Postnatal Lung Development and Modulation of Lung Growth

The human lung at birth is not simply a miniaturized version of the adult one. A newborn lung has to undergo very marked structural changes in order to match adult morphology. We know that between 17 and 50 million alveoli are present at birth, whereas the lung of a man of 75 kg body weight contains around 300 million alveoli. Furthermore, the pulmonary microvasculature is completely restructured in the early postnatal period. Although this process markedly transforms septal morphology, very little is known about it. The late E. A. Boyden, an expert in structural pulmonary development, whose careful three-dimensional reconstructions of the developing lung represented pioneering work, seriously argued that the ‘alveoli’ counted at birth were saccules1. Therefore the alveolar stage of lung development has been placed after birth (Fig. 4.1). Recently Langston and co-workers2 have published a study on 42 fetal lungs showing that shallow alveoli were present sometimes as early as the 32nd or 34th week. At birth they counted on average 50 million alveoli, a finding which renewed the discussion about the degree of alveolization of the newborn human lung. Practically, this would mean that the human lung could be born in a more mature state than assumed so far. Nevertheless, these new data do not overthrow the fact that the bulk of alveoli develop after birth. Indeed, on average only 15–20% of their final number is present in the first days of postnatal life.