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Cytokeratin immunotyping of primary prostatic epithelial spheroids grown in Matrigel and media supplemented with serum. The epithelial cells stain predominantly for luminal cytokeratin (18, green), which additionally co-localizes with basal cytokeratins (1, 5, 10, 14; red). Nuclei are counterstained with DAPI (blue). In the absence of serum, spheroids develop that are composed of solid masses of cells. In the presence of serum, spheroids develop lumen with one or two epithelial layers (shown here), and stromal co-culture results in polarization of the epithelium. For details, see the article by Lang et al in this issue.