Lung Development and Maturation

Embryonic Period

The lung first appears as a ventral bud off the esophagus just caudal to the laryngotracheal sulcus. The grooves between the lung bud and the esophagus deepen, and the bud elongates within the surrounding mesenchyme and divides to form the main stem bronchi ( Fig. 62.1 ). Subsequent dichotomous branching forms to the conducting airways. The branching of the endodermal endothelium is controlled by the underlying mesenchyme, because removal of the mesenchyme stops branching. Transplantation of the mesenchyme from a branching airway to more proximal airway induces airway development in the new location.

The commitment of endodermal cells to epithelial cell lineages requires the expression of families of transcription factors that include thyroid transcription factor-1, forkhead gene family members, and others. At least 15 homeobox domain-containing genes ( HOX genes) also contribute to lung morphogenesis. Multiple other growth and differentiation factors, such as retinoic acid and the fibroblast growth factor family members (FGF)-7 and FGF-10 and their receptors, are temporally and spatially expressed and are critical to early lung morphogenesis and subsequent development. Genetic ablation of these transcription and growth factors causes lung developmental abnormalities that range from trachea-esophageal fistula and altered branching morphogenesis to severe lung hypoplasia and complete aplasia of the lungs. Lobar airways are formed by about 37 days, with progression to segmental airways by 42 days and subsegmental bronchi by 48 days in the human fetus. The pulmonary vasculature branches off the sixth aortic arch to form a vascular plexus in the mesenchyme of the lung bud. Major regulators of vascular development are vascular endothelial growth factor (VEGF) and its receptors in the mesenchyme. Vascular development additionally requires extracellular matrix (fibronectin, laminin, type IV collagen) and other growth factors such as platelet-derived growth factor. The pulmonary artery can be identified by about 37 days, and venous structures appear somewhat later. Abnormalities in early lung embryogenesis cause trachea-esophageal syndromes, branching morphogenesis abnormalities, and aplasia.

Pseudoglandular Stage

The 15-20 generations of airway branching occur in the pseudoglandular period of lung development from about the fifth to the eighteenth week, when airway branching is complete ( Fig. 62.2 ). The developing airways are lined with simple cuboidal cells that contain large amounts of glycogen. Neuroepithelial bodies and cartilage appear by 9-10 weeks. Ciliated cells, goblet cells, and basal cells are in the epithelium of proximal airways by 13 weeks. In general, epithelial differentiation is centrifugal in that the most distal tubules are lined with undifferentiated cells with progressive differentiation of the more proximal airways. There are multiple regulators of branching morphogenesis such as FGF-10, FGF-7, endothelial growth factor, transforming growth factor-α, and their receptors. Upper lobar development occurs earlier than lower lobe development in animals, and a similar pattern of development probably occurs in humans. Early in the pseudoglandular stage, the airways are surrounded by a loose mesenchyme with the developing vasculature and capillaries. Pulmonary arteries grow in conjunction with the airways, with the principal arterial pathways being present by 14 weeks. Pulmonary venous development is in parallel but with a different pattern that demarcates lung segments and subsegments. By the end of the pseudoglandular stage, airways, arteries, and veins have developed in the pattern corresponding to that found in the adult.

Canalicular Stage

The canalicular stage between 16 and 25 weeks’ gestation represents the transformation of the previable lung to the potentially viable lung that can exchange gas ( Fig. 62.3 ). The bronchial tree has completely branched, and respiratory bronchioles are forming. The three major events during this stage are the appearance of the acinus, epithelial differentiation with the development of the potential air-blood barrier, and the start of surfactant synthesis within recognizable type II cells. The acinus in the mature lung is the tuft of about 6 branching generations of respiratory bronchioles, alveolar ducts, and alveoli originating from a terminal bronchiole (see Fig. 62.2 ). This saccular branching is the critical first step for the development of the future gas exchange surface of the lung. The mesenchyme surrounding the airways becomes more vascular and more closely approximated to the airway epithelial cells ( Fig. 62.4 ). Capillaries initially form as a double capillary network between future airspaces and subsequently fuse to form a single capillary. With fusion of the vascular and epithelial basement membranes, a structure comparable to the adult air-blood barrier forms. If the double capillary network fails to fuse, the infant will have severe hypoxemia resulting from alveolar-capillary dysplasia. The total surface area occupied by the air-blood barrier begins to increase exponentially toward the end of the canalicular stage, with a resultant fall in the mean wall thickness and an increased potential for gas exchange.

