A morphological study of acute respiratory tract lesions in a lipopolysaccharide instilled rat model

Acute inflammatory reactions in different respiratory regions were studied in a lipopolysaccharide (LPS) instilled rat model. Rats were sacrificed at 1, 2, 4, 8, 12, 24 and 48 h after administration of Escherichia coli LPS to the lumen of the trachea, and changes in the trachea, bronchioles and alveoli were observed by light and electron microscopy. In the trachea, neutrophils were markedly increased in subepithelial connective tissue and between tracheal epithelial cells, from 2 h on, showing a peak at 8 h. The number of tracheal mast cells increased at 4 and 8 h. Different structural features of secretory granules between mast cells in the tracheal epithelium and subepithelium were noticed, and the possible involvement of mast cells in airway acute inflammation is discussed. In the bronchioles, Clara cells showed characteristic morphological alterations. At 8 to 12 h, Clara cells revealed well-developed smooth and rough endoplasmic reticula and many free ribosomes, possibly for enhanced synthesis of secretory substances. Many Clara cells possessed a large apocrine-like protrusion filled with an amorphous substance, likely indicating an apocrine secretion of the cells. At 24 h, Clara cells were observed to take on a pseudostratified arrangement, suggesting enhanced proliferation of progenitor cells and their differentiation into Clara cells. In the alveoli, neutrophils infiltrated alveolar walls and pulmonary venules at 4 h and disappeared at 12 h. Prior to morphological changes in bronchioles and alveolar walls at 2 h, the macrophages with well-developed filopodia and free-ribosomes appeared in alveolar sacs and ducts. They may contribute to histological changes especially at the initial stage of acute inflammation in alveoli.