Lung development of monotremes: evidence for the mammalian morphotype

The reproductive strategies and the extent of development of neonates differ markedly between the three extant mammalian groups: the Monotremata, Marsupialia, and Eutheria. Monotremes and marsupials produce highly altricial offspring whereas the neonates of eutherian mammals range from altricial to precocial. The ability of the newborn mammal to leave the environment in which it developed depends highly on the degree of maturation of the cardio-respiratory system at the time of birth. The lung structure is thus a reflection of the metabolic capacity of neonates. The lung development in monotremes (Ornithorhynchus anatinus, Tachyglossus aculeatus), in one marsupial (Monodelphis domestica), and one altricial eutherian (Suncus murinus) species was examined. The results and additional data from the literature were integrated into a morphotype reconstruction of the lung structure of the mammalian neonate. The lung parenchyma of monotremes and marsupials was at the early terminal air sac stage at birth, with large terminal air sacs. The lung developed slowly. In contrast, altricial eutherian neonates had more advanced lungs at the late terminal air sac stage and postnatally, lung maturation proceeded rapidly. The mammalian lung is highly conserved in many respects between monotreme, marsupial, and eutherian species and the structural differences in the neonatal lungs can be explained mainly by different developmental rates. The lung structure of newborn marsupials and monotremes thus resembles the ancestral condition of the mammalian lung at birth, whereas the eutherian newborns have a more mature lung structure.