Miller and Marty evidence in nonhuman primates suggests that alveoli increase in number through young adulthood (Hyde et al. 2007). If confirmed, it would expand the sensitive time period dur- ing which disruption of molecular mecha- nisms of alveolar development is of special concern, from the first few years of life until termination of longitudinal growth. Cellular differentiation and repair. With > 40 distinct cell phenotypes represented in the mature lung, undisturbed cellular differ- entiation is important to future lung func- tion. In general, differentiation occurs in a proximal to distal sequence, with the tracheal epithelial cells differentiating first. Thus, iden- tical critical processes involving cell signal- ing in development are occurring at different times, depending on the location proximal to distal in the respiratory tree. Cross-talk between endothelial cells and epithelial cells is essential for differentiation of these cell types. VEGF-A expressed by the lung epithelium is essential to pulmonary cap- illary development. In mice, deficient capil- lary development by selective inactivation of the Vegf-A gene in the respiratory epithelium results in disruption of primary septae for- mation during alveolarization (Gerber et al. 1999; Yamamoto et al. 2007). Yamamoto et al. (2007) also demonstrated that hepato- cyte growth factor (Hgf) expression in the developing endothelium is essential for nor- mal epithelial development. The transcription regulatory protein GATA binding protein 6 (GATA-6), expressed in lung epithelial cells, interacts with TTF1. In mouse models, inhibition of GATA-6 during fetal life inhibited terminal differentiation of respi- ratory epithelial cells (Liu et al. 2002; Yang et al. 2002). Elevated levels of GATA-6 post- natally (when levels normally are decreased) resulted in alterations in alveolar septation causing increased lung volumes and air space enlargement that persisted into adulthood (Liu et al. 2003). Airway resistance and airway and tissue elastance were significantly decreased. This provides an example of morphogenesis dependent on the precise timing of expres- sion of GATA-6. Few studies have examined the impact of environmental chemicals on GATA-6 expression. Clara cells are nonciliated bronchiolar cells and are the principal epithelial cells in the distal airway, whose products include Clara cell secretory protein and some compo- nents of surfactant. Clara cells are important in the metabolism of airborne toxicants due to their high cytochrome P450 (CYP450) content and are particularly sensitive to injury by xenobiotics, in part as a result of their abil- ity to transform xenobiotics to reactive inter- mediates (Massaro et al. 1994). Clara cells are considered multipotent progenitor cells that regenerate airway epithelium after oxidant injury (Evans et al. 1976). Xenobiotic Disruption of Lung Development Chemical exposures that affect expression of important growth regulators can result in more severe effects or even lethality when the exposure occurs during susceptible periods of lung development. Table 1 summarizes these cellular and subcellular impacts, along with associated alterations in lung structure or func- tion (and possible clinical implications) for individual chemicals discussed here. Potential windows of susceptibility for exposure can be inferred by comparing the information in Table 1 with the timeline of key develop- mental periods in humans shown in Figure 2. Below we describe examples of xenobiotics that alter lung development by disrupting essential processes such as branching morpho- genesis, alveolarization, and cellular differen- tiation and repair. hTe congenital diaphragmatic hernia model and nitrofen. Congenital diaphragmatic hernia (CDH) is a serious condition in newborns, with an incidence of about 1 in 3,000 live births in the United States. Many die in utero; morbidity and mortality in the first days after birth are high (Hartman 2004). Although Table 1. Cellular, structural, and functional impacts on lung development of xenobiotics. Structural or functional impact Possible clinical implications References Chemical Nitrofen (2,4- dichlorophenyl-p- nitrophenyl ether) TCDD Nicotine 4-Ipomeanol, naphthalene, 1- nitronaphthalene Ozone Arsenic DEHP 1158 Cellular and subcellular level impacts Down-regulates GATA-6, Wnt7, BMP4, FGF, and RALDH2; decreases RA synthesis; inhibits T3 receptor binding AhR, thyroid hormone Suppresses glycolysis and glycogenolysis, reduces synthesis of phosphorylase and phosphofructokinase, inhibits Na+/K+-ATPase Inhibits bronchiolar cell differentiation and repair Depletes proteoglycan and Fgf2, alters Fgfr1, thinning basement membrane zone Increases ER-α expression, dysregulates matrix genes; β-catenin up-regulates EGFR, L-myc, and AFP Binds to PPARγ, altering airway cell differentiation and surfactant protein production Decreased branching; altered smooth muscle, surfactant, and alveolar septation Delayed lung development, decreased total lung space, increased septal area Slower septal formation, bleb formation, decreased number of alveoli, increased alveolar volume Injury/loss of Clara cells Altered bronchiolar growth (longer/decreased diameter), fewer branches, alters orientation of bronchiolar smooth muscle Altered branching and cell migration, decreased elasticity and structural support Thickened primary septa, fewer/more dilated airspaces, increased type II pneumocytes Pulmonary hypoplasia, immature lung Chronic bronchitis,a decreased functional capacity, chronic obstructive pulmonary disease (?) Increased susceptibility to inhaled toxicants, alteration in surfactant Increased airway hyperreactivity,a emphysema (?), decrease in lung function growth,a asthma inductiona Bronchiectasis,a airway hyperreactivity, lung cancera Bronchopulmonary dysplasia, altered lung mechanics, altered surfactant regulation, asthmaa Brandsma et al. 1994; Leinwand et al. 2002; Major et al. 1998; Mendelsohn et al. 1994; Montedonico et al. 2008; Pongracz and Stockley 2006; Shu et al. 2002; Takayasu et al. 2007a; Wang et al. 2005 Kransler et al. 2009; Rogan et al. 1988a; ten Tusscher et al. 2001 Fanucchi et al. 1997, 2004; Plopper et al. 1994; Smiley-Jewell et al. 1998, 2000 Fanucchi et al. 2006; Galizia and Kinney 1999a; McConnell et al. 2002a; Plopper et al. 2007; Tager et al. 2005a Guha Mazumder 2007a; Hays et al. 2008; Lantz et al. 2009; Petrick et al. 2009; Shen et al. 2007; Smith et al. 2006a; Waalkes et al. 2003; Von Ehrenstein et al. 2005a Bornehag et al. 2004a; Chang and Szabo 2000; Magliozzi et al. 2003; Rosicarelli and Stefanini 2009; Yang et al. 2003; Decreased functional capacity, Kordom 2004; Kordom et al. 2002; Maritz 1988, emphysematous changes 1986, 2002; Maritz and Windvogel 2003 Abbreviations: AFP, α-fetoprotein; AhR, aryl hydrocarbon receptor; BMP4, bone morphogenetic protein 4; EGFR, epidermal growth factor receptor; ERα, estrogen receptor-α; L-myc, lung associated myc oncogene; PPARγ, peroxisome proliferator–activated receptor γ; RALDH2, retinal dehydrogenase 2; T3, triiodothyronine. aIncludes evidence in humans. ovlume 118 | number 8 | August 2010 • Environmental Health Perspectives