In a recent report on human pulmonary fibrosis, Brody and Craighead (1976) drew conclusions from cellular identifications that are inconsistent with a large body of literature. The major cell shown in Figure 7b of that paper was labeled as a fibroblast in close structural relationship with the basal lamina and associated collagen fibrils. This cell contained a few round cytoplasmic vesicles of coarsely electron dense material, purportedly "pre-collagen fibrils". There is no documented report of similar collagen secretory vesicles in fibroblasts. The cell shown has striking similarities to micrographs of mast cells (Wilson, 1974;Serafini-Francassini et al., 1969;Said and Luri, 1971).
The number of pulmonary mast cells increases in response to a wide variety of conditions, including pulmonary injury or respiratory distress (Wilson, 1974). We have completed an ultrastructure study of a lung biopsy from a case of idiopathic spontaneous pneumothorax (Tueller et al.). This lung had considerable thickening of the basal lamina such as that demonstrated by Brody and Craighead and, in addition, many mast cells were found in the lung parenchyma (Fig. 1).
The degranulation of cytoplasmic vesicles of mast cells and the resulting release of histamine is capable of initiating pathological changes in lung structure (Said and Luri, 1971). It is possible, although not proven, that histamine may have contributed much to the abnormal lung structure described by Brody and Craighead.
The misidentification of mast cells was not the only area of doubt raised by this article on human pulmonary fibrosis. There were portions of two other cells, identified as fibroblasts (Figs. 3 and4), which have many of the characteristics of pulmonary macrophages. It is well documented that macrophages increase in number and migrate through the interstitial tissue in response to a wide variety of pulmonary insults, including pulmonary fibrosis (Hance and Crystal, 1975). The cells in question have a very electron-dense cytoplasm and appear to contain many lysosorne-like granules.