflammatory cells. The thickened alveolar septa are lined by hyperplastic pneumocytes.

High-power photomicrograph shows alveoli containing capillaries within a narrow interstitium. The alveoli are lined with thin, elongated type I pneumocytes (arrow) and smaller numbers of cuboidal type II pneumocytes (dashed arrow).