Human lung development is normally divided into five distinct stages of organogenesis, which describe the histologic changes that the lung undergoes during morphogenesis and maturation of its structural elements (1,2). These five stages include the embryonic, pseudoglandular, canalicular, saccular, and alveolar stages of lung development. Human lung development is initiated during the early embryonic period of gestation (3 to 7 weeks postcoitum) as a small saccular outgrowth of the ventral foregut called the respiratory diverticulum. During the subsequent pseudoglandular stage of lung development (5 to 17 weeks postcoitum), formation of the conducting airways, that is, the tracheobronchial tree, occurs by elongation and repetitive branching of the primitive bronchial tubules. Vascularization of the surrounding mesenchyme with formation of the air-blood barrier, that is, the alveolar-capillary membrane, occurs during the canalicular stage of lung development (16 to 26 weeks postcoitum). Cytodifferentiation of bronchiolar and alveolar epithelial cells is also initiated during this stage. Enlargement and expansion of the peripheral air spaces occur during the saccular stage of lung development (24 to 38 weeks postcoitum), resulting in the formation of primitive sac-like alveoli and thick interalveolar septa. Formation of thin secondary alveolar septa and remodeling of the capillary bed occur during the alveolar stage of lung development (36 weeks postcoitum to 8 years of age), giving rise to the mature alveolar organization of the adult lung (Fig. 26.1).

The human lung is a derivative of the primitive foregut endoderm and the surrounding splanchnic mesoderm (3). The primitive respiratory diverticulum appears at 3 weeks of gestation as an enlargement of the caudal end of the laryngotracheal sulcus located in the median pharyngeal groove, which is an outgrowth of the ventral wall of the primitive esophagus. During the 4th week of gestation, the respiratory diverticulum enlarges and subdivides into the left and right primary bronchi (see Fig. 26.1A and B). The primitive lung grows caudally, expanding into the mesenchyme surrounding the primitive foregut, while the trachea becomes separated from the esophagus by a band of mesenchymal tissue called the tracheoesophageal septum. Between 4 and 5 weeks of gestation, the left and right primary bronchi subdivide to produce secondary, or lobar, bronchi (see Fig. 26.1C and D). Further subdivision of lobar bronchi into tertiary or segmental bronchi occurs during the 6th week of gestation, with the lung taking on a lobulated appearance as the segmental buds are formed (see Fig. 26.1E and F). The developing respiratory tract is lined by endodermally derived epithelium that forms the conducting airways and alveoli. The surrounding mesoderm is composed of mesenchymal cells that will differentiate into connective tissue components, including blood and lymphatic vessels, fibroblasts, smooth muscle cells, and cartilage.

Preacinar blood vessels first appear at the end of week 4 (4,5). Pulmonary arteries arise from the sixth pair of aortic arches and grow into the mesenchyme, where they accompany the developing airways, segmenting with each bronchial subdivision. Pulmonary veins develop as outgrowths of the left atrium of the heart and subdivide several times before connecting to the pulmonary vascular bed. Intra-acinar vessels develop later, in parallel with alveolar formation.

The pseudoglandular stage of fetal lung development extends from about 5 to 17 weeks of gestation and is marked by the formation of the bronchial portion of the lung. This occurs through a process known as branching morphogenesis (6), during which the segmental tubules of the developing lung undergo repetitive lateral and terminal dichotomous branching to form the primitive bronchial tree (see Fig. 26.1G and H). By week 17 of gestation, the segmental bronchi have subdivided to produce approximately 23 generations of bronchial tubules ending in the terminal bronchioles. These bronchial tubules are lined initially by a pseudostratified columnar epithelium containing large pools of glycogen. A prominent basement membrane underlies the epithelium, and mesenchymal cells adjacent to these tubules differentiate into fibroblasts, which become organized in a circumferential orientation, perpendicular to the long axis of the tubules. As branching progresses, pseudostratified columnar epithelium is reduced to a tall columnar epithelium, especially in distal regions of the bronchial tree. During this period, cytodifferentiation of the airway epithelium occurs in a centrifugal direction with ciliated, nonciliated secretory, goblet, neuroendocrine, and basal cells appearing first in the more proximal airways (7,8). Cartilage, smooth muscle cells, and mucous glands are also found in the trachea during the pseudoglandular stage of development and extend as far as the segmental bronchi.

The canalicular stage of fetal lung development extends from week 16 to 26 of gestation. By the end of week 16, the terminal bronchioles have divided into two or more respiratory bronchioles that have subdivided into small clusters of short acinar tubules and buds lined by cuboidal epithelium. These structures undergo further differentiation and maturation to become the adult respiratory unit, or pulmonary acinus, consisting of the alveolated respiratory bronchiole, alveolar ducts, and alveoli. Clusters of acinar tubules and buds continue to grow by lengthening, subdividing, and widening at the expense of the surrounding mesenchyme (Fig. 26.2A). This peripheral growth is accompanied by the formation of intraacinar capillaries, which align themselves around the air spaces, establishing contact with the overlying cuboidal epithelium. During this stage of lung development, type II epithelial cell differentiation occurs in acinar tubules with formation of intracellular multivesicular bodies and multilamellar bodies, the storage form of pulmonary surfactant phospholipids (9). Type I epithelial cell differentiation occurs in conjunction with development of the airblood barrier, wherever endothelial cells of the developing capillary system come into contact with the overlying epithelial cells.

During the saccular stage of fetal lung development, which extends from week 24 to 38 of gestation, the terminal clusters of acinar tubules and buds begin to dilate and expand into thin, smooth-walled, transitory ducts and saccules that later become the true alveolar ducts and alveoli of the adult (Fig. 26.2B). During this stage, there is a marked reduction in the amount of interstitial tissue. Intersaccular and interductal septa develop, which contain

a delicate network of collagen fibers and the intra-acinar capillary bed. Near the end of this stage, elastin is deposited in regions in which future interalveolar septa will form (10). Increasing amounts of tubular myelin, the secretory form of pulmonary surfactant, are seen in the air spaces.

