Embryology and Maturation of the Respiratory System

Formation of the Lung Buds and Early Respiratory Development

Around the $4^{th}$ week of embryonic development, the respiratory diverticulum, also known as the lung bud, appears as an outgrowth or evagination from the ventral wall of the foregut. The emergence and specific localization of this lung bud are heavily dependent on an increase in retinoic acid ($AR$), which is synthesized by the adjacent mesoderm. This rise in $AR$ levels induces a positive regulation of the transcription factor $TBX4$. This factor is expressed in the endoderm of the intestinal tube at the specific site where the respiratory diverticulum originates. $TBX4$ is responsible for inducing the formation of the bud as well as regulating the continuous growth and differentiation of the lungs. Consequently, the internal epithelial lining of the larynx, trachea, and bronchi, as well as that of the lungs, is entirely of endodermal origin. In contrast, the cartilaginous, muscular, and connective tissue components of the trachea and lungs are derived from the visceral (splanchnic) mesoderm that surrounds the foregut.

Partitioning of the Foregut and the Tracheoesophageal Septum

Initially, the lung bud maintains open communication with the foregut. However, as the diverticulum expands in a caudal direction, two longitudinal ridges known as the tracheoesophageal ridges begin to separate it from the foregut. Subsequently, these ridges fuse to form the tracheoesophageal septum. This fusion divides the foregut into two distinct portions: a dorsal portion, which becomes the esophagus, and a ventral portion, which becomes the trachea. Despite this separation, the respiratory primordium maintains communication with the pharynx through the laryngeal orifice. The developmental stages illustrate the precise transition from a shared tube to partitioned systems, ensuring the respiratory and digestive tracts are distinct yet connected at the proximal end.

Clinical Correlations: Esophageal Atresia and Tracheoesophageal Fistulas

Abnormal separation of the esophagus and trachea by the tracheoesophageal septum results in esophageal atresia, which may occur with or without tracheoesophageal fistulas ($FTE$). These defects occur in approximately $1/3,000$ births. In $90\%$ of these cases, the proximal portion of the esophagus ends in a blind sac, while the lower segment forms a fistula with the trachea. Isolated esophageal atresia and $H$-type $FTE$ without esophageal atresia each account for approximately $4\%$ of the total defects. Other variations represent about $1\%$ of these malformations. These anomalies are often linked with other congenital defects, including cardiac malformations in $33\%$ of cases. Furthermore, $FTE$ is a component of the $VACTERL$ association, which includes vertebral anomalies, anal atresia, cardiac defects, tracheoesophageal fistula, esophageal atresia, renal anomalies, and limb defects. This clustering of anomalies is of unknown cause but occurs more frequently than would be expected by chance. A significant complication of some $FTE$ cases is polyhydramnios, occurring because amniotic fluid cannot pass into the stomach or intestine after being swallowed. Additionally, at birth, gastric contents or amniotic fluid may enter the trachea through the fistula, triggering conditions such as pneumonitis and pneumonia.

Embryology of the Larynx

The internal lining of the larynx originates from the endoderm, while its cartilaginous and muscular structures derive from the mesenchyme of the fourth and sixth pharyngeal arches. Rapid proliferation of this mesenchyme causes the laryngeal orifice to change from a sagittal slit into a $T$-shaped opening. As the mesenchyme of these arches transforms into the thyroid, cricoid, and arytenoid cartilages, the characteristic adult configuration of the laryngeal orifice becomes recognizable. Around the same time these cartilages form, the laryngeal epithelium also proliferates rapidly, leading to a temporary occlusion of its lumen. Subsequent vacuolization and recanalization create a pair of lateral recesses known as laryngeal ventricles. These recesses are bounded by tissue folds that differentiate into the false and true vocal cords. Because the laryngeal musculature derives from the fourth and sixth arches, all laryngeal muscles are innervated by branches of the tenth cranial nerve, the vagus nerve. The superior laryngeal nerve innervates derivatives of the fourth arch, while the recurrent laryngeal nerve innervates those of the sixth arch.

Development of the Trachea, Bronchi, and Lungs

As it separates from the foregut, the lung bud forms the trachea and two lateral outpocketings called primary bronchial buds. At the start of the fifth week, each of these buds enlarges to form the right and left primary bronchi. The right primary bronchus then generates three secondary bronchi, while the left generates two. This pattern foreshadows the formation of three lobes in the right lung and two in the left. With subsequent growth in caudal and lateral directions, the lungs expand into the body cavity. The available spaces, the pericardioperitoneal canals, are narrow and located on either side of the foregut. These spaces are gradually occupied by the growing lungs. Eventually, the pleuroperitoneal and pleuropericardial folds separate these canals from the peritoneal and pericardial cavities, leaving the remaining spaces as the primitive pleural cavities. The mesoderm covering the exterior of the lung becomes the visceral pleura, while the somatic mesoderm layer covering the interior of the body wall becomes the parietal pleura. The space between these layers is the pleural cavity.

