18.1 Phases of lung development

• Introduction
  
• Embryonic phase
  
• Pseudoglandular phase
  
• Canalicular phase
  
• Saccular phase
  
• Alveolar phase
  
• Classification in the adult lung

Embryonic phase

The embryonic phase of lung development begins with the formation of a groove in the ventral lower pharynx, the sulcus laryngotrachealis (stage 10, ca. 28 days, 10). After a couple of days - from the lower part - a bud forms, the true lung primordium (stage 12, ca. 30 days,12). In the further subdivision into the two main bronchi (stage 14, ca. 33 days,14) the smaller bud on the left is directed more laterally than the somewhat larger one on the right that - parallel to the esophagus - is directed more caudally. Thus the asymmetry of the main bronchi, as they present in adults, is already established. The subsequent divisions of the endodermal branches also take place unequally in that on the right three further buds form and, on the left, only two, corresponding to the later pulmonary lobes. In the next division step, which occurs at the end of the embryonic period, the segments of the individual pulmonary lobes arise.

At the end of the embryonic period the first segments appear in the five (three right and two left) lobes of the lungs. With their distended ends the lungs resemble an exocrine gland.
At this time the pulmonary vessels have formed themselves.

Stage 14 (33 day)

Stage 16 (39 day)

Stage 23 (56 day)

The pulmonary circulation system (smaller circulation system) is formed out of the 6th pharyngeal arch artery. These develop somewhat differently than the other 4 aortic arches in that first a vessel plexus forms around the lung anlage, originating from the aortic sac. The true 6th aortic arch is only then formed after vessels - also from the dorsal aorta - grow into this plexus and thus a connection between the truncus pulmonalis and dorsal aorta has arisen.

Fig. 3
From the aortic sac a vessel plexus arises around the lung anlagen that with the lung buds extends caudally.

Fig. 4
Also from the dorsal aorta a vessel plexus forms that soon communicates with the ventral one and thus creates a connection between the ventral aortic sac and the dorsal aorta. On the left the arterial duct arises from it.