Introduction

Development of the lower respiratory tract begins on day 22 and continues to form the trachea, lungs, bronchi, and alveoli. The process divides into five stages: embryonic, pseudoglandular, canalicular, saccular, and alveolar stage. Although the process begins early on in fetal development, complete maturation does not take place until the child is approximately 8 years of age. This developmental delay is vital in premature babies where their survival is intricately linked to which developmental stage their respiratory tract has reached at the time of birth.

Development

Embryonic Stage – 3-6 weeks

The respiratory diverticulum appears on the ventral wall of the primitive foregut endoderm, posterior to the pharynx.  The ventral wall rostral to the lung buds now pinches off from the dorsal wall to form a parallel tube which, after caudal elongation, forms the trachea anteriorly and the esophagus posteriorly. Following the fourth week of development, the caudal end of the trachea bifurcates to form the left and right primary bronchial buds which continue to grow into the adjacent layer of splanchnopleuric mesoderm-derived pleural mesenchyme. The formation of the primary bronchial buds marks the beginning of branching morphogenesis. At the end of the fifth week, the primary bronchial buds divide asymmetrically to form the secondary bronchial buds, two on the left and three on the right, which will give rise to future lobes of each lung. The final round of branching in the embryonic stage occurs at the end of the sixth week of development with the division of the secondary bronchial buds into tertiary bronchial buds on each side which will eventually give rise to the bronchopulmonary segments of the mature lung.

Parietal and visceral pleura of the lungs form from somatopleuric and splanchnopleuric layers of mesoderm respectively during weeks 5 to 7. These pleuroperitoneal will later extend caudally and fuse with the posterior edge of the septum transversum to close the thoracic cavity and separate it from the abdominal cavity below.

At the end of the embryonic stage, the larynx, trachea, lung primordia, lobe of the lungs and the bronchopulmonary segments have formed.

Pseudoglandular Stage – 5-17 weeks

This stage is primarily responsible for the generation of the bronchial tree. The cuboidal epithelium lends the lungs to look similar to exocrine glands, which loosely embed in the extensive pleural mesenchyme in this phase of development. During this stage, the tertiary bronchial buds undergo extensive branching morphogenesis to form the first 20 generations of the respiratory tree in humans by the end of week 16.[1]  Differentiation of the respiratory epithelium begins in the proximal airways to form cilia on the surface of the columnar epithelial cells. The splanchnopleuric mesoderm begins to differentiate to give rise to the intrapulmonary arteries, which branch in a similar yet more extensive fashion to the bronchi, supportive cartilage in the airways and surrounding layer of alpha actin.[2]

At the end of this stage, the respiratory tree has developed as far as the terminal bronchioles, with the formation of an arterial system, cartilage, and smooth muscle. As the respiratory bronchioles have not developed yet, infants born at this stage will not be able to facilitate gas exchange and hence unable to survive.

Canalicular Stage – 16-25 weeks

This stage marks the division between the conducting and respiratory units in the respiratory tree. Elongation and growth of the existing terminal bronchioles form an acinus composed of respiratory bronchioles, each of which gives rise to 3 to 6 alveolar ducts. Intensive angiogenesis of the splanchnic mesenchyme in the mesodermal tissue surrounding the acinus creates a dense capillary network that begins to form the blood-air barrier. During week 20, lamellar bodies begin to appear in the cytoplasm of the cuboidal type II pneumocytes lining the distal epithelium. Lamellar bodies store pulmonary surfactant, composed of lipids and Surfactant Proteins A-C, before exocytotic release into the alveoli.[3] There is little differentiation of type II pneumocytes into squamous type I pneumocytes, which will later form the structural epithelium of alveoli.

At this stage, some respiration is possible due to the formation of the gas exchanging portion of the lungs; therefore, infants born at this stage can survive if provided with intensive care. However, they often do not survive due to the lack of surface area for gas exchange and limited production of pulmonary surfactant by type II pneumocytes.[1]

Saccular Stage – 24 weeks-birth

During this stage, the gas-exchange surface area of the lungs significantly expands. Growth of the terminal airways reduces the amount of surrounding mesodermal tissue and forms clusters of enlarged airspaces known as terminal sacs or ‘saccules.’ Each saccule is separated by thick primary septa containing a double capillary network, with a central connective tissue layer. Maturation and differentiation of type II pneumocytes into type I pneumocytes results in thin-walled terminal sacs. Capillaries invade the thin walls of the sacculi to form the blood-air barrier composed of type I pneumocytes, thin basement membrane of the capillaries, and endothelium, producing a functional surface for efficient gas exchange.

