mCMV infection, cell proliferation, IL-6 expression, and mucin expression. A, B. Cell proliferation. Cell proliferation was determined by the distribution of PCNA (brown color). In control E15 + 6 SMGs (A), PCNA positive nuclei are primarily seen in branching epithelia (double arrows) and rarely in mesenchyme (m). With mCMV infection (B), PCNA-positive nuclei are primarily seen in mesenchymal cells (arrowheads) and, to a lesser degree, in epithelial cells (e). C, D. Immunolocalization of IL-6. A substantial increase in immunodetectable IL-6 is seen in mCMV-infected SMGs ductal epithelia (D) compared to controls (C). E-I. Immunolocalization of mucin protein. In E15 + 6 (E) and E15 + 12 (G) SMGs, mucin is localized to the cytoplasm of terminal bud epithelia (e). In mCMV-infected E15 + 6 SMGs (F), there is an increase in mucin localized to epithelial apical surfaces surrounding dilated lumina (arrowheads). By day 12 (H, I), mCMV-infected SMGs are characterized by a notable decline in epithelial-localized mucin; however, mucin is found in a subpopulation of metaplastic stromal cells (arrows). Sections C, D were counterstained with DAPI. Bar: A, B: 23 μm; C-F: 20 μm; G-I: 16 μm.