Structural Aspects of Postnatal Lung Development – Alveolar Formation and Growth

Abstract

The human lung is born with a fraction of the adult complement of alveoli. The postnatal stages of human lung development comprise an alveolar stage, a stage of microvascular 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, alveolar formation occurs rapidly and is typically very conspicuous 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 important role in supporting gas exchange. In view of reports that alveoli are added after the stage of microvascular maturation, the question arises whether the present concept of alveolar formation needs revision. On the basis of morphological and experimental findings we can state that mature lungs contain all the features needed for ‘late alveolarization’ by the classical septation process. Because of the high plasticity of the lung tissues, late alveolarization or some forms of compensatory alveolar formation may be considered for the human lung.

Perinatal Activation of the Interleukin-33 Pathway Promotes Type 2 Immunity in the Developing Lung

“…occurs through the second postnatal week in mice and between 2 and 3 years in humans, a time frame where many atopic children also become allergic (Burri, 2006;Wahn and von Mutius, 2001). We here found highly increased production of the epithelial cytokine IL-33 and accumulation of innate type 2 cells during the alveolar period when the lung is maximally remodeling.…”

Antenatal inflammation and lung injury: prenatal origin of neonatal disease

“…5 The embryonic development of the lung is a well-defined sequence of proliferation and differentiation, which has been histologically characterized. 6 The different phases of gestational lung development is shown in Figure 1. The developing lung histologically resembles an exocrine gland during the pseudoglandular phase from 6 to 16 weeks.…”

“…The developing lung histologically resembles an exocrine gland during the pseudoglandular phase from 6 to 16 weeks. 6 By 16 weeks, all major elements of the lung have formed except those involved in gas exchange. Respiration is therefore not possible.…”

“…Preterm babies who develop BPD are usually born when the lung development is in the canalicular and/or early saccular phase. 6 The later phases of lung development occur postnatally in the presence of injurious stimuli such as resuscitation, oxygen toxicity, sepsis and persistent ductus arteriosus. 7 The preterm babies who were described by Northway and colleagues some 40 years ago were born in a later phase of lung development.…”

Role for ephrinB2 in postnatal lung alveolar development and elastic matrix integrity

“…Mice are born with lungs composed of simple tissue walls, termed primary septa, comprising a double capillary layer and interstitial cells encased by an epithelium. During subsequent development, the lung airspace rapidly becomes subdivided by means of insertion of vascularized tissue folds, termed secondary septa, which eventually mature to form the alveolar walls (Burri, 2006). Motive force is provided by interstitial myofibroblasts, which express contractile proteins in concert with secondary septum formation (Adler et al, 1989;Jostarndt-Fogen et al, 1998;Wagner et al, 2003;Yamada et al, 2005).…”