The correct answer is F. This is a photomicrograph of lung alveolar walls or septa. Type II pneumocytes are cuboidal-like with round nuclei and a washed-out or foamy cytoplasm. This cytoplasmic appearance is due to the lipid content. Lamellar bodies (secretory granules) of these cells contain a complex of phospholipids, proteins, and carbohydrates known as surfactant. Surfactant reduces alveolar surface tension and thereby prevents alveolar collapse during expiration. Type II pneumocytes are also stem cells that produce type I pneumocytes. The latter form about 90% of alveolar surface area and are the cells across which gas exchange occurs. Alveolar macrophages are also present in the lumen of the alveolus.
Osteoblasts, which produce alkaline phosphatase (choice A), might be confused with these cells. However, osteoblasts line bone and have more of an epithelial-lining appearance.
Angiotensin-converting enzyme (ACE; choice B) is produced in the lungs by pulmonary endothelial cells, not by alveolar cells. ACE converts angiotensin I to angiotensin II, a vasoconstrictor which is an important regulator of blood pressure.
Immunoglobulins (choice C) are secreted by plasma cells, which are lymphoid cells and consequently, connective-tissue cells. The cells in the micrograph are attached to a basal lamina, so they cannot be plasma cells.
Major basic protein (choice D) is produced by eosinophils. The labeled cells do not have the nuclear lobulation that is typical of eosinophils.
Mucus (choice E) is produced by mucous cells that have a similar cytoplasmic appearance; however, the nuclei of such cells are usually flattened and pushed to the side next to the basement membrane.
Tubular myelin (choice G) is a highly-ordered lattice that forms within the alveolar airspace through reorganization of lamellar bodies. Although tubular myelin is rich in surfactant lipids and apoproteins, both of which are secreted by type II pneumocytes, the meshwork forms after secretion and, thus, is not produced by pneumocytes directly.
This is a multi-step question.
What is the question asking?
The question is asking you to select the product secreted by the cells labeled by arrows on the photomicrograph.
What is the first step?
The first step is to remind ourselves that we are looking at a lung biopsy sample. The section contains air-filled spaces, so we are in the respiratory zone. The photomicrograph shows that the air spaces are lined by a single layer of cells, which is characteristic of alveoli.
What is the next step?
The next step is to recall what types of cell we might expect to find within alveoli. There are three principal cell types:
- Type I pneumocytes. These are squamous epithelial cells, which appear to be flattened. They account for approximately 95% of the alveolar lining. Their thin profile facilitates gas exchange between blood and the atmosphere.
- Type II pneumocytes. These are cuboidal-shaped epithelial cells with a foamy cytoplasm and round nuclei. These cells produce surfactant and are also responsible for replacing damaged type I and type II pneumocytes.
- Alveolar macrophages. These are large cells that can be found between the lumen and connective tissue septum of the alveoli. They are the scavengers of foreign particles in the lung.
What is the next step?
The next step is to identify the cells indicated by the arrows in the vignette. Based on the descriptions above, these are type II pneumocytes. The foamy cytoplasm is due to their high lipid content, which is one of the principal constituents of surfactant (choice F).
Can other answers be eliminated?
- Type II pneumocytes do not produce alkaline phosphatase (choice A), immunoglobulin (choice C), or major basic protein (choice D).
- Angiotensin-converting enzyme (choice B) is produced by the vascular endothelial cells that line pulmonary capillaries, but not type II pneumocytes..
- Mucus (choice E) is produced by cells located in the conducting airways, but they are not found in the respiratory zone.
- Tubular myelin (choice G) forms from secreted lamellar bodies. Tubular myelin is not a direct product of pneumocytes.
What is the single best answer and why?
Surfactant (choice F) is the single best answer. The arrows in the micrograph point to cells attached to the basement membrane of the alveolar wall. They have a characteristically cuboidal shape, round nuclei, and foamy cytoplasm, which is characteristic of type II pneumocytes. Type II pneumocytes produce surfactant.
MedEssentials (4th Ed.): pp. 274
First Aid (2019): pp. 647.2
First Aid (2018): pp. 643.2
First Aid (2017): pp. 627.2
