Physicians involved in regenerative medicine research conduct a series of animal experiments to determine pulmonary tissue regeneration capacity. During one of the experiments, lung alveoli are exposed to NO2 and massive necrosis of the epithelial lining ensues. Histologic examination of the injured tissues a month later shows partial recovery of the alveolar epithelial lining. This regenerated tissue is most likely derived from which of the following cell types?
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Ninety-five percent of the alveolar surface is covered by flat type I pneumocytes. The remaining 5% is covered by interspersed cuboidal type II pneumocytes that make up more than half of the total number of alveolar cells. Type II pneumocytes are the source of pulmonary surfactant and have the ability to proliferate in response to injury. They also act as progenitor cells for type I pneumocytes, which cannot regenerate on their own (Choice E).
(Choice A) Alveolar macrophages are a self-maintaining population derived from fetal monocytes during lung development. They are the principal cells involved in clearing inhaled foreign particles from the terminal respiratory units.
(Choice B) The pseudostratified, ciliated, columnar epithelium of the tracheobronchial tree gradually tapers to become a simple ciliated cuboidal epithelium at the level of the terminal bronchioles. Ciliated cells are absent from the lining of alveolar ducts and alveolar sacs.
(Choice C) Club cells are nonciliated, secretory cells found predominantly in the terminal portions of the bronchioles. These cells can act as a regenerative source of ciliated cells in the bronchioles.
(Choice D) Goblet cells are found in the respiratory epithelium of the bronchi and larger bronchioles; they are absent from the alveolar mucosa. Goblet cells are considered terminally-differentiated, mucin-producing cells.
Educational objective:
Type II pneumocytes have 2 important functions: regeneration of the alveolar lining following injury and surfactant production.