The Maturation of Airway Structure and Function (2002)

After completing this article, readers should be able to: Airway formation is one of the earliest events in the development of the respiratory system. However, continued maturation of the airway continues well into postnatal life. With advances in the medical care of increasingly preterm infants, an understanding of developmental airway physiology becomes imperative. This article reviews maturation of the airway and its components as well as their contributions to airway function during development.Airway development in the human begins during the fourth week of gestation when the respiratory diverticulum, or lung bud, branches from the embryonic foregut. The esophagotracheal septum forms and separates the foregut into the esophagus dorsally and the trachea ventrally. Thus, the developing airway is of foregut endodermal origin. Elongation of the respiratory diverticulum forms the trachea; branching forms the main stem bronchi. Growth and elongation continues in the caudal direction under the influence of airway secretions and physical forces. By the end of the 16th week of gestation, branching of the conducting airways is complete.The airway matures initially in the trachea and proceeds distally. Tracheal cartilage begins to form during the seventh week of gestation. Maturation of the distal airway cartilage is not complete until after birth. Epithelial differentiation begins in the trachea during the 10th week of gestation. Together with phasic contractions of the fetal airway smooth muscle, which are present following the 23rd gestational week, lung fluid secreted from the epithelial cells promotes growth and development of the respiratory system. Fetal lung fluid is a stable, isotonic solution that has a low protein content and a high concentration of chloride ions. In addition to maintaining the lung in an expanded state, an important role in respiratory development, fetal lung fluid keeps the lungs free of amniotic fluid and clears the airway lumen of mucus and cellular debris.It is well established that mature airway smooth muscle behaves tonically. However, throughout fetal development, airway smooth muscle displays a rhythmic, phasic activity similar to that of the gastrointestinal tract. This phasic activity produces peristaltic waves that begin in the trachea and proceed distally. It has been proposed that these waves displace the fetal lung fluid distally, creating a positive, distending pressure in the developing airways and playing a vital role in lung and airway morphogenesis.In addition to the difference in tonic versus phasic physiology displayed by mature and immature airway smooth muscle, respectively, these tissues also differ in their functional phenotype. Mature airway smooth muscle has been described as possessing a “contractile” phenotype, in which the major proteins expressed are related to the predominantly contractile activities of this tissue. In contrast, fetal airway smooth muscle has been characterized as possessing a “synthetic/proliferative” phenotype. In these fetal cells, protein expression and cellular activity are focused on proliferation and secretion at the expense of contractile protein expression. Similar descriptions have been attributed to other smooth muscle tissues.Unlike skeletal muscle, which undergoes terminal differentiation into a mature phenotype, smooth muscle is capable of modulating between mature and immature phenotypes. In fact, the changes seen in the vascular smooth muscle of atherosclerotic plaques have been described as a regression of the vascular smooth muscle to its fetal phenotype. It is thought that a similar event may occur in airway disease states, where airway smooth muscle shifts from its mature “contractile” phenotype to an immature “proliferative” or “synthetic” phenotype. This would correlate with the airway remodeling that is characteristic of chronic airway diseases.The airway tree is a branched conducting system whose major functions include delivery, distribution, and conditioning of gas to the gas exchange units of the lung. The lower airway is comprised of three primary components: epithelium, cartilage, and smooth muscle. The epithelium lines the entire length of the airway. From the trachea to the large bronchioles, it is composed of a pseudostratified ciliated columnar epithelium, referred to as respiratory epithelium. The pseudostratified nomenclature applies because the respiratory epithelium is one cell layer thick, and all cells attach to the basement membrane. However, not all cells extend completely to the airway lumen, which gives the histologic appearance of multiple cell layers. Beyond the large bronchioles, there is a gradual transition to a simple ciliated columnar epithelium and finally a cuboidal epithelium. The epithelium is composed of approximately eight different cell types, the primary one of which is columnar epithelial cell, which contains a layer of cilia on its apical surface. Other cell types include mucus-secreting cells, brush cells, small granule cells, and basal cells. The brush cells are columnar cells with extensions of the plasma membrane termed microvilli. The basal surfaces