An ultrastructural study of spontaneous chronic lung lesions in asymptomatic rabbits.

An electron microscopical study of chronic lung lesions in 12 clinically healthy, purpose bred laboratory rabbits (eight of which were free from infections with known respiratory pathogens but 4 of them carried a natural B. bronchiseptica infection) revealed focal chronic interstitial pneumonia, vascular changes and focal chronic bronchiolitis. In addition, severe endothelial changes and intravascular deposition of collagen were observed in septal capillaries. In type I pneumocytes and septal capillary endothelium we noticed numerous rounded structures, 70-90 nm in diameter, which consisted of a limiting two layer membrane enclosing an irregularly rounded electron-dense centre surrounded by a more electron-lucent halo. These structures appeared free in the cytoplasm, or they were attached to or apparently budding from membranes other than the plasmalemma. Particles located extracellularly were not found. Whether the structures described were involved in the genesis of the lesions found remains to be elucidated.