18.1 Phases of lung development

Classification in the adult lung


Quiz 06

In the adult lung one distinguishes between conducting and respiratory zones.
In the conducting zone, all branches of the bronchial tree, the walls of which contain cartilage tissue and seromucous glands, are bronchi. As soon as cartilage and glands are no longer present, bronchioli are involved.


Quiz 13


Quiz 07

Fig. 11 - Overview of the wall construction in the lungs  Legend

Ciliated epithelium
Goblet cell
Smooth muscle cell
Clara cell
Basal membrane
Type I pneumocyte
Alveolar septum
Type II pneumocyte

Fig. 11
Diagrams for comparing the constructions of the walls in the respiratory tract.

According to their function the respiratory tract passages are divided into conducting and respiratory zones:

Conducting zone = 16 generations

  • Segmental bronchi are continued by several generations of
  • Intersegmental bronchi (up to ca. 1 mm diameter). After these follow the
  • Bronchioli (< 1mm diameter) that after several divisions go over into
  • Terminal bronchioli (ca. 0.4 mm diameter). They subdivide numerous times and represent the end stretch of the purely conductive respiratory tract. The measurements come from histological findings.

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Histological image of respiratory epithelium.

Respiratory zone = 7 generations

  • Out of the terminal bronchioli several generation of
  • Respiratory bronchioli (= 3 generations) proceed. From them follow several generations of
  • Alveolar ducts (= 3 generations) that in
  • Alveolar sacculi (last generation = 23rd generation) end

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Histological image of an alveolar duct.

Fig. 12 - Overview of prenatal lung development

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For the branching out of ever new lung buds an interaction between the respiratory endodermal epithelium and the surrounding pulmonary mesenchyma is primarily responsible. Mainly the epidermal growth factor (EGF) and the extracellular form of the transforming growth factors (TGF-b) appear to be important for lung development.
In addition, one finds specific extracellular matrix components like collagen of types I and III, as well as proteoglycan and the fibronectin and syndecan glycoproteins. These molecules are found around the passages and in the forks of the bronchial tree. They are responsible for the stabilization of the already formed structures - these are not present in the regions of the newly formed branches.
Epimorphine, a further protein, appears to promote the formation of epithelial passages. If epimorphine is blocked by antibodies, the epithelium that lies above it can not form itself into tubes and remains unorganized. (5)

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