of these cells have synaptic contact (epitheliodendritic synapse) and are considered receptor cells. Small granule cells are a class of enteroendocrine cells seen in the gut and gut derivatives. Some of these cells are associated with nerves and function in reflex arcs. Basal cells are cuboidal cells whose nuclei are located adjacent to the basement membrane. They are the reserve population of cells whose function is to replace old or displaced cells, enabling the epithelium to regenerate.Cartilage is present from the trachea to the bronchioles. In the trachea, cartilage exists as C-shaped rings that are open posteriorly. In the bronchi, the cartilage forms plates that encompass the entire airway circumference. At more distal airway generations, the cartilage plates become smaller and more discontinuous, gradually disappearing before the bronchioles. Like the airway cartilage, the airway smooth muscle varies along progressive airway generations. In the trachea, the airway smooth muscle is confined to the trachealis muscle. The trachealis, along with fibroelastic tissue, bridges the gap between the tips of the C-shaped cartilage rings and forms the posterior wall of the trachea. In contrast, bronchial airway smooth muscle forms a complete circumferential layer, which gradually diminishes and becomes discontinuous at lower generations.A mucosal layer covers the inner surface of the trachea. The mucosa consists of an epithelial layer supported by a basement membrane. The basement membrane is part of a loose connective tissue layer referred to as the lamina propria. The lamina propria is very cellular, containing lymphocytes and lymphatic tissue, plasma and mast cells, eosinophils, and fibroblasts. Also included within the mucosal layer are glands that send ducts to the epithelial surface. Below the mucosa lies the submucosa, a connective tissue layer containing the distributing blood vessels, lymphatics, and mucus-secreting glands. The submucosa ends when it blends into the perichondrium of the cartilage rings. The outer layer of the trachea consists of adventitia, which binds the trachea to adjacent structures, such as the esophagus, neck musculature, and mediastinal structures. The adventitia contains the large blood vessels and nerves that supply the components of the tracheal wall.The primary innervation of the trachea is cholinergic from the vagus nerve. Vagal stimulation results in contraction of airway smooth muscle. Additionally, there is nonadrenergic-noncholinergic innervation of the trachea, which mediates both bronchoconstrictive and bronchodilatory actions through mediators such as nitric oxide, vasoactive intestinal peptide, neurokinins, and substance P. There is little, if any, direct sympathetic innervation of the airway. Sympathetic control is primarily through circulating catecholamines.The primary function of the airway is to serve as a conduit for gas delivery to the respiratory apparatus. In addition to facilitating the transfer of gas, the airway conditions the gas so it is heated to body temperature and saturated with water vapor when it reaches the gas exchange units. The airway also plays a defensive role against inhaled pathogens. Airway secretions, such as immunoglobulin A and mucus, inactivate and trap inhaled particles. Cilia then participate in transporting the particles to the upper airway, where they are expelled.Among the airway epithelium functions are defense mechanisms and modulation of airway smooth muscle function. Although the epithelium is capable of releasing both contractile and relaxant mediators, its primary effect involves smooth muscle relaxation. Nitric oxide appears to be the primary epithelial neurotransmitter and has two major functions on airway smooth muscle. By activating the secondary messenger guanylate cyclase, nitric oxide results in both smooth muscle relaxation and a reduction in smooth muscle proliferation. The influence of the airway epithelium on tracheal smooth muscle is present both in immature and mature airways (Fig. 1).Airway cartilage also participates in airway function. The cartilage confers rigidity to the airway wall, affecting airway distensibility and compressibility. This has been demonstrated experimentally in animal models by exposing cartilage to papain. Following softening of the cartilage, airway resistance increases. Additionally, the C-shaped cartilage rings of the trachea apply a distending force to the posterior wall and the trachealis muscle. This distending force contributes to the intrinsic afterload seen by airway smooth muscle in vivo, thereby influencing airway caliber.The airway smooth muscle is the primary airway component that influences airway function. Embryonically, phasic contractions of airway smooth muscle modulate and maintain a positive transmural pressure in the respiratory system, facilitating development. Postnatally, the tonic nature of mature airway smooth muscle regulates the functional characteristics of the airway. When airway smooth muscle contracts in the lower airways, it narrows the airway lumen and increases the resistance to airflow. Although the large airways in the adult contribute approximately 80% of the total airway