Figure 3. Bronchus from animal MPX-9, which was infected intranasally. A) Cross-section of a bronchus, showing focal metaplasia and proliferation (between the arrows) of the luminal epithelium. B) Higher magnification showing the details of the metaplastic epithelium, accompanied by focal necrosis. Compare to the adjacent unaffected area, which is lined by normal ciliated columnar epithelial cells. C and D) Immunohistochemical staining of the corresponding field shows presence of viral antigen limited to the region of epithelial abnormality. A and B, hematoxylin and eosin stain; C and D, immunoperoxidase staining with vaccinia antibody. Original magnification: A and C, 4× objectives; B and D, 20× objectives.