Structural Aspects of Postnatal Lung Development – Alveolar Formation and Growth

Abstract

The human lung is born with a fraction of the adult com- plement of alveoli. The postnatal stages of human lung development comprise an alveolar stage, a stage of mi- crovascular maturation, and very likely a stage of late alveolarization. The characteristic structural features of the alveolar stage are well known; they are very alike in human and rat lungs. The bases for alveolar formation are represented by immature interairspace walls with two capillary layers with a central sheet of connective tissue. Interalveolar septa are formed by folding up of one of the two capillary layers. In the alveolar stage, al- veolar formation occurs rapidly and is typically very con- spicuous in both species; it has therefore been termed 'bulk alveolarization'. During and after alveolarization the septa with double capillary networks are restructured to the mature form with a single network. This happens in the stage of microvascular maturation. After these steps the lung proceeds to a phase of growth during which capillary growth by intussusception plays an im- portant role in supporting gas exchange. In view of re- ports that alveoli are added after the stage of microvas- cular maturation, the question arises whether the present concept of alveolar formation needs revision. On the ba-