The alveolar stage, which extends from 36 weeks of gestation to between 2 and 8 years of age, is the last stage of lung development and is marked by the formation of secondary alveolar septa, which partition the transitional ducts and saccules into true alveolar ducts and alveoli (Fig. 26.2C and D). This process of alveolarization greatly increases the surface area of the lung available for gas exchange. At the beginning of this period, the secondary interalveolar septa consist of short buds or projections of connective tissue that contain a double capillary network and interstitial cells that are actively synthesizing collagen and elastic fibers. By 5 months of age, these secondary interalveolar septa have lengthened and thinned, and now contain only a single capillary network. Although definitive alveoli can be found in the human lung by 36 weeks of gestation, 85% to 90% of all alveoli are formed within the first 6 months of life (11). Overall the number of alveoli increases by about sixfold between birth and adulthood, that is, from an average of 50 million alveoli in the term lung to 480 million (range: 274 to 790 million) in the adult human lung (12). After the first 6 months of life, alveolar formation occurs at a slower pace until about 2 to 8 years of age, when further growth of the lung becomes proportional to growth of the body (13). The surface area available for gas exchange, and its diffusion capacity, increases linearly with body weight up to about 18 years of age (13). The conducting airways also increase in length and diameter, while airspace and capillary volumes increase coordinately at the expense of interstitial volume.

Each of these stages of lung development includes distinct changes in tissue organization and cellular differentiation that are important for subsequent growth and maturation of the lung. Structural and functional defects in lung development at birth can often be traced to arrested or aberrant development during one of these periods of organogenesis, often as a result of mutations in genes critical for patterning and growth of the lung, such as the GLI3 gene (Pallister Hall syndrome) (14), which is a component of the Sonic Hedgehog signaling pathway, and the fibroblast growth factor receptor, FGFR2, gene (Pfeiffer, Apert, and Crouzon syndromes) (15,16) that is required for FGF signaling. Lung morphogenesis is determined by complex interactions among cells in the splanchnic mesenchyme and foregut endoderm that are regulated by multiple signaling pathways and transcriptional mechanisms (17). Developmental anomalies of the lung occur through defective division and differentiation of the lung bud, of the left or right bronchial bud or of the trachea and esophagus. Pulmonary agenesis, tracheal and bronchial malformations, tracheoesophageal fistulas, ectopic
lobes, bronchogenic, cysts, and cystic pulmonary adenomatoid malformations (CPAM) arise during the embryonic and pseudoglandular stages of lung development (2). Defects in lung growth and morphogenesis underlie common clinical disorders associated with pulmonary hypoplasia or dysplasia, such as congenital diaphragmatic hernia (CDH) and alveolar capillary dysplasia with misalignment of pulmonary veins (ACD/MPV). Pulmonary hypoplasia can be caused by a reduction of space within the pleural cavity, usually as a consequence with another primary developmental defect, such as CDH or by a reduction in the amount of amniotic fluid following premature rupture of membranes or in association with renal dysgenesis Potter syndrome (18,19). RDS and bronchopulmonary dysplasia (BPD) are associated with premature birth at a time when biochemical functions (e.g., surfactant production) and structural functions (e.g., elasticity) of the lung are still underdeveloped.

The unique physical-chemical boundary between the alveolar gases and the highly solvated molecules at the apical surface of the respiratory epithelium generates a region of high surface tension produced by the unequal distribution of molecular forces among water molecules at an air-liquid interface. Surface-active material at this interface in the alveoli provides surface tension-lowering activity that contributes to the remarkable pressure-volume associations characteristic of the lung. This surface-active material, called surfactant, has been subject to intense study in recent decades (9,28,29).

Deficiency or dysfunction of pulmonary surfactant plays a critical role in the pathogenesis of respiratory diseases in the newborn period. Pulmonary surfactant exists in a variety of physical forms when isolated from the alveolar wash of the lung. These physical forms include lamellated and vesicular forms and highly organized tubular myelin. Tubular myelin is highly surface active and, although composed predominately of phospholipids, its unique structure depends on Ca2+ and lung surfactant proteins A (SP-A), B (SP-B), and D (SP-D). Tubular myelin represents the major extracellular pool of surfactant from which a lipid monolayered/multilayered film is generated to produce an interface between the hydrated cellular surfaces and alveolar gas (Fig. 26.3). Lamellated and vesicular forms of surfactant represent nascent or catabolic forms of surfactant material, respectively; the latter is taken up by type II epithelial cells and recycled. Surfactant proteins A, B, C, and D play important roles in the organization and function of the surfactant complex regulating surfactant homeostasis. Alveolar surfactant concentrations are tightly controlled by a variety of mechanisms that modulate lipid and protein synthesis, storage, secretion, and recycling (30,31).

RDS, previously called hyaline membrane disease, is a common cause of morbidity and mortality associated with premature delivery. RDS is a developmental disorder rather than a disease process per se, and it is usually associated with premature birth. The incidence and severity of RDS generally increase with decreasing gestational age at birth and are usually worse in male infants. Infants of diabetic mothers with poor metabolic control and infants born after fetal asphyxia, maternofetal hemorrhage, or after pregnancies complicated by multiple births are at higher risk for RDS. RDS affects approximately 20,000 to 30,000 infants each year in the United States and complicates about 1% of pregnancies. Approximately 50% of the infants born between 26 and 28 weeks of gestation develop RDS, whereas fewer than 20% to 30% of premature infants at 30 to 31 weeks have the disorder.