Electron Microscope Studies on Alveolar Cells from Mammals 



A. PoLiCARD, A. Collet and S. Pregermain 

 with technical assistance of C. Reuet 



Centre d' Etudes et Recherches des Charbommges de Fiance, 

 Paris- Vernenil en Halatte, France 



The general organization of alveolar walls studied 

 with electron microscopy seems now to have been 

 made quite clear as a result of recent investigations 

 (1-10, 14-16). 



Our work, based on the findings of these authors 

 and on our own results, led us to the schematic 

 representation shown in figure 1. The true alveolar 

 wall consists of a connective space containing an 

 amorphous substance of weak electron density, of 

 collagen fibrils and septal histiocytic cells. The septal 

 space is limited on either side by a vitreous basement 

 membrane. This basement membrane forms an Y- 

 shaped connection with the capillary basement mem- 

 brane, thus incorporating in a peculiar way the blood 

 vessels in the alveolar wall. The basement mem- 

 branes are covered continuously with a thin cytoplas- 

 mic layer produced from the small alveolar cells and 

 having the appearance of very long prolongations. 

 At long intervals, large alveolar cells, partly capped 

 by the cytoplasmic layer from the small cells, are 

 attached to the basement membrane. Finally, free 

 cells are seen in the alveolar cavity. 



The object of this paper is to discuss some of the 

 aspects of the ultrastructure of the large alveolar 

 cells and to define the problems arising from them. 



Fig. L Semi-diagrammatic representation of the general 

 structure of the lung alveolus. 1, capillary endothelium; 

 2, cytoplasmic layer: .?, septal space: 4, vitreous basement 

 membrane; 5, endothelial ceil junction: 6, intraseptal cell; 

 7, alveolar cells (large, small and free); 8, collagen fibrils; 

 9, elastic fibers. 



On the other hand, evidence is presented on the 



pathological changes of these elements under the 



action of an irritating process. 



Rats narcotised with Nembutal were injected intra- 

 tracheally with Palade mixture. Small pieces of tissue 

 were fixed for 2 hours and embedded in prepolymerised 

 butyl methacrylate containing 1 per cent benzoyl per- 

 oxide. Sectioning was done by means of a Porter micro- 

 tome. An RCA EMU 3A electron microscope was used. 



The relations between the cytoplasmic alveolar 

 lining and the large cells are rather peculiar. The 

 large alveolar cell is partly capped, sometimes on a 

 great length (5 microns) by the cytoplasmic layer. 

 The large alveolar cell is then set in the prolongations 

 of the small cells and if the observer could look at the 

 front, only a part of its front surface would be seen. 



It will be noted that the microvilli of the large 

 alveolar cell appear only at the extremity of the 

 cytoplasmic lining. The boundary between the two 

 cellular surfaces seems to be the continuation of the 

 basement membrane which inserts the cytoplasm. 

 It is thought that they probably have the same 

 mucopolysaccharidic nature. 



The appearance of the mitochondria is variable. 

 Cristae are numerous and sometimes pressed close 

 together. Their aspect varies according to the orienta- 

 tion of the section. Images of tubules in cross-section 



Fig. 2. Rat lung. Large alveolar cell: relations of the cyto- 

 plasm with basement membrane and cytoplasmic layer (CD. 

 Microvilli (Mv). Osmiophilic bodies (Ob). Mitochondria 

 (M). Prolongation of the basement membrane by the inter- 

 cellular limits. 



