INTRODUCTIONThe developmental origin and functional identity of certain cells in the pulmonary alveolar walls, thought by some to be epithelial and therefore entodermal and by others to be of mesodermal origin, has long been a problem of theoretical interest (Bremer, ' 3 5 ) . A new approach to this question is now available through the electron microscope, with its higher practical resolving power. It is now possible to demonstrate the fine structure of the pulmonary alveolar epithelium (Low and Daniels, '52; Low, '53, '54; Karrer, '56; van Breeman and Neustein, '56). As a further aid, an ideal medium for the study of phagocytosis at the electron level of magnification has been developed (Sampaio, '55, '56). These two circumstances have made it practical to compare the reactions of pulmonary epithelium and alveolar macrophages to an artificially introduced foreign substance. This paper reports an investigation conducted along these lines, and collates the results with certain other non-experimental preparations.
OBSERVATIONSAll alveoli containing Thorotrast have basically the same appearance. Crystals of Tho2 are clearly visible, scattered lightly or heavily as the case may be, on the surface of the attenuated epithelium (figs. 1, 2, 4, 5, 6). The Tho, is never within the epithelial cytoplasm. The same holds true for the cell bodies of the epithelial cells where a considerable amount of perinuclear cytoplasm is present ( fig. 3). The alveolar macrophages present an entirely different appearance (figs.
4, 5 ) .The Tho, has been phagocytosed and segregated in vacuoles which are scattered randomly throughout the cell. These vacuoles are, €or the most part, lightly stippled but occasional opaque ones with heavy concentrations of Tho2 may be observed (fig. 4). The general picture is typical of an early stage of phagocytosis as described by Sampaio ( '56).The macrophages are free in the alveoli and rest on an uninterrupted layer of attenuated epithelium ( fig. 4), even when they are located in a recess in the alveolar wall as frequently occurs ( fig. 5). The Tho, crystals on the surface of the epithelium serve to delineate this relationship much more effectively than in lung not containing Tho,, where the thinly attenuated epithelium is often difficult to distinguish from the macrophage cytoplasm. Doubt was expressed concerning this point in a previous publication (Low, '53, p. 248) A relationship of considerable theoretic interest in normal untreated lung is that between the cuboidal cells lining the respiratory bronchioles and alveolar ducts (and sacs) and the primarily respiratory epithelium of the alveoli. Fortuitous fields have been observed in this laboratory on three occasions and are illustrated in plate 3. Figure 6 shows the situation in a respiratory bronchiole where the cytoplasm of the last cuboidal cell attenuates abruptly to a thinness characteristic of alveoli and continues as such over the adjacent capillary. Figure 7 represents the tip of an alveolar wall facing on an alveolar duct (or sac). Here t...
