What feature(s) of mesothelial cells do not typically occur in macrophages (macrophage cells)/histiocytes (immune cells)?

Distinguishing Features of Mesothelial Cells from Macrophages/Histiocytes

The feature that occurs in mesothelial cells but NOT typically in macrophages/histiocytes is an eccentric round/oval nucleus and overall smooth, round membrane/cell shape (option B).

Morphological Comparison of Mesothelial Cells vs. Macrophages/Histiocytes

Mesothelial Cell Characteristics

• Cell Shape and Membrane: Smooth, round cell membrane with polyhedral shape 1
• Nuclear Features: Eccentric round to oval nuclei with finely reticular chromatin 2
• Cytoplasm: Uniform blue-gray cytoplasm that is moderately abundant 3
• Size: 15-30 μm (can be up to 50 μm) 3
• Arrangement: Often form cohesive groups or sheets 3

Macrophage/Histiocyte Characteristics

• Cell Shape and Membrane: Irregular cell membrane with curved, irregular nuclei 1
• Nuclear Features: Nuclei are often kidney-shaped or indented rather than round/oval 1
• Cytoplasm: Granular cytoplasm, often with microvacuoles 1
• Size: Typically smaller than mesothelial cells (about half the size of ALCL cells) 1
• Arrangement: Typically discohesive, not forming sheets 1

Features Shared by Both Cell Types

Vacuoles (Option A)

Both mesothelial cells and macrophages/histiocytes can display cytoplasmic vacuolation:

• Mesothelial cells may show vacuolation, especially when transformed during chronic inflammation 2
• Macrophages/histiocytes characteristically show abundant cytoplasmic microvacuoles 1

Hemosiderin Granules (Option C)

Both cell types can contain hemosiderin granules:

• Macrophages/histiocytes commonly contain hemosiderin granules from phagocytosis of red blood cells
• Mesothelial cells can also contain hemosiderin granules, particularly in hemorrhagic conditions 3

Uniform Blue-Gray Cytoplasm (Option D)

Both cell types can display blue-gray cytoplasm:

• Mesothelial cells typically have pale blue cytoplasm 2
• Macrophages can also have blue-gray cytoplasm, though often more granular 1

Diagnostic Pitfalls

1. Reactive Changes: Mesothelial cells are extremely sensitive to pathologic processes and can undergo significant morphological changes, making them difficult to distinguish from other cell types 3

2. Transformation: Mesothelial cells can transform and become phagocytic with conspicuous cytoplasmic vacuolation in chronic inflammatory conditions, resembling macrophages 2

3. Activation: Activated mesothelial cells may show pleomorphism and intense basophilia that can mimic neoplasia 2

4. Surface Configuration: Mesothelial cells detached from their epithelial setting can lose their characteristic microvilli and acquire surface blebs, altering their appearance 4

5. Transitional Forms: Some studies suggest a possible transition between mesothelial cells and macrophages, further complicating distinction 5

The most reliable distinguishing feature between mesothelial cells and macrophages/histiocytes is the eccentric round/oval nucleus and overall smooth, round cell shape seen in mesothelial cells but not typically in macrophages/histiocytes.