Morphological Study On The Lung Of The Yak Fetal

Lung tissues from40yak fetuses of different age group were collected toinvestigate their development process by using the methods of histology and histochemistry. Theresults showed that the developmental period of yak lung can be divided into five stages.①Embryologic stage (30～50days): a lung bud was found. Its branches formed into the primarybronchi, and then into the lobar bronchus. The bud had the pseudostratified columnar epitheliumand was full of glycogen.②Pseudoglandular stage (50～120days): the bronchial tree developedapparently and its terminal bud shaped like the gland. The bud possessed the pseudostratifiedcolumnar epithelium characterized with the nuclei locating in the top of the epithelial cells. Mostof the terminal buds were surrounded by a thin smooth muscle which circled by mesenchymecells，and the elastic fibers would be found in late developmental period of terminal bud.③Canalicular stage (120～180days): the respiratory region grown rapidly, no glandular structurewould be found in terminal buds. There existed lots of tubular branches with the single-walledcolumnar epithelium or cuboid epithelium. Only a few of glycogen would be found in theepithelium.④Saccular stage (180～220days): terminal sac (original alveolar) was formed by theridge of dissepiment, and some primitive alveoli epitheliums differentiated into flat type I alveolarcells and cubical type II alveolar cells.⑤Alveolar stage (220～260days): the alveolar wasformed. Many of epitheliums differentiated into flat type I alveolar cells and cubical type IIalveolar cells, and even more elastic fibers of alveolar ducts would be found. It would beconcluded from the present study that the developmental process of lung in yak fetuses is similarto that in cattle, while the yak has a longer saccular stage and a shorter alveolar stage. The resultssuggest that the lung of yak fetus matures even earlier than that of cattle.