Postnatal development of the lung parenchyma in a marsupial: the tammar wallaby.

BACKGROUND: Marsupials are born at an early stage of development, and lung development from an air-sac stage to maturity occurs in the air-breathing environment, the pouch.
METHODS: The morphology of the lung parenchyma in pouch young of the tammar wallaby, Macropus eugenii, at four stages of development (0, 30, 70, and 180 days) after birth and in the adult animal was investigated using light, scanning, and transmission electron microscopy, and developmental changes were documented.
RESULTS: The lung at birth is at the terminal-sac stage of development. In the first 70 days, development involves tissue proliferation and septal development leading to subdivision of the terminal sacs, terminal-sac expansion, and septal thinning. Between 70 and 180 days, when the pouch young make the transition from ectothermy to endothermy, remodelling of the septa and microvascular maturation occurs. There is an overlap with further tissue proliferation and terminal-sac subdivision and expansion. Alveoli are found at 180 days, but they are small in comparison to those in the adult lung.
CONCLUSIONS: Lung growth occurs slowly, and the lung remains at the terminal-sac stage of development during the ectothermic period. True alveoli appear only during the transition from ectothermy to endothermy.