Morphological analysis of the postnatally developing marsupial lung: The quokka wallaby

We investigated the events that take place during the postnatal morphogenes is of the lung of the quokka wallaby, Setonix brachyurus, using the light m icroscope and both the scanning and transmission electron microscopes. The lung of term, newborn babies (joeys) at 3-days of postnatal life was at lat e canalicular stage and comprised large airways and tubules separated by th ick mesenchymal interstitium. The tubules were lined by a low cuboidal epit helium but had few portions with true gas exchange barrier where capillarie s came into close contact with squamous type of epithelium. By the fifth da y postpartum, the lung entered the early saccular stage characterised by la rge air sacs, thinner septa, a better developed double capillary system and conversion of the cuboidal epithelium into a squamous one of type I cells interrupted by groups of cuboidal type II cells with lamellar bodies. Trans itory respiratory bronchioles were recognisable toward the end of this stag e. Formation of secondary septa started by Day 15, dividing the saccules in to several generations of smaller air spaces. There were alternating and co ncurrent periods of tissue proliferation and air space expansion, followed by septal thinning. Alveolization started from about 125 days postpartum wh en the first burst of small sized air spaces bounded by septa with a single capillary layer were encountered. By Day 180 the process of alveolization was completed with only occasional septa showing a double capillary system and by Day 210 postnatally, the lung resembled that of an adult. For the fi rst time in a mammal, the canalicular stage was encountered postnatally dur ing lung development. Anat Rec 262: 253-265, 2001. (C) 2001 Wiley-Liss, Inc .