An in vitro model of intra‐epithelial expansion of transformed urothelial cells

Replacement of normal urothelium by pre‐cancerous epithelium may explain the high recurrence rate of human bladder cancer. An in vitro model was designed in order to study the mechanisms of expansion of transformed urothelial cells at the expense of normal urothelium. For this purpose, mouse bladder explants were allowed to expand on a transparent porous membrane. Subsequently, cell sheets of the non‐tumorigenic mouse urothelial cell line NUC‐5, the tumorigenic mouse urothelial cell line NUC‐5 Py and the human bladder‐carcinoma cell line T24 were inoculated adjacent to the primary explant. Daily measurements of the outgrowth of the bladder explant were performed, and all cultures were terminated on day 24. At this time the post‐confluent primary urothelial cell outgrowths still showed proliferative activity, as demonstrated by bromodeoxyuridine incorporation. In due course the non‐tumorigenic NUC‐5 cells were replaced by the bladder outgrowth. T24 and NUC‐5 Py cells were able to inhibit the bladder outgrowth, or even infiltrate or replace the explant. This was confirmed by immunohistochemistry with (species‐specific) anti‐cytokeratin antibodies and by microscopic evaluation of cross‐sections of the porous membrane. This co‐cultivation model appears to be suitable for the in vitro study of the mechanisms of intra‐epithelial expansion of transformed urothelial cell lines.