Microanatomy of the lung of the bowhead whale Balaena mysticetus.

The lungs from six bowhead whales harvested by Alaskan Eskimos have been examined with light and electron microscopes. Airways ranging from 1 to 40 mm in luminal diameter are lined by a pseudostratified ciliated epithelium containing numerous mucus-secreting cells. The underlying lamina propria-tela submucosa of these airways contains tubuloalveolar glands, plasma cells, and lymphatic accumulations in addition to both elastic and collagenous fibrillar elements. Cartilage extends to the level of the respiratory airways, but smooth muscle is absent from airways larger than 3 mm, and tubuloalveolar glands are absent from airways smaller than 3 mm. Respiratory airways are lined by pseudostratified, simple cuboidal, and simple squamous epithelia. Alveolar ducts are lined by simple squamous epithelium exclusively. A connective tissue core composed mostly of elastic fibers supports the walls of the alveolar ducts. Neither smooth muscle nor cartilage has been observed in these structures. Alveoli contain the typeical cetacean double capillary bed separated by a thick septum composed mainly of collagenous connective tissue. Alveoli are lined by a simple squamous epithelium similar to that encountered in alveolar ducts and respiratory airways. This epithelium is composed of type I and II pneumocytes closely appressed to an underlying capillary network. The type II pneumocytes contain typical lamellar bodies and tubular myelin can be seen in the air spaces. The lung is surrounded by a thick (X = 2.5 mm) visceral pleura rich in blood vessels and elastic fibers.