Morphometric analysis of postnatal lung development in the gray short‐tailed opossum ( <i>Monodelphis domestica</i> ): An ultrastructural study

An ultrastructural morphometric analysis of the postnatal development of the lung in the gray short-tailed opossum (Monodelphis domestica) has been conducted to evaluate the morphofunctional status of this poorly developed marsupial lung immediately following parturition. Allometric scaling of the volumes of the parenchymal components against body mass is carried out. In the parenchymal septa, volume proportions of pulmonary capillaries and solid septal tissue components (type I and type II epithelial cells, capillary endothelium, and interstitium) as well as alveolar and capillary surface densities are determined in transmission electron microscopic images using morphometry. Most parameters show a positive correlation with body mass over postnatal development. The lung of the neonate is at the late canalicular stage and consists of large terminal airspaces. The thick airspace septa are lined by cuboidal epithelium, interspersed with respiratory capillaries, and have a massive interstitial layer. A mature surfactant system is present at birth. The volume density of type II epithelial cells, which contain surfactant producing lamellar bodies, is highest in the late saccular stage, when septal growth is accelerated. The rapid development of the lung is indicated by an increase in the septal proportion of the parenchyma around postnatal day 4 and attainment of the saccular stage between day 4 and 7. The saccular stage, indicated by formation of a double capillary system, is characterized by repetitive steps of septation, increasing the number of saccular generations. By day 28 alveolarization starts and the process of microvascular maturation begins. The reduction of the interstitial tissue causes the two capillary beds to converge; finally, the two capillary layers begin to fuse, leading to a single capillary septum. In adults, capillaries and alveolar epithelia are the predominant septal components; the interstitium is greatly reduced. Despite the immaturity of the gray short-tailed opossum lung, it appears to be structurally and quantitatively sufficient for gas exchange. The structural transformation of the septa is accompanied with an increase in gas exchange surface area and thinning of the diffusion barrier, in order to meet the metabolic requirements of the developing young.