Morphometric analysis of the postnatally developing marsupial lung: the gray short-tailed opossum (Monodelphis domestica)

The postnatal lung development of the gray short-tailed opossum (Monodelphis domestica) was investigated to assess the morphofunctional status in this immature born marsupial. Lung volume, surface densities and surface areas, parenchymal (air spaces and septa) and non-parenchymal (airways, blood vessels, connective tissue) volume proportions were determined in Micro-CT images using morphometry. The lung of the neonate was at the canalicular stage and consisted of large terminal air spaces and a rudimentary bronchial tree. The absolute lung volume was only 0.002 cm³ with an air space surface density of 145 cm^-1 and a surface area of 0.301 cm². The rapid development of the lung was indicated by an increase in the septal proportion of the parenchyma around day 4. Between day 4 and 7, the lung entered the saccular stage with a reduction in septal thickness and development of a double capillary system. The subsequent saccular period was characterized by repetitive steps of septation, increasing the number of airway generations. Between day 28 and 35 the lung entered the alveolar stage, accompanied by microvascular maturation leading to single capillary septa, resulting in a marked increase in lung volume and surface area. The lung of the newborn Monodelphis domestica is structurally and quantitatively sufficiently developed to meet the respiratory requirements of the small neonate, however, supported by cutaneous respiration during the first postnatal days. The lung has to mature rapidly in the postnatal period to support the increasing metabolic requirements of the developing young.