resistance, alterations in the remaining 20% by changes in the diameter of smaller airways and bronchioles significantly modulate resistance changes. This has a significant effect on airflow under both normal and pathologic conditions. Developmentally, the proportion of large to small airway contribution is approximately 50:50. This emphasizes why changes in both respiratory compliance and resistance in the neonate are strongly suggestive of both upper and lower airway involvement.Although airway smooth muscle contraction is believed to increase lower airway resistance by decreasing the luminal diameter, trachealis contraction actually has been demonstrated to decrease tracheal resistance. This phenomenon is due to multiple mechanisms. Resistance of the airway to collapsing forces is influenced by manipulation of the tracheal cartilage. Trachealis contraction causes the ends of the tracheal cartilage to come together and overlap, essentially creating a complete cartilaginous ring around the entire circumference of the trachea. This results in a more rigid structure that is less susceptible to collapse. Because there is a direct relationship between the rigidity of a tube and the flow that it can support, decreasing the compliance of the trachea allows it to support higher flows. In addition, trachealis contraction affects both the elastic and viscoelastic properties of the trachea. Elastic properties are responsible for dimensional changes that are in phase with the pressure changes causing the deformation; viscoelastic properties involve dimensional changes that are out of phase with the pressure changes. Trachealis contraction stiffens the posterior wall, making it more rigid and less compliant. By increasing the wall tension of the airway, the LaPlace relationship states that an increased collapsing pressure would be required for deformation. This decrease in compliance is important in making the trachea less susceptible to collapse and distension.In addition to influencing the elastic properties of the trachea, smooth muscle contraction also alters the viscoelastic properties. In response to a rapid volume increase, intratracheal pressure increases to a peak, followed by decay to a new steady state. This decay has been characterized into three phases. The first phase is a rapid decay and is due to collagen. The second phase is a longer and slower phase, representing the smooth muscle. The third phase is the final steady state, representing the passive stress inherent in the elastin component. The relaxation time constant characterizes the time required for dissipation of peak pressures to 63% of a new steady-state plateau following a volume increase. The relaxation time constant is decreased by trachealis contraction, and the hysteresis is increased. Taken together, these smooth muscle-induced changes allow the trachea to function more like a rigid tube that is less vulnerable to collapse or distension and reduces the resistance to air flow.Considerable research has been devoted to studying the effects of development on airway structure and function. Maturation of the airway occurs from its conception, through parturition, and well into postnatal development. Structurally, there is an obvious increase in airway dimensions. Concomitantly, the content and dimensions of airway cartilage and smooth muscle also increase. However, there is no developmental change in the ratio of contractile to connective tissue within the trachealis muscle or in the proportion of smooth muscle in the posterior wall of the trachea.In addition to developmental differences in structure, there are similar differences in both the passive and active aspects of airway function. Airway compliance and relaxation time constants are increased in prematurity and decrease with maturation (Figs. 2 through 4). It has been proposed that the increased compliance of preterm airways is due to immaturity of the individual components of the airway. For example, cartilage from premature airways is more compliant than that from mature airways (Fig. 5). Additionally, the properties of paracrine responses to local neuropeptides have been shown to change with development in some species, resulting in a developmental decrease in the relaxant effect of these peptides, which may contribute to the maturational decrease in airway compliance. Furthermore, the characteristics of airway smooth muscle change with development; the smooth muscle becomes stiffer with maturation. The stiffening in the posterior wall contributes to the developmental decrease in compliance. In addition to affecting the elastic properties of the airway, the developmental changes in airway smooth muscle also affect the airway’s viscoelastic characteristics. The increased tone of mature muscle results in a developmental decrease in the relaxation time constant. A shorter relaxation time constant allows the airway to recover to its plateau tension more rapidly following a volume change.The airway undergoes maturational changes in active, force-generating properties as well. The maximal force-generating capacity of the tracheal smooth muscle increases with development (Fig. 6). This increase in functional performance allows the