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18.1 Phases of lung development
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At this stage the lungs resemble the development of a tubulo-acinous gland. According to the classical view, the entire air-conducting bronchial tree up to the terminal bronchioli are set down in this phase (16 generations). Recent morphometric studies (3) have shown that with the end of the pseudoglandular phase 20 generations are partially present in the lungs, which means that at this point in time the respiratory ducts have already been formed.
The primordial system of passages, the air-conducting bronchial tree, is initially coated by cubic epithelium. These are the precursor cells of the ciliated epithelium and of the secretory cells. In humans, the first ciliated epithelial cells can be found in the 13th week of pregnancy (7).
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In the respiratory part the first typically lung-specific cells, connected to the terminal bronchioli, appear:
the type II pneumocytes (alveolar cells) (3).
The developing broncho-pulmonary epithelium begins to produce amniotic fluid, which is also found in the lungs up to the time of birth. |
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Fig. 5 - Lung tissue in the pseudoglandular phase |
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Lung mesenchyma
Type II pneumocytes
Capillaries |
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Fig. 5
In the pseudoglandular phase the lungs resemble a gland. At the end of this phase the precursors of the pneumocytes can be discerned in the respiratory sections as cubic epithelium.
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Relatively early in the development of the lungs, endocrine-active cells (Kultschitsky cells) appear that produce bombesin and serotonin. In contrast to the precursors of the pneumocytes, which originate from the endoderm, they stem from the neural crest (neuroectoderm). Via paracrine mechanisms bombesin probably plays a decisive role for lung development in that mainly the type II pneumocytes proliferate. (1)
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The differentiation of the lungs takes place in a centrifugal direction. In the central, air-conducting portions of the lungs the epithelium begins to differentiate into cilia-carrying cells and goblet cells. After the 10th week cartilage and smooth muscle cells as well as bronchial glands can be found in the wall of the bronchi. The peripheral sections partially retain - until far beyond the pseudoglandular phase - cubic epithelium that is still little differentiated. This is important for a further proliferation of the bronchial tree into the surrounding mesenchymal tissue.
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If one begins, roughly estimated, with a number of 15'000 terminal bronchioli (8) per lung in adults and thereby ca. 15,000 acini and with a theoretical assumption of a dichotomous division of the pulmonary branches, one has the result that this stage is attained after little fewer than 2 14 generations. In the late pseudoglandular stage one finds, however, far more than 15'000 end pieces. Thus the lung end pieces at this stage already represent the respiratory portions of the lungs.
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