Pulmonary surfactant production begins at 24 weeks; however, the production of adequate amounts to prevent atelectasis is not until 32 weeks. Therefore infants born after 32 weeks have a much higher chance of survival than those born at 24 weeks.

Alveolar Stage – 36 weeks – 8 years

Prior to birth, immature alveoli appear as bulges from the sacculi which invade the primary septa. As the sacculi continue to increase in size, the protrusions in the primary septa become larger; these new longer and thinner septations are known as secondary septa and are responsible for the final division of the respiratory tree of sacculi into alveoli. Septation occurs at sites where there is increased fibroblast activity and secretion of collagen and elastin fibers into the interstitium. The process of alveolar division continues until 3 years of age, with the majority of divisions occurring within the first 6 months. To create a thinner diffusion barrier. The double-layer capillary network fuse into a single network, each one closely associated with two alveoli as maturation progresses.

Until the third year of life, enlargement of lungs is a consequence of the increasing number of alveoli; after this point, both the number and size of alveoli increases until the mature lungs form at around 8 years of age.

Cellular

The endoderm of the primitive foregut forms the respiratory epithelium.

Splanchnopleuric layer of lateral plate mesoderm forms the connective tissue, muscle, vasculature, and bronchial cartilage of the respiratory tract.

Neural crest cells of the ectoderm form the laryngeal cartilage.

Molecular Level

The molecular signals regulating lung development are complex and involve members of BMP, EGF, FGF Hedgehog, TGFB, WNT families, and retinoic acid. Nkx2.1 encodes a transcription factor essential in lung development; The Wnt/beta-catenin pathway regulates Nkx2.1 expression in ventral endoderm of the foregut. Inactivation of Wnt2a, Wnt2b, or beta-catenin causes lung aplasia.[4]

Branching within the respiratory tree is caused by reciprocal interactions between the ventral endoderm of the foregut and the adjacent splanchnopleuric mesoderm. FGF10 is a crucial ligand in facilitating this process by inducing the formation of the trachea from the lung primordia, as well as the formation and subsequent branching of the primary bronchial buds. FGF10 signals to the FGF2b receptor in the mesenchymal cells at the tips of the bronchial buds to stimulate mitosis in the adjacent endoderm, thus stimulating its outgrowth.[5] Additionally, FGF10 expressing mesenchyme distal to a branching tip is solely responsible for generating airway smooth muscle. Conversely, Sonic Hedgehog in the ventral foregut endoderm stimulates proliferation of the adjacent mesenchyme; hence the absence of either Shh or FGF10 from embryos prevents branching morphogenesis of the primary bronchial buds.[4]

Mechanism

During fetal development, the placenta acts as the sole exchange surface. Hence the pulmonary circuit plays a minor role before birth. Oxygenated, nutrient-rich blood enters the placenta via the umbilical vein and into the inferior vena cava, bypassing the liver through a shunt known as ductus venosus. A rudimentary valve at the opening of the inferior vena cava shifts the blood from the right atrium to the left atrium through the foramen ovale and septum secundum, before passing into the aorta to supply the brain. The deoxygenated blood from the brain returns into the right atrium via the superior vena cava and subsequently gets shifted to the right ventricle. Typically, the deoxygenated blood would enter the pulmonary circulation via the pulmonary arteries, however as the lungs are not necessary for respiration during gestation, the blood is diverted into the aorta through the ductus arteriosus and returns to the placenta via the umbilical arteries.

Upon cutting the umbilical cord at birth, and the baby takes its first breath, the lungs rapidly expand to open the pulmonary circuit. The increased pressure in the left atrium due to the return of oxygenated blood pushes the septum primum against the septum secundum to close the foramen ovale. The increase in the blood flow and oxygen levels soon cause the ductus arteriosus and ductus venosus to constrict and close. This process takes approximately 6 hours and creates a circulation that is well oxygenated with little carbon dioxide.[6] Hence the pulmonary circuit is functional and now essential for survival.

