Postnatal Development and Growth of the Pulmonary Microvasculature

Abstract

In the current literature (although not officially sanctioned by the third edition of Nomina Embryologica, 1989) lung development is generally divided into four successive, but partially overlapping, phases, known as the pseudo-glandular, canalicular, saccular, and alveolar stages. Based on recent structural observations, we have advocated the idea that alveolization does not represent the final step in lung development and have proposed the introduction into the nomenclature of a stage of microvascular maturation [1]. As will become evident in this chapter, the process of alveoli formation depends upon the presence of a specific morphology of the parenchymal¹ capillaries, i.e., on the presence of a double capillary network within the intersaccular and interalveolar walls [2–5]. The adult mature interalveolar septa, however, are slender and contain only a single capillary system, allowing gas exchange on both of its faces. The transformation of the postalveolar to the adult pulmonary capillary system therefore requires a stage of microvascular maturation before pulmonary development may be considered to be complete.