N-methyl-N-nitrosourea induces dysplasia and cell surface markers of neoplasia in long-term rat bladder organ cultures.

Untreated organ cultures of normal rat bladder can be maintained for long periods, up to 160 days, in a supplemented Waymouth's medium MB 752/1. During that time, the urothelium retains a similar appearance to that seen in vivo, namely a three-cell thick epithelium with specialised superficial cells whose characteristic surface features are identifiable by scanning electron microscopy. These superficial cells cover the major part of the explants, but the surface features of basal and intermediate cells can be observed on the cut edges and re-epithelialising surfaces of the explant. These are described and illustrated. When normal cultures are treated with the direct-acting carcinogen, N-methyl-N-nitrosourea (MNU), the in vitro response of the urothelium resembles to a certain extent the in vivo response to MNU instilled directly into the bladder. A histological progression is seen through mild to severe dysplasia which resembles carcinoma in situ. However, no marked changes in growth pattern, such as the development of papillary or nodular hyperplasia are seen. It is suggested that this is related to the failure of the vascular and stromal elements of the explants to respond to MNU treatment in vitro. The development of urothelial dysplasia is reflected by marked changes in cell surface differentiation, including development of pleomorphic microvilli, which closely resemble those seen following MNU treatment in vivo. These changes appeared earlier and were far more severe in vitro than in vivo. The significance of pleomorphic microvilli in bladder cultures is considered. A few were seen in control cultures but primarily on epithelial outgrowths onto the Millipore filter support, suggesting a relationship to the distance of the urothelium from viable stromal support. In the MNU-treated cultures, they were numerous and found on the surface of cells covering most of the explants. They were not solely related to the proliferative state of the urothelium in these cultures. This in vitro culture system provides a useful model with which to study the effects on the urothelium of various known and suspect carcinogens.