Identification of serous-like cells in the surface epithelium of human bronchioles

The conductive airways of the mammalian lung are lined by several morphologically distinct cell types, both ciliated and non-ciliated. Of the non-ciliated secretory cells, mucous (goblet), Clara and neuroendocrine cells have been identified in adult human bronchiolar epithelium but, thus far, the serous cell has not. We have examined human membranous and respiratory bronchioles from macroscopically normal peripheral lung (n = 5 cases). For ultrastructural studies, a minimum of five nucleated, non-ciliated cell profiles, containing electron-dense secretory granules, were selected at random from both bronchiolar levels in each case, such that a total of 60 cells was examined. Data from the computer-aided image analysis of the cells indicated that two populations existed, differing in both granule area and granule number per cell p < 0.0005 by discriminant analysis. By visual inspection, the cells fell neatly into two groups: those which, were predominantly serous-like in type, and those which were "Clara". In the membranous bronchioles, serous-like cells had a mean(SEM) granule area per cell of 3.67(0.62) microns 2 and Clara cells 0.47(0.07) microns 2 (p < 0.001). Also, in the membranous bronchiole, the mean(SEM) number of granules in serous-like cells was 40(4.7) and in Clara cells 10(1.1) (p < 0.001). At the respiratory bronchiolar level, the corresponding means were similar to those of the membranous bronchioles and, likewise, serous and Clara cells were significantly different. Thus, our data, indicate that serous cells are present in the adult human bronchiole.