An osmiophilic bilaminar lining film at the respiratory surfaces of avian lungs

Summary

The ultrastructure of the lungs of an adult pigeon, sparrow and hen, and of foetal chickens and chicks of different ages were studied. The presence of a noncellular continuous film on the air capillary surfaces, i.e. on the effective respiratory surfaces, was established at all stages of development as well as in three observed avian species. This film was bilaminar, of a thickness of approximately 100–150 Å. The upper and lower layers are strongly osmiophilic, the intermediate layer is of low electron density. This membrane is lacking in all respiratory passages with the exception, mentioned above, of air capillaries. The lung tissue was examined by the periodic acid-silver methenamine technique for the demonstration of polysaccharides and by the colloidal iron method (Mowry's modification of Hale's method) for the detection of acid mucopolysaccharides. After the first mentioned reaction, the picture corresponding to the visualization of “carbohydrate-rich cell coat” (Rambourg, 1967) was obtained. The reaction with colloidal iron was strongly positive at the air capillary surfaces. Nevertheless, the weakly expressed positive reaction was also present at the endothelial cell surfaces. It was assumed that the bilaminar osmiophilic lining film did not contain polysaccharides. Furthermore, this film was also shown to be present in the chicken's lung which had been rinsed several times with saline before fixation. It seems that this layer is secreted by the air capillary epithelial cells. We consider the type of secretion to be merocrine. In view of its behaviour in response to certain histochemical techniques and on the basis of electron microscope investigation its lipoproteinous nature is most probable.