Morphology of the Tracheobronchial Tree and the Route of the Pulmonary Artery in the Fetal Minke Whale (Balaenoptera acutorostrata)

Molecular and statistical studies support the close phylogenetic relation between Cetacea and Artiodactyla. The presence of the tracheal bronchus has been pointed out as one of the common traits between only these groups. Nakakuki (1980) has investigated the mammalian tracheobronchial trees in 50 species based on his new nomenclature, and his study has consequently demonstrated the above-mentioned indication. Therefore, comparative anatomy of the tracheobronchial tree based on the nomenclature seems to be useful for investigations of the cetacean phylogeny. Three pairs of fetal lungs of the minke whale (Balaenoptera acutorostrata) were supplied from the Institute of Cetacean Research, Tokyo (1991) to observe their tracheobronchial trees and the ramification of the pulmonary artery. The fetal tracheobronchial tree consisted of one tracheal, four lateral, four dorsal and one medial secondary bronchi in the right lung, and five or six lateral and four or five dorsal secondary bronchi in the left, using the new nomenclature. The right pulmonary artery crossed the right axial bronchus from the ventral side to the dorsal over the third lateral bronchus. The tracheal bronchus in the fetal minke whale seems to belong to the type II of Nakakuki's nomenclature, and this type is seen in one species of the river dolphins. The other conspicuous traits of the fetal lungs are the defect of the second lateral broncus and the route of the pulmonary artery in the right side. This route is very different from that of many other mammals.