The development of the lung starts at 4 weeks of gestation and continues through young adulthood. Any developmental abnormality during the growth of the lung may lead to anatomic and physiologic abnormalities, which in turn may lead to significant symptoms and a reduction in quality of life. Prenatal ultrasonography and Doppler technology as screening tools for fetal growth have led to early identification of most developmental anomalies, including pulmonary vascular abnormalities, and revolutionized pre-emptive management of these disorders.1,2 Nonetheless, some developmental disorders of the lung may remain subclinical and escape diagnosis until adulthood.
Research using animal models and in vitro models has led to the identification of several genetic factors and signaling molecules that play a significant role in lung growth and morphogenesis.3 Some lung abnormalities are part of genetic syndromes involving multiple organs that share the same molecular pathways of embryogenesis. While some lung abnormalities resolve spontaneously before birth, others persist, with a spectrum of severity ranging from asymptomatic to severe postnatal respiratory distress. Asymptomatic abnormalities often go undetected until adulthood when they are incidentally discovered on chest imaging and often misidentified as malignant or infectious lesions. Thus, an understanding of developmental defects of the respiratory system, with a clinicoradiologic approach, is critical for accurate identification and management of these patients. For ease of understanding, the developmental disorders of the lung can be classified as airway disorders, parenchymal disorders, and vascular disorders (Fig. 105-1).
Classification of developmental disorders of the lung.
Important considerations in lung development include stages of lung development, the role of fetal lung fluid in lung development, and associated molecular mechanisms.
Lung development starts at 4 weeks of gestation and is divided into five developmental stages known as the embryonic stage, pseudoglandular stage (5–16 weeks), canalicular stage (16–26 weeks), saccular stage (26 weeks until term), and alveolar stage (postnatal stage).
The lung originates from the laryngotracheal groove of the endoderm located at the fourth pharyngeal arch. The splanchnic mesoderm provides the muscular and cartilaginous covering to the lung and also forms the visceral pleura, while the somatic mesoderm forms the parietal pleura.4 The laryngotracheal groove out-pouches from the foregut, forming the respiratory diverticulum that forms the tracheal bud separating it from the esophagus. The tracheal bud further branches into the two primary bronchi (left and right), which then divide into secondary or lobar bronchi (three bronchi on the right and two on the left for each lung lobe) by 5 weeks of gestation.
The lung continues to grow laterally and caudally on exposure to the splanchnic mesoderm. The conducting airways ...