PERSPECTIVE
Fetal Airway Smooth-Muscle Contractility and Lung Development
A Player in the Band or Just Someone in the Audience?

Kenneth T. Nakamura and Paul B. McCray Jr.

Kapiolani Medical Center for Women and Children, John A. Burns School of Medicine, Department of Pediatrics, Honolulu, Hawaii; and Department of Pediatrics, University of Iowa College of Medicine, Iowa City, Iowa

Lung development proceeds like a well orchestrated symphony, featuring contributions from a variety of members. When the music's over, movements encompassing lung bud formation, branching morphogenesis, epithelial cell differentiation, vascular development, and alveolarization will all have been played. Key instrumentalists include a genetic program implemented by a hierarchy of transcription factors and gradients of growth factors, epithelial- mesenchymal interactions, and circulating hormones, with possible contributions from many physical factors (1). The important contributions of several transcription factors, such as TTF-1, HNF-3 and , HFH-4, N-myc, GATA-6, Gli2 and 3, Shh, Bmps, HOX genes, and many growth factors, including FGF-2, FGF-10, KGF, EGF, TGF-, and PDGF-AA, have been elegantly demonstrated by a variety of techniques, including gene targeting (3, 6). In contrast, the contribution of physical factors, such as fetal airway smooth-muscle contractility, to the sound of the symphony has been more difficult to discern.

	Physical Factors and Lung Development

Several physical factors have been implicated in lung development (14): liquid secretion by the pulmonary epithelium (15, 16), the maintenance of a positive intraluminal pressure by the fetal glottis and upper airways (17, 18), patency of the fetal airways (19, 20), adequate intrathoracic and amniotic liquid volumes (21, 22), and fetal breathing movements (14, 23). In fetal lambs, chronic drainage of tracheal liquid by tracheostomy produces lung hypoplasia, whereas ligation of the trachea induces lung growth and cell maturation (15, 16, 24). Because both the intraluminal liquid volume and pressure are altered by these experimental manipulations, lung liquid secretion and the maintenance of a positive intraluminal pressure may be interrelated in their effects. Fetal breathing movements produced by phasic contraction of the respiratory muscles cause distention of the developing air spaces and increase the intrathoracic liquid volume on inspiration (25). If fetal breathing is stopped by phrenic nerve section or cervical cord lesions, the lungs are hypoplastic (26, 27). Therefore, fetal breathing movements also provide physical signals that influence lung development.

	Spontaneous Contractility of Fetal Airway Smooth Muscle

In this issue of the Journal, Schittny and colleagues present compelling evidence that fetal airway smooth muscle is spontaneously contractile throughout gestation and that this phasic activity is associated with the maintenance of a positive intraluminal pressure (28). Airway smooth-muscle development begins early in gestation. Smooth-muscle cells are present in human fetal trachea, primary, and lobar bronchi by the sixth to eighth week of gestation (29). In fetal rat airways, myoblast-to-smooth-muscle-cell transformation occurs in the mesenchymal condensation next to the primitive bronchi during the pseudoglandular stage (30). Furthermore, early in gestation the airway smooth muscle becomes innervated and responsive to contractile and relaxing stimuli (31, 32). Recent studies of the anatomy of airway innervation in the developing lung show that an extensive plexus of nerve trunks containing nerve bundles, forming ganglia, and Schwann cells ensheathes airway smooth muscle as it envelopes the fetal airways (33, 34).

Several investigators previously noted that airway smooth muscle is spontaneously contractile and pharmacologically responsive from early in fetal life. Lewis first reported fetal airway and air sac smooth-muscle contractility in chick embryos and cultured chick lung and noted that contractions were temperature-dependent, had a frequency of ~ 2/min, and occurred without histologic evidence of innervation (35). Schopper later documented spontaneous contractility of fetal airway smooth muscle and noted similar findings in chick and guinea pig embryos and cultured guinea pig lung tissue (36, 37). Sollman and Gilbert described spontaneous contractility of airway smooth muscle in preparations from puppies and midgestation human fetal lung and reported the pharmacologic responses to adrenergic and cholinergic stimuli and other agents (38). Sorokin observed that contractions of fetal airway smooth muscle were present after two to three days in culture in cultured rat lungs and speculated that the distention of the distal lung units produced by such contractions might influence the development of the respiratory portion of the lungs (39). More recently, several laboratories have turned their attention to this interesting aspect of developmental physiology (31, 40, 41). These reports of similar spontaneous airway smooth-muscle contractility throughout gestation in several species and in many models including embryos, cultured fetal lung explants, and isolated adult smooth-muscle preparations support the notion that such contractions are a physiologic feature of the developing lung.

