Module
18
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Respiration tract

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). After a couple of days - from the lower part - a bud forms, the true lung primordium (stage 12, ca. 30 days). In the further subdivision into the two main bronchi (stage 14, ca. 33 days) 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.

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It appears that the branching and growth of the lung anlage is under the direction of the mesenchyma on which it lies. Like in the kidney, it appears also here that an epithelio-mesenchymal interaction is important for the normal development of the lungs. This is shown by recombination experiments in which the tracheal epithelium begins to fork when it is grown on a layer of bronchial mesenchyma.

At the end of the embryonic period the first segments ppear 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.

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Synoptique

Overview of the illustrations stage 14, stage 16 and stage 23

Fig. pulmo14 - Development of the lungs up to
the end of the embryonic period
media/module18/r1b_entwicklung_st14.gif

  1. Right main bronchus
  2. Upper right pulmonary lobe
  3. Middle pulmonary lobe
  4. Lower right pulmonary lobe
  5. Left main bronchus
  6. Upper left pulmonary lobe
  7. Lower left pulmonary lobe

Legend
Fig. pulmo14
Fig. pulmo16 - Development of the lungs up to
the end of the embryonic period
media/module18/r1b_entwicklung_st16.gif

  1. Right main bronchus
  2. Upper right pulmonary lobe
  3. Middle pulmonary lobe
  4. Lower right pulmonary lobe
  5. Left main bronchus
  6. Upper left pulmonary lobe
  7. Lower left pulmonary lobe

Legend
Fig. pulmo16
Fig. pulmo23 - Development of the lungs up to
the end of the embryonic period
media/module18/r1b_entwicklung_st23.gif

  1. Right main bronchus
  2. Upper right pulmonary lobe
  3. Middle pulmonary lobe
  4. Lower right pulmonary lobe
  5. Left main bronchus
  6. Upper left pulmonary lobe
  7. Lower left pulmonary lobe

Legend
Fig. pulmo23

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 - Development of the pulmonary vessels in stage 13
media/module18/r1d_plexus_st13.gif

  1. First aortic arch (atrophying)
  2. Second aortic arch
  3. Third aortic arch (internal carotid artery forms from the ventral part)
  4. Fourth aortic arch (on the right: part of the subclavian artery, left: arcus aortae)

Fig. 4 - Development of the pulmonary vessels in stage 15
media/module18/r1d_plexus_st15.gif

5
Dorsal aorta
6
Lung buds
7
Aortic sac
8
Pulmonary plexus

Legend
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.