Cotton-wool fibres are not uncommonly introduced into systemic veins during medical procedures and are swept into the pulmonary circulation as emboli. To ascertain the histological changes which occur in the pulmonary vasculature, and the time scale of the various cellular reactions we induced experimental cotton-fibre pulmonary embolism in 51 rats. Within an hour the fibres became covered by platelets and plasma proteins and neutrophil polymorphs were attracted around them. By two hours macrophages congregated around the emboli and, with accumulation of epithelioid cells, granulomas were formed. By 16 hours these had increased in size with the appearance of foreign body giant cells, and reached their maximum size in three days. By 16 hours the granulomas were in the wall of the pulmonary artery with rupture of the elastic laminae. At the end of a week granulomas had passed through the wall to reach the alveolar spaces. By the sixth month all were extravascular, lying within the alveolar walls or in the peribronchiolar fibrous tissue. The continuity of the arterial media through which the granuloma had passed was restored without haemorrhage.
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