The contractile activity of airway smooth muscle in the fetal lung is phasic (like gastrointestinal muscle) in contrast to the tonic activity in postnatal lung. This property is retained even in culture. Importantly, in the study by Schittny and colleagues, fetal airway smooth-muscle activity was shown to be peristalsis-like, clearly demonstrating that the luminal liquid was propelled along the length of the airways and distending the distal ends (28). Spontaneous smooth-muscle activity was carefully quantified in this study. Interestingly, although nerve fibers are present in developing lung, spontaneous smooth-muscle activity continues in the presence of atropine or tetrodotoxin (31, 41). Along with evidence of spontaneous activity in vitro, these data indicate independence from central nervous system control. Whether there is any relationship between the secretion of fetal lung liquid and smooth-muscle contractility is unknown.

	Mechanical Stretch as a Growth-Inducing Signal

Spontaneous fetal airway smooth-muscle contractions result in mechanical distortion or stretching of the fetal pulmonary epithelium and mesenchyme (28, 31, 32). How might the effects of stretch and distention produced by fetal airway smooth-muscle contractility influence lung growth and development? Wirtz and Dobbs reported that a single mechanical stretch of alveolar type II cells stimulated release of surfactant and a sustained increase in intracellular calcium, and they hypothesized that phasic distention of the alveolar epithelium is a mechanical signal for surfactant release mediated through calcium-induced exocytosis (42). Changes in lung inflation appear to be an important signal contributing to compensatory lung growth. Pneumonectomy or experimental inflation of the lung induced the expression of the immediate early genes, c-fos and junB, suggesting that they may play an upstream role in the signal transduction pathway leading to compensatory lung growth (43). Perhaps similar mechanisms exist in the fetal lung, where changes in intraluminal pressure produce a mechanical signal, resulting in the release of growth factors, signaling molecules, or the regulation of gene expression.

In the last decade, many in vitro studies have shown that pulsatile mechanical stretching of fetal lung organotypic cultures or cells can induce a number of changes that could impact lung development. For example, intermittant stretching of fetal lung organotypic cultures or isolated fetal cells stimulates prostacyclin production and increases cyclic adenosine monophosphate (cAMP) production while also increasing lung cell growth (44, 45). Mechanical stretch also increases the expression of extracellular matrix components (46), surfactant protein (SP)-A, SP-B, and SP-C mRNAs (46, 47), and parathyroid hormone-related peptide (48). The signal transduction pathways underlying stretch-mediated changes have also been investigated. Evidence suggests that the cellular changes induced experimentally by mechanical strain involve pp60 activation and are mediated by phospholipase C and protein kinase C (49, 50). Thus, there is growing evidence for stretch-induced signals in the developing lung.

	Possible Interventions for the Treatment of Developmental Lung Abnormalities

Although the basic biology of spontaneous fetal airway smooth-muscle contractility remains to be determined, the interplay among physical factors and lung growth has fostered the notion that lung development might be manipulated therapeutically. A significant research effort has been devoted to the study of interventions that specifically alter intratracheal resistance to accelerate lung growth (51). Studies to date have investigated the timing (54) and duration (55, 56) of fetal tracheal occlusion necessary to cause a beneficial effect, employing various animal paradigms (51, 57). These efforts hold the promise that approaches such as fetal tracheal occlusion may be applicable for the antenatal treatment of various conditions associated with hypoplastic lungs (52). In the present study by Schittny and coworkers, it is clearly shown that the active tone of airway smooth muscle maintains a positive intraluminal pressure (28). The authors speculate that such a pressure signal acts as a stimulus to lung growth via the force exerted across the airway wall and adjacent parenchyma. However, while the rates of fetal lung cell division are influenced by local distension, airway pressure is not the sole determinant of fetal lung growth. Biochemical indices of lung maturation, including surfactant maturation, are also influenced by growth factors in lung liquid or systemic hormonal signals (24, 58, 59). Moreover, this carefully orchestrated program (Figure 1), when interrupted by preterm birth, may result in incomplete structural development of the lungs (60).

View larger version (111K): [in this window] [in a new window]   Figure 1.   Hypothetical scheme of lung development. Fetal airway contractility is likely one of the important physical factors influencing lung growth and development; however, its proper place in this complex process awaits future study.

	Areas for Future Studies

This present study, as all good research, raises more questions than it answers. For example, when does airway smooth muscle lose its capacity to generate spontaneous activity? What are the mechanisms underlying spontaneous contractility? What happens to smooth-muscle contractility after tracheal obstruction? Further studies are required to define the links between lung tissue stress, increased growth, structural remodeling, and the endocrine environment (60). For example, in vivo experiments designed to selectively inhibit fetal airway smooth-muscle contractility at different times during gestation could help determine its relative role in influencing normal lung growth and development. Such knowledge will help us to better understand how novel therapeutic approaches such as tracheal occlusion promote increased intraluminal airway pressure and induce lung growth in conditions such as diaphragmatic hernia (57). Further experimental study will provide new insights into how physical forces such as airway smooth-muscle contractility influence lung development.

	Footnotes

Abbreviation: surfactant protein, SP.

(Received in original form May 11, 2000).

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