Bronchial Tree Division and Signaling

Development continues as secondary bronchi divide repeatedly in a dichotomous pattern, giving rise to $10$ tertiary (segmental) bronchi in the right lung and $8$ in the left, establishing the bronchopulmonary segments of the adult lung. By the end of the sixth month, approximately $17$ generations of subdivisions have occurred. To reach the final configuration of the bronchial tree, six additional divisions must occur during postnatal life. Branching is regulated by epithelial-mesenchymal interactions between the endoderm of the lung buds and the surrounding visceral mesoderm. Signaling for branching is emitted by the mesoderm and involves members of the fibroblast growth factor ($FGF$) family. As these subdivisions form, the lungs assume a more caudal position, so that by the time of birth, the bifurcation of the trachea coincides with the level of the fourth thoracic vertebra ($T4$).

Maturation of the Lungs and Alveolar Cells

Until the seventh month of gestation, bronchioles undergo continuous division into smaller channels during the canalicular phase, accompanied by a steady increase in vascular supply. Terminal bronchioles divide into respiratory bronchioles, which each divide into $3$ to $6$ alveolar ducts. These ducts end in terminal sacs (primitive alveoli), surrounded by flat alveolar cells in close contact with adjacent capillaries. By the end of the seventh month, the number of alveolar sacs and mature capillaries is sufficient to guarantee adequate gas exchange and survival for a premature neonate. During the final two months of intrauterine life and for several years after birth, the number of terminal sacs increases steadily. The cells lining the sacs, known as type $I$ alveolar epithelial cells (pneumocytes), become increasingly thin, allowing surrounding capillaries to protrude into the lumen. This contact forms the blood-air barrier. Mature alveoli do not exist before birth. Additionally, at the end of the sixth month, type $II$ alveolar epithelial cells develop and synthesize surfactant, a fluid rich in phospholipids that reduces surface tension at the alveolar-capillary interface.

Fetal Environment and the Initiation of Parturition

Before birth, the lungs are filled with a fluid containing high chloride levels, few proteins, some mucus from bronchial glands, and surfactant. Surfactant concentrations increase significantly during the final $2$ weeks before birth. During the $34^{th}$ week of gestation, surfactant enters the amniotic fluid and acts on macrophages in the amniotic cavity. Evidence suggests these "activated" macrophages migrate through the chorion to the uterus, where they begin synthesizing immune system proteins like interleukin $1\beta$ ($IL-1\beta$). The upregulation of these proteins leads to increased prostaglandin synthesis, which triggers uterine contractions. This suggests that signals from the fetus may participate in the initiation of labor. Fetal respiratory movements begin before birth, causing the aspiration of amniotic fluid, which is vital for stimulating lung development and conditioning respiratory muscles.

Respiratory Adaptation at Birth and Postnatal Growth

At birth, most lung fluid is rapidly absorbed by blood and lymphatic capillaries, while a small amount may be expelled through the trachea and bronchi during delivery. Once the fluid is absorbed from the alveolar sacs, surfactant remains as a thin phospholipid layer on the alveolar cell membranes. As air enters the alveoli during the first breath, this surfactant layer prevents the formation of a high-tension air-water interface. Without surfactant, the alveoli would collapse during expiration, a condition known as atelectasis. Postnatal lung growth is primarily due to an increase in the number of respiratory bronchioles and alveoli rather than an increase in the size of existing alveoli. Only about one-sixth of the adult number of alveoli are present at birth. The remainder form during the first $10$ years of postnatal life through the continuous generation of new primary alveoli.

Summary of Lung Maturation Periods

Clinical Perspectives on Respiratory Distress and Malformations

Surfactant is critical for the survival of premature infants. Insufficient surfactant causes high surface tension in the alveolar-capillary membrane, leading to alveolar collapse during expiration and resulting in Respiratory Distress Syndrome ($SDR$). Previously known as hyaline membrane disease, $SDR$ accounts for about $20\%$ of neonatal deaths. Deceased infants' alveoli often contain protein-rich fluid, hyaline membranes, and lamellar bodies. Treatment involves artificial surfactant for the neonate and glucocorticoid administration to mothers in preterm labor to stimulate surfactant synthesis. Other rare anomalies include lung agenesis (unilateral or bilateral) and ectopic lung lobes originating from the trachea or esophagus, likely formed from additional respiratory buds. Congenital lung cysts, formed by the dilation of terminal or larger bronchi, can give the lung a honeycomb appearance on X-rays. These cysts often drain poorly and lead to chronic infections.

Questions & Discussion

Question 1: A prenatal ultrasound reveals polyhydramnios and, at birth, the neonate has an excessive amount of fluid in their mouth. What type of congenital defect could exist, and what is its embryonic origin? Should the neonate be carefully examined to rule out other congenital defects? Why?

Answer: The presence of polyhydramnios and excessive oral fluid suggests esophageal atresia, often accompanied by a tracheoesophageal fistula ($FTE$). This originates from an abnormal partitioning of the foregut by the tracheoesophageal septum during the $4^{th}$ week of development. Yes, the neonate should be carefully examined because $FTE$ is frequently associated with other anomalies, such as cardiac malformations ($33\%$ of cases) or the $VACTERL$ association (vertebral, anal, cardiac, renal, and limb defects).

Question 2: A newborn at $6$ months of gestation has difficulty breathing. What is this due to?

Answer: At $6$ months ($24$ weeks), the lungs are in the canalicular or early terminal sac stage. The difficulty breathing is likely due to Respiratory Distress Syndrome ($SDR$) caused by insufficient surfactant production. Type $II$ pneumocytes only begin producing surfactant at the end of the sixth month, and the concentrations are often not high enough to prevent alveolar collapse (atelectasis) until later in gestation.