Pathophysiology

Reduced pulmonary segments or terminal sacs, commonly known as pulmonary hypoplasia, is due to insufficient lung fluid during gestation. Lung fluid is produced by epithelial cells as early as 6 weeks and plays a vital role in maintaining fetal lung expansion.[7] Once inhaled during fetal breathing movements, peristaltic waves shift this fluid distally resulting in the stretching of the lower airway, subsequently stimulating the release of various growth factors involved in the regulation of lung development. Decreased volumes of lung fluid result in reduced branches in the pulmonary tree and fewer terminal sacs, thus compromising the structural integrity of the lungs. Pulmonary hypoplasia usually occurs secondary to a defect which limits the pleural cavity, consequently reducing lung expansion and adversely affecting development. Commonly associated abnormalities include musculoskeletal defects of the chest wall, congenital diaphragmatic hernia, and oligohydramnios due to renal agenesis. 

Surfactant deficiency is the most common cause of neonatal respiratory distress syndrome. Pulmonary surfactant production is via type II pneumocytes and is responsible for maintaining lung surface tension. It spreads as a thin layer over the air-liquid barrier of alveoli, thus preventing atelectasis at the end of expiration. Approximately 90% is formed by phospholipids (phosphatidylcholine, phosphatidylglycerol, phosphatidylethanolamine) with the remaining 10% formed by proteins. These are primarily four surfactant proteins (surfactant proteins A-D), each with its particular function; surfactant protein A and D (SP-A, SP-D) function in innate immunity. Hence degradation of these proteins may increase susceptibility to lung inflammation and infection. Surfactant protein B and C (SP-B, SP-C) contribute to the surface properties of surfactant. SP-B and SP-C organize the surfactant protein into tubular myelin, which is essential in reducing surface tension. Production of pulmonary surfactant begins as early as 24 weeks, however the production of sufficient amounts does not occur until late gestation, thus the risk of neonatal respiratory distress syndrome is inversely proportional to gestational age; the risk decreases from 60% at less than 24 weeks gestation to less than 5% for those infants born after 34 weeks [8]. Infants typically present in the first few hours of life, with tachypnoea, hypoxia, hypercapnia, breathing difficulties, cyanosis, and nasal flaring.[6]

Esophageal atresia is a blind pouch in the esophagus and is commonly associated with an abnormal communication with the trachea known as a tracheoesophageal fistula. This condition occurs when the laryngotracheal tube fails to bud off to form the esophagus dorsal to the trachea. Possible causes include the failure of the esophageal endoderm to proliferate rapidly during the fourth to fifth weeks of embryonic development due to mutations in the SHH signaling pathway. Tracheoesophageal fistulas are commonly associated with polyhydramnios as the abnormal communication with the trachea prevents the fetus from swallowing, thus impairing its ability to return the amniotic fluid to the maternal circulation. Infants typically present with coughing, gagging or choking when attempts to feed are made, and may also have bluish skin.

Clinical Significance

Tracheoesophageal fistula is detectable on prenatal ultrasounds around 20 weeks; however, as the scan has a sensitivity of 26%, it is commonly diagnosed after birth when the previously mentioned symptoms first present. Surgical correction is the preferred method of treatment and is usually successful in preventing recurrent fistulas and/or tracheomalacia. The most common post-operative complication occurring within 48 hours is an anastomotic leak which can resolve with a course of antibiotics and close monitoring.[9]

For those mothers at risk of preterm delivery, corticosteroids should be provided from 23 to 34 weeks gestation to accelerate lung maturation and surfactant production. After birth, the infant can receive an exogenous surfactant preparation; the surfactant preparations can be natural, such as those derived from animal lungs or human amniotic fluid or entirely synthetic. Current guidelines state that in preterm infants with respiratory distress syndrome, a natural surfactant preparation should be administered. Additionally, the infant is given oxygen, prophylactic antibiotics for sepsis, and intubated if necessary.[10]