mature airway to be more capable of withstanding distending pressures than an immature airway.The maturation of multiple parameters may be responsible for the maturation of airway function. Developmental changes at the cellular level may alter cellular function, which affects tissue and overall airway function. Age-related differences in the contraction velocities of individual smooth muscle cells would likely affect the performance of both the tissue and organ. Driska et al demonstrated that isolated adult sheep airway smooth muscle cells contracted at a faster velocity than neonatal cells. However, the rate of shortening was similar between adult and preterm cells, with newborn cells being slower than either of these age groups.In addition to developmental changes of the airway at the tissue and cellular levels, changes may be occurring at the molecular level. Research is ongoing to understand the changes in contractile protein expression and signal transduction that may contribute to the developmental changes in airway smooth muscle and influence overall airway function in both health and disease.The two primary contractile proteins in airway smooth muscle are actin and myosin. We focus on the developmental expression of myosin isoforms. Structurally, myosin is a hexameric protein that is composed of two pairs of light chains and two heavy chains (Fig. 7). Both pairs of light chains are located in the head region of the molecule. The 20-kDa regulatory light chain (LC20) possesses the site of phosphorylation by myosin light chain kinase. There are multiple known isoforms of the regulatory light chain. The 17-kDa essential light chain (LC17) consists of two isoforms: LC17a and LC17b. Their isoelectric points allow for this differentiation. Myosin can be separated functionally into the head and the rod domains. The head is the motor domain. The rod domain, or tail, is divided into two parts by the hinge region: light meromyosin and the S2 fragment.There are four known myosin heavy-chain isoforms. These variations all are derived from a single gene and result from alternative splicing of the mRNA at the 5′ N-terminal head region or the 3′ C-terminal tail region. Splicing at the N-terminal region of the mRNA results in the insertion or omission of a 21-nucleotide (7-amino acid) insert in the head region of the myosin heavy-chain molecule. The inserted isoform is referred to as SMB; the noninserted form is referred to as SMA. Additionally, splicing of the mRNA at the C-terminal tail region results in either a 204-kDa isoform (SM1) with a 43-amino acid carboxyl terminus or a 200-kDa isoform (SM2) with a 9-amino acid C-terminus. Combinations of the head and tail isoforms result in four possible myosin heavy-chain isoforms (Fig. 7).The head regions of the myosin heavy chains possess the actin-activated magnesium-ATPase activity of the molecule. The 7-amino acid insert, which begins at amino acid 212 of the myosin heavy chain molecule, is located within the 25- to 50-kDa junction of the primary sequence. In the tertiary structure of the protein, this insert is located in a flexible loop segment near the ATP-binding pocket of the myosin heavy chain. Thus, the location of the insert confers functional significance. Due to its proximity to the ATP-binding site, the insert may affect ATP binding, hydrolysis, or release of the products of hydrolysis: ADP and inorganic phosphate. It is believed that ADP release is the rate-limiting step of the cross-bridge cycle. Studies examining the functional influence of the insert have suggested that its presence confers a higher enzymatic activity to the myosin.In addition to altering enzymatic activity at the molecular level, the presence of the insert may influence the functional phenotype of the smooth muscle tissue. The SMB myosin heavy chain isoform, with its N-terminal head insert, occurs primarily in phasic smooth muscle; the noninserted SMA isoform is found predominantly in tonic smooth muscle. Although mature airway smooth muscle is a multiunit smooth muscle that normally displays tonic characteristics, the presence of the inserted isoform has been reported in the trachealis.The C-terminal tail region of the myosin heavy chain is an alpha-helix. The alpha-helices of two heavy chains are wound around one another in an alpha-helical coiled coil. Acting through hydrophobic interactions, this tail region participates in filament formation. The two tail isoforms (SM1 and SM2) play a role in filament formation and overall molecular function that is unclear. The SM1 isoform of smooth muscle myosin heavy chain is constitutively expressed throughout development in a number of species; the SM2 isoform often appears in late gestation or after birth. Although the relative proportion of the two tail isoforms has been examined in a number of tissues and species, the functional ramifications of tail isoform expression presently are unclear.Considerable progress has been made over the past several decades in understanding the age-related differences in airway We are to the of the individual components of the airway and the they play in the maturation of airway function. 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