Comparative Study of Respiratory Portion of the Lung. ∗

Abstract

All authors agree that the respiratory portions of lungs of Amphibia and Reptilia are lined by continuous, flattened, nucleated, epithelial membranes. Some investigators maintain that there is a continuous, flattened nucleated and non-nucleated epithelial membrane lining the respiratory portions of mammalian lungs, while others deny the presence of an epithelium. Some believe that the small, nucleated cells found in the meshes of the capillaries in the alveolar septa are of mesenchymal origin. For a review of the literature on this subject see Fried. 1

This work comprises a study made on the nature of the lining of frog, turtle, chicken, rabbit, guinea pig, and rat lungs. The lungs of the animals, after their lung capillaries had been filled with blood by ligating their pulmonary veins before the animals were killed, were fixed in Zenker-formol solution, embedded in nitrocellulose and stained to show collagenous, reticular and elastic fibers and cellular details. In addition, rabbit lungs, made atelectatic by phrenicotomy and pneumothorax were kept in a state of collapse from one to 40 days and were studied with the same histological methods. To demonstrate cell outlines, intratracheal and intravascular injections of 2% silver nitrate were also made into the lungs of other animals in each of the above groups.

A continuous layer of flattened, nucleated epithelium covering the alveolar surfaces in frog and turtle lungs is easily demonstrated both in the silver nitrate and Zenker-formol fixed preparations after they are stained respectively with hematoxylin eosin-azure and Mallory's phosphotungstic hematoxylin. The nuclei of the flattened cells are usually located in the meshes of the capillaries. Intratracheally silver-injected preparations show a system of black lines on the surfaces of the alveolar septa continuous with the limits of the cuboidal cells which line the main air passages.