The laryngo-tracheal diverticulum, which is the original base of the larynx, trachea, bronchi and lungs, can also be called respira-tory diverticulum, is formed by a longitudinal shallow groove in the middle of the endoderm of the caudal wall of the primitive pharynx at the beginning of the fourth week of human embryo. The end of the laryngeal diverticulum develops and expands and divides into two lung buds, later, the left lung bud divides into two branches and the right lung bud divides into three branches, and continues to divide in a dendritic manner. By the 6th month of fetal life, the branches reach about 17 levels, forming the terminal bronchioles and respiratory bronchioles with gas exchange function, alveolar ducts and alveolar sacs; by the 7th month, the number of alveoli increases, the alveolar epithelium begins to differentiate into type I alveolar cells and type II alveolar cells, and type II alveolar cells begin to secrete alveolar surface active substances, at which time the fetus has the function of normal respiration. Branching of the bronchial tree continues after birth, reaching 24 levels of branching in early childhood, when the morphological development of the bronchial tree is truly complete. Pulmonary differentiation There are generally three stages of lung differentiation: the pseudoglandular stage, the tubular stage, and the primitive alveolar stage. The left and right lung buds are constantly branching, forming bronchioles and terminal fine bronchioles, the air-conducting part of the lung. By 17 weeks, the other major components of the lung have largely formed, except for the gas exchange portion. At this time, the trachea is still blind, lined with columnar epithelial cells, and is not yet capable of gas exchange. During weeks 17-25, the tubular phase, the bronchial and terminal bronchial lumen is enlarged. Two or more respiratory bronchioles grow at the end of each terminal bronchus. At the end of the latter, thin-walled vesicles called terminal vesicles are formed, which is the basis of the primitive alveoli; in the tubular stage, the interstitial capillaries develop faster, increase in density, and project into the lumen of the terminal vesicles, and begin to morphologically establish the structural basis for gas exchange between the capillaries and the alveoli. By the primitive alveolar stage (25th week to birth), a large number of terminal alveoli develop further into primitive alveoli. The inner surface of the primitive alveoli is covered with epithelial cells of endodermal origin, which are considered to be the stem cells of the alveolar epithelium. Initially, these epithelial cells are rectangular in shape, and at approximately 26 weeks, some of them become a single layer of flattened epithelium and develop into type I alveolar cells. At the same time, the capillary network surrounding the alveoli within the mesenchyme proliferates rapidly, approaching the alveoli and gradually separating the capillaries from the alveolar lumen with two very thin layers of cells. By the time of birth, the alveoli and capillaries are quite developed, so the fetus has respiratory function at birth. During the late fetal period until about 8 years of age, alveoli still continue to develop and differentiate as the final stage of lung genesis. Later on, the typical mature alveoli are truly developed.