Lung Organogenesis

Abstract

Developmental lung biology is a field that has the potential for significant human impact: lung disease at the extremes of age continues to cause major morbidity and mortality worldwide. Understanding how the lung develops holds the promise that investigators can use this knowledge to aid lung repair and regeneration. In the decade since the “molecular embryology” of the lung was first comprehensively reviewed, new challenges have emerged—and it is on these that we focus the current review. Firstly, there is a critical need to understand the progenitor cell biology of the lung in order to exploit the potential of stem cells for the treatment of lung disease. Secondly, the current familiar descriptions of lung morphogenesis governed by growth and transcription factors need to be elaborated upon with the reinclusion and reconsideration of other factors, such as mechanics, in lung growth. Thirdly, efforts to parse the finer detail of lung bud signaling may need to be combined with broader consideration of overarching mechanisms that may be therapeutically easier to target: in this arena, we advance the proposal that looking at the lung in general (and branching in particular) in terms of clocks may yield unexpected benefits.

Introduction

The concept that lung organogenesis is instructed by coordinated mesenchymal-to-epithelial crosstalk originates in the classical recombination experiments of Alescio and Cassini (1962), in which replacing tracheal mesenchyme with mesenchyme from the lung periphery induced ectopic branching of tracheal epithelium in murine embryonic lung organ culture. This idea was extended in an early review by Warburton and Olver (1997) to include the coordination of genetic, epigenetic, and environmental factors in lung development, injury, and repair. Thereafter, a molecular basis of lung morphogenesis was attempted by Warburton et al. (2000). Over the last decade, significant progress has been made in this field as reviewed by Cardoso and Lu, 2006, Maeda et al., 2007, and others. Nevertheless, the ultimate goal remains as stated by Warburton and Olver (1997), “to devise new rational and gene therapeutic approaches to ameliorate lung injury and augment lung repair … the ideal agent or agents would therefore mimic the instructive role of lung mesenchyme and would correctly induce the temporospatial pattern of lung-specific gene expression necessary to instruct lung regeneration.” To this overall strategy, we can now add (i) the modulation of lung mechanobiology to favor appropriate lung regeneration and (ii) the stimulation of endogenous stem/progenitor cells or supply of exogenous ones for lung regeneration. Therefore, the current review draws together three important strands of information on lung organogenesis as of April 2010: (i) molecular embryology of the lung, (ii) mechanobiology of the developing lung, and (iii) pulmonary stem/progenitor cell biology. Applying advances in these complementary areas of research to lung regeneration and correction of lung diseases remains the therapeutic goal of this field. With the recent human transplanation of a stem/progenitor cell-derived tissue-engineered major airway (Macchiarini et al., 2008), we can clearly see the potential of this field, while recognizing the many problems yet to be solved.

Before concentrating on the molecular biology, mechanobiology, and stem cell biology of the lung, a first step in regenerative strategies is to consider the developmental anatomy of the lung. From this, we can at least see what type of structures we need to generate.