ABSTRACT
The lung appears as a ventral diverticulum from the endodermal foregut in the fourth week after ovulation. The complete lining epithelium of the lung is derived from the endoderm. This bud is formed within the splanchnic mesoderm surrounding the gut and the dorsal aorta; it is from this mesenchyme that the airway walls and blood vessels are derived. A division produces the left and right bronchi by 26-28 days of gestational age and segmental airways are present by 6 weeks. Further division of airways into the surrounding mesenchyme continues until the end of the pseudoglandular stage (17 weeks of gestation) by which time all pre-acinar airways to the level of the terminal bronchiolus are present. The majority of divisions occur during the tenth to fourteenth weeks of gestation5
(Figures 1.1 and 1.2). During the canalicular period (16-27 weeks of gesta-
tion) the pre-acinar airways increase in diameter and length. The peripheral airways continue to divide to form the prospective respiratory bronchioli (two to three generations in humans) and beyond these the prospect-
airways thins and capillaries come to lie beneath the epithelium of the peripheral airways, apparently causing the epithelium to become thinner. The larger airways (prospective bronchi) are lined by columnar epithelium, but the distal bronchioli are lined by cuboidal cells. At the level of the prospective respiratory bronchioli, part of the wall is lined by flattened cells, as are the prospective alveolar ducts which at this stage are sac shaped (saccules). By 20-22 weeks of gestation, type I and II alveolar epithelial cells can be identified lining all saccular air spaces. The type I cells are flat and elongated and cover the majority of the surface. The type II cells maintain a cuboidal shape and develop lamellar bodies around 24 weeks of gestation, which is 4-5 weeks before surfactant can be detected in the amniotic fluid. By the end of the canalicular stage, the air to blood barrier is thin enough to support gas exchange (about 0.6 m) but the gas exchange units are the large thin-walled saccules. True alveoli develop later (p. 7).
