Morphological studies on proliferation and desquamation of the alveolar lining epithelium in carrageenan-induced experimental pneumonia.

An experimental model of carrageenan-induced pneumonia in rabbits was used to study the proliferation and desquamation of alveolar lining epithelium, light and electron microscopically. Observation at 5 hr after intrapulmonary injection of 0.5% lambda carrageenan solution revealed epithelial cell (predominantly type 1 pneumocyte) injury, exposing the basement membrane of the alveolar wall, and that at 40 hr demonstrated an appearance of many large pneumocytes among the alveolar lining epithelium as a reparative reaction. This proliferative response of immature type 2 pneumocytes reached to the maximum in 60 and 72 hr followed by an intense desquamative response of the excessive alveolar epithelium in 5 and 6 days. The sloughed off epithelial cells were demonstrated as cell sheets or as clusters of epithelial cells. Inflammatory infiltrates in alveolar and interstitial spaces consisted of neutrophils and monocytes (or macrophages) at early stages, and mononuclear cells or macrophages at latter stages. Observation in 2nd and 3rd week showed accumulation of macrophages with many osmiophilic lamellar bodies in the alveolar spaces. The alveolar wall lining epithelium was still cuboidal or elliptical in shape in some alveoli and membranous in the others. Thickened alveolar septa at this stage were also noted. Thus this animal model appears to be a valuable tool for further understanding of the events leading to proliferation and desquamation of the alveolar lining epithelium.