Morphological Development of the Human Fetal Lung

Abstract

The morphological development of the human fetal lung was studied analysing a total of 198 human fetal lungs, including lungs of 16 embryos. The weight of the lung increased progressively from 0.7 gm at 9-12 weeks of gestation to 45.2 gm at 37-40 weeks, and the right lung constituted 54 % of total lung weight. The lung, fixed in the inflated state with negative pressure of 24-25 em H20 , increased rapidly in its volume at 24-25 weeks of gestation, coinciding with the development of saccules. Reconstruction of the embryonal lungs revealed the presence of lung buds at 4 weeks, stem bronchi at 5 weeks, segmental bronchi at 6 weeks, and subsegmental bronchi thereafter. Histological characteristics were analysed with special emphasis on sequential development of the functioning respiratory units. The period of 4-8 weeks was characterized by budding of large bronchi and appearance of pleura and lobar pattern. During the period of 9-16 weeks, conducting airways were laid down associated with development of cilia and mucous glands. At the period of 17 -28 weeks acinar canals were proliferating and were followed by flattening of epithelium and vascularization. Well developed saccules appeared by the end of this period. During the period of 29-36 weeks, progressive thinning of septa was noted, accompanied by segmentation of saccules by developing secondary crests. As a result well developed alveoli with delicate septum and a single capillary system could be identified at the period of 37 -40 weeks. The height of the epithelium at the terminal buds showed gradual decrease from 36 f1 m at 5 weeks to 5.8 J1 m at 21 weeks of gestation.