Epithelial differentiation is characterized by proximal to distal thinning of the epithelium by transformation of cuboidal cells into thin cells that line tubes. The tubes grow both in length and in width with attenuation of the mesenchyme, which is simultaneously becoming vascularized. During the canalicular stage, many of the cells would best be characterized as intermediary cells, because they are neither mature type I nor type II epithelial cells. These epithelial cells develop characteristics of mature type II cells such as lamellar bodies, supporting the concept that type I cells are derived from type II cells or intermediary cells that then further differentiate into type I cells. After about 20 weeks in the human fetus, cuboidal cells rich in glycogen begin to have lamellar bodies in their cytoplasm. The transcription factors TTF-1, FOXa1, FOXa2, and GATA 6 mediate type II cell differentiation. The glycogen in type II cells provides substrate for surfactant synthesis as the lamellar body content increases. In the adult human lung, the thin type I cells occupy about 93% of the alveolar surface versus 7% for type II cells. About 8% of lung cells are type I cells, and about 16% of lung cells are type II cells.

Saccular and Alveolar Stages

The saccular stage encompasses the period of lung development during the potentially viable stages of prematurity from about 24 weeks to term. The terminal sac or saccule is the developing respiratory bronchiole or alveolar duct that is elongating, branching, and widening prior to the initiation of alveolarization at about 32 weeks in the fetal human lung. Alveolarization is initiated from these terminal saccules by the appearance of septa in association with capillaries, elastin fibers, and collagen fibers (see Fig. 62.4 ). Shallow alveolar structures with crests (or septa) with elastin at the free margin of the crests can be identified by 28 weeks’ gestation in the human. Alveolar numbers increase rapidly from about 32 weeks’ gestation to term when the human lung contains between about 50 and 150 million alveoli. For comparison, the adult human lung has about 500 million alveoli. The most rapid rate of accumulation of alveoli occurs between 32 weeks’ gestational age and the first months after term delivery ( Fig. 62.5 ). The potential lung gas volume and surface area increases from about 25 weeks’ gestation to term. This increase in lung volume, and the surface area of sacculi, establishes the anatomic potential for gas exchange and thus for fetal viability. There is a wide range of lung volumes and surface areas at a given gestational age. Therefore, the gas exchange potential of different fetuses at the same gestational age will be determined in part by the structural development of the lung. Human newborns born at 22 or 23 weeks’ gestation can have a sufficiently mature lung structure to support gas exchange, indicating that structural lung maturation can be induced at very early gestational ages. Since antenatal corticosteroids increase survival at these early gestational ages, their use must support maturation of this early gestational potential for gas exchange.

Alveolarization progresses rapidly from late fetal to early neonatal life. The traditional view was that alveolar development was completed by early childhood and that new alveoli do not develop in adults. Newer evidence in rats using 3-D visualization with high-resolution synchrotron radiation x-ray tomographic microscopy demonstrated that new alveoli continued to be formed well into adulthood. Using hyperpolarized helium MRI, Narayanan et al. demonstrated that alveolar formation continues until at least 20 years of age. This potential for continued alveolar growth explains why many preterm infants with interrupted lung development at birth can have relatively good lung function later in life.

A number of factors that can stimulate or interfere with alveolarization have been identified ( Box 62.1 ). Chronic mechanical ventilation of preterm animals using modest tidal volumes and low oxygen exposures interrupts both alveolarization and vascular development ( Fig. 62.6 ). Mechanical ventilation of the saccular lung disrupts elastin at the developing crests and induces an “arrest in development” characterized by lack of secondary crest formation and resulting in fewer and larger alveoli without much fibrosis or inflammation ( Fig. 62.7 ). Preterm infants who have died after long-term ventilation or after the development of bronchopulmonary dysplasia (BPD) also have decreased alveolar numbers and an attenuated microvasculature with less prominent airway injury and fibrosis than in the past. Factors that likely contribute to this arrest of lung development include many of the components of care of preterm infants. Antenatal glucocorticoid treatments in monkeys and sheep cause thinning of the interstitium and an increased surface area for gas exchange with delayed alveolar septation. Postnatal glucocorticoid treatments of the saccular lung also interrupt alveolarization and capillary development. Hyperoxia or hypoxia and poor nutrition can interfere with alveolarization. In transgenic mice, overexpression of proinflammatory mediators in the pulmonary epithelium interferes with alveolar development. Antenatal lung inflammation associated with chorioamnionitis in sheep causes delayed alveolar and microvascular development. Thus, multiple factors may contribute to delayed alveolarization in preterm infants. Because lung growth following the completion of alveolarization is by increase in airway and alveolar size, any event that decreases alveolar number could impact lung function as the individual ages. However, the growth potential of the human lung is remarkable, and infants with mild BPD had normal alveolar numbers at 10 to 14 years of age as estimated by hyperpolarized helium MRI. The residual lung structural abnormalities in the lungs of infants who have survived severe BPD remain to be characterized.