Morphometric analysis of the remodeling of the rat pulmonary epithelium during early postnatal development

In the rat lung the alveoli are formed in the early postnatal period. Within 3 weeks the alveolar surface area increases more than five-fold and the saccular lung reaches its mature structure. In order to analyse the changes in the compartments of the parenchymal tissue, the lungs of three animals per group aged 4, 13, and 21 days, respectively, were investigated by means of electron microscopic morphometry. Whereas the capillarisation of the interalveolar septa almost doubled in the period investigated, the endothelium and alveolar epithelium increased their share of the septal tissue at the expense of the interstitial compartment. The cellularity of the interstitium decreased markedly with age, the ratio of cells to interstitial spaces falling from 5.1 to 1.7. Within the epithelium, the type II cell population increased its mass almost six times, representing 58% of the alveolar epithelial volume at 3 weeks. Accordingly, the portion of alveolar surface area covered by type II cells was highest on day 21, with 9% versus 5.6% on day 4, and around 3% in a normal adult lung. The observed quantitative structural changes are discussed in the light of the functional demands of the organ: the need for an expanding alveolar surface area to be covered by a thin epithelial lining layer, the maintenance of a thin air-blood barrier, and a quantitatively adapted secretion of alveolar surfactant.