EMBRYOLOGY OF THE LUNGS AND PULMONARY VESSELS
Fetal lung development can be categorized into the embryonic, pseudoglandular, canalicular, and saccular phases.1 Embryonic development begins at 24 to 26 days gestation when a diverticulum arises from the ventral wall of the foregut. Over the next 2 days, the right and left lung buds arise from this diverticulum. The developing airways become separated from the esophageal portion of the foregut by ingrowths of adjacent mesoderm that form the tracheoesophageal septum. The lung buds elongate into primary lung sacs, and the 5 secondary bronchi develop as outgrowths of the primary bronchi. This completes the embryonic period, at approximately the end of the fifth week.
The pseudoglandular phase predominantly consists of development of the bronchial tree. During this phase, the airways are blind tubules lined with columnar or cuboidal epithelium. The pseudoglandular phase occurs between the fifth and 16th weeks of gestation. Nearly all of the conducting airways are present by the end of the pseudoglandular phase.
The canalicular phase represents the early stage of development of transitional airways. There is decrease in mesenchymal tissue within the developing lungs, and newly formed capillaries and air spaces approximate one another. The canalicular phase occurs between the 17th week and the 25th to 28th weeks.
The saccular (or alveolar) phase relates to development of the alveoli. Defined acinar morphology is present by the 28th week of gestation. During the final weeks of fetal development, there is prolific development of alveoli. Alveolar development continues in postnatal life to approximately the age of 8 years.
Tracheal cartilage development predominantly occurs during the pseudoglandular and canalicular periods. Initial cartilage development occurs during the seventh to eighth weeks of gestation. Bronchial cartilage development occurs in a centrifugal direction.
Anomalies of the lung related to abnormal bronchopulmonary (lung bud) development include agenesis, bronchial atresia, tracheal atresia, some instances of congenital lobar emphysema, congenital cystic adenomatoid malformation, pulmonary bronchogenic cyst, tracheal bronchus, and accessory cardiac bronchus. The pathogenesis of bronchogenic cysts apparently involves abnormal epithelial budding caused by local defects in the mesenchymal substrate.2 The faulty development that results in cystic adenomatoid malformation occurs later in gestation, and is characterized by disordered development of the bronchioles and failure of differentiation of the epithelium into a mature form. This may be related to faulty signaling between the bronchioles and peribronchial mesenchyme during the period of active bronchial development, which occurs between the fifth and eighth weeks.3 The developmental mechanism of pulmonary sequestration involves both abnormal bronchial budding (supernumerary budding from the foregut, or pinching off from the developing bronchial tree) and failure of normal mesenchymal maturation (persistent systemic arterial supply).4,5 Congenital lobar emphysema can be caused by any developmental abnormality that results in lobar air trapping, such as a focal anomaly of airway cartilage, intrinsic or extrinsic bronchial obstruction, or abnormal supporting stroma of ...