The development of the lung in mammals: an analysis of concepts and findings.

To evaluate one model of mammalian-lung development, i.e., division into periods, pre- and postnatal lung development in the CPB-S mouse strain was divided into the currently distinguished periods: the pseudoglandular period, covering establishment of the air-conducting portion; and the canalicular, terminal-sac, and alveolar or postnatal periods, in which the respiratory portion develops. The last three periods would each cover the formation of a different component of the respiratory unit or pulmonary acinus (acinus pulmonaris) (nonalveolated respiratory bronchiole, nonalveolated duct and sac, and alveolar pouch). However, determination of the nature of the relevant structures on the basis of recent findings concerning the epithelia showed that these hypotheses are not tenable. Since the tubule with cuboidal epithelium (appearing in the pseudoglandular and following periods) is the basic structure in the genesis of the pulmonary acinus, the development of the respiratory portion must start in the pseudoglandular period. Likewise, since the definitive components of the acinus are derived from this acinar tubule, their establishment may not be restricted to one of the other periods. Because other postulated divisions of mammalian-lung development were based on similar histological interpretations, they cannot reflect the course of mouse-lung development either. Therefore, a developmental scheme based on the recent findings concerning the epithelia is given as well as a tentative scheme for the human lung. The respiratory portion proved to develop by budding of acinar tubules, the mode of budding being not restricted to any particular pattern.