Histological Studies on the Respiratory Tissue in the Dolphin Lung

Abstract

In the terminal portion of the bronchial tree of the dolphin lung the bronchioli, bronchioli terminales, respiratorii and alveolar ducts were identified. The simple cuboidal epithelium of the bronchioli respiratorii contained blood capillaries which appeared as if exposed directly to the bronchiolar lumen. In the wall of the bronchioli, down to the distal end of the bronchioli respiratorii, there existed numerous small cartilages. Numerous well-developed sphincters occurred especially in the wall of the bronchioli terminales and respiratorii at regular intervals and the bronchiolar lumen formed many large round dilations (ampullae) between the sphincters, which were communicated by constricted portions (isthmi) induced by the sphincters.
Each alveolar septum of the dolphin lung possessed two independent respiratory surfaces facing the neighboring alveolar lumina, which were completely separated by a thin continuous axial connective tissue layer. Both respiratory surfaces have their proper blood capillaries and alveolar epithelial cells respectively.
The cuboidal alveolar epithelial cells were classified into small and large ones, of which only the former protruded the thin cytoplasmic projection covering the alveolar surface of the blood capillaries. The large cytoplasm-rich cells might be transformed into round alveolar phagocytes storing a large amount of fat droplets and scant dust particles which they might have probably taken up from the blood and inhaled air respectively. Similar transformation and fat storing were also sometimes observed in the simple cuboidal epithelium of the bronchioli. These epithelial alveolar phagocytes were easily detached from the alveolar wall into the alveolar lumina. Besides, there existed mesenchymal histiocytes; those in the connective tissue of the alveolar septa and in the small lymphoid tissue found in the bronchiolar mucosa stored mainly dust particles, while those in other connective tissue portions stored mainly fat droplets as did the alveolar phagocytes. Thus the amount of fat droplets stored in epithelial and mesenchymal phagocytes exceeded in general by far that of dust particles, the latter being as compared with that in other common mammals, extraordinally small. These morphological facts suggested on the one hand that the dolphin lung may participate much in fat metabolism and on the other that the air contamination on the sea surface, where the dolphin lives, may be very low.
In the dolphin lung the development of the elastic tissue was not particularly good. The thick elastic strands found in the wall of the alveolar duct contained few smooth muscle fibers. The functional significances of the bronchiolar cartilages and sphincters were also discussed.