Electron microscopic studies of lung biopsy specimens reveal the presence of pulmonary alveolar capillaries of the fenestrated type (containing diaphragm-like openings or fenestrae in their cytoplasm) in 20 of 28 patients with a variety of fibrotic lung disorders. Such capillaries are normally present in bronchial mucosa, but not in alveoli. Consideration is given to the possibility that this phenotypic change represents a metaplastic process analogous to that which occurs in pulmonary alveolar epithelium in fibrotic lung disorders.
