Experimental lesions in rats corresponding to advanced human asbestosis.

Rats inhaling chrysotile asbestos developed a progressive interstitial fibrosis similar in most respects to human asbestosis. The earliest lesions were focal deposits of fibrous tissue in the walls of respiratory bronchioles and alveolar ducts. Later alveolar septa between adjacent bronchioles became progressively thickened to produce lesions with similarities to human honeycombing. The thickened septa between alveoli or "micro-honeycomb" spaces were mainly surfaced with cuboidal epithelial cells although some spaces lined by ciliated columnar epithelium were also found. Transmission electron microscopy of these advanced lesions showed that the cuboidal epithelial cells retained most of the characteristics of type 2 pneumocytes but that they frequently exhibited apical cytoplasmic blebs normally associated with the apocrine secretion of Clara cells. Columnar cells exhibited all stages from fully cilitated to cells with only an occasional cilium among the normal cell surface microvilli. Alveolar or micro-honeycomb spaces frequently contained clusters of pulmonary macrophages with their surface processes interdigitated but with no signs of fusion to giant cells. At more than 18 months after the end of dust inhalation these macrophages contained no chrysotile asbestos. The basement membranes beneath the epithelial layers of thickened septa were irregular and often convoluted as well as being much thicker than normal. Microscopic deposits of calcification were frequently found within the basement membrane material. Some thickened septa were relatively acellular, consisting mainly of masses of collagen fibrils but others were cellular and contained many macrophages, fibroblasts, plasma cells and mast cells. In these advanced lesions extremely little chrysotile asbestos was found and this was present in two sites only. Some chrysotile, always as individual fibrils and usually of short length, was present among collagen fibrils in areas of fibrosis and some was present within the thickened basement membranes.