Lung Development: From Embryogenesis to Adulthood

Gurkan Mollaoglu^*
*Correspondence: Gurkan Mollaoglu, Department of Oncological Sciences, University of Utah, Salt Lake City, USA, Email:

Keywords

Embryogenesis • Lung buds • Pseudoglandular stage

Introduction

During the early weeks of embryonic development, a series of intricate processes lead to the formation of the lungs. Lung development begins with the formation of two lung buds, one on each side of the primitive foregut. These lung buds arise from the ventral wall of the foregut, which is a primitive tube that eventually gives rise to the digestive and respiratory systems. This initial step is orchestrated by a group of transcription factors, including Nkx2.1 (also known as thyroid transcription factor-1 or TTF-1), which plays a crucial role in lung specification. Once the lung buds form, they continue to undergo branching morphogenesis. This process involves the repeated branching and division of the lung buds into smaller tubules, forming the bronchial tree [1].

Literature Review

This stage is known as the pseudoglandular stage because the lung tissue resembles a glandular structure due to the extensive branching. The epithelium that lines these tubules differentiates into various cell types, including ciliated cells, mucus-producing cells and neuroendocrine cells. As lung development progresses, the pseudoglandular stage gives way to the canalicular stage, typically occurring during the second trimester of pregnancy. During this stage, the lung undergoes further branching and differentiation. The capillaries that will facilitate gas exchange start to invade the developing lung tissue and the thinning of the alveolar walls begins. Specialized cells called type I pneumocytes, responsible for gas exchange, start to emerge and increase in number.

Discussion

The saccular stage takes place during the late second trimester and continues into the third trimester of pregnancy. At this stage, the primitive alveoli, or air sacs, become more prominent. The thinning of the alveolar walls accelerates and the development of type II pneumocytes, which produce surfactant, intensifies. Surfactant is a crucial substance that reduces surface tension in the alveoli, preventing their collapse during exhalation. The final stage of fetal lung development is the alveolar stage. It primarily occurs during the third trimester and extends into the postnatal period. In this stage, the number of alveoli increases significantly and their walls continue to thin out. Type II pneumocytes become the predominant cell type, secreting surfactant to facilitate proper lung function. The lungs are still not fully mature at birth, but they are capable of supporting gas exchange. The expression of specific genes, such as Nkx2.1, Sox2 and Foxp2, is essential for lung specification and development. Mutations in these genes can lead to congenital lung abnormalities [2].

Fetal breathing movements, though not directly related to respiration, play a role in lung development. These movements help to stimulate the lung tissue and promote proper growth. Hormones, such as glucocorticoids, thyroid hormones and growth factors, are involved in lung development. Corticosteroids are sometimes administered to pregnant women at risk of preterm delivery to enhance fetal lung maturation. Mechanical forces, including amniotic fluid dynamics and pressure from the chest wall, are essential for lung development. Absence or insufficient mechanical forces can result in lung hypoplasia (underdevelopment). Appropriate oxygen levels are necessary for lung development. Both hypoxia (low oxygen) and hyperoxia (high oxygen) can disrupt normal lung development. Adequate nutrition, particularly during pregnancy, is crucial for the development of the fetal lung. Malnutrition can lead to growth restriction and lung underdevelopment [3].

After birth, the lungs undergo additional changes and maturation processes to support independent breathing. These postnatal changes are particularly critical for preterm infants, as their lungs are often underdeveloped. The first breath taken by a newborn is a pivotal moment in postnatal lung development. The expansion of the lungs with air initiates a series of changes, including the closure of certain fetal structures like the ductus arteriosus and foramen ovale. Surfactant production by type II pneumocytes continues after birth, ensuring that the alveoli remain stable and functional during breathing. In preterm infants, surfactant deficiency can lead to Respiratory Distress Syndrome (RDS). Over the first few years of life, the number of alveoli in the lungs increases significantly. This process, known as alveolarization, continues into childhood and is essential for achieving optimal lung function [4].

Postnatal lung growth continues well into adolescence. Both the size and number of alveoli increase, enhancing the lung's capacity for gas exchange. Factors such as nutrition, environmental factors and overall health play a role in postnatal lung growth. Several factors can influence postnatal lung development. Preterm infants are born with underdeveloped lungs, leading to respiratory difficulties and a higher risk of lung-related complications. Preterm infants often require oxygen therapy, but prolonged exposure to high levels of oxygen can lead to lung damage, a condition known as bronchopulmonary dysplasia. Infants who require mechanical ventilation can experience lung injuries due to the pressure applied to their fragile lungs. Infections, especially in the early postnatal period, can negatively impact lung development and function. Exposure to pollutants, allergens and smoking can impair postnatal lung development in infants and children [5,6].

Conclusion

While lung development is an intricate and highly regulated process, sometimes it can go awry, leading to congenital lung abnormalities. These can range from minor structural anomalies to life-threatening conditions. CDH occurs when there is a hole in the diaphragm, allowing abdominal organs to move into the chest cavity and compress the developing lungs. In this condition, a mass of non-functioning lung tissue forms outside of the normal lung.

Acknowledgement

None.

Conflict of Interest

There are no conflicts of interest by author.

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