Urothelial transformation into functional glandular tissue in situ by instructive mesenchymal induction. It is generally believed that adult tissue is terminally differentiated. The ureter is derived from the metanephric diverticulum which, along with the derivatives of the metanephric mesoderm, forms the kidney. In our experiments, the left ureters of adult male athymic mouse hosts were severed below the kidney, and mesenchyme from neonatal rat seminal vesicles (SVM) was grafted to the cut end of the ureter, thus bringing adult mouse ureter epithelium (URE) in contact with neonatal rat SVM. After four to eight weeks, the in situ tissue recombinants were harvested, and the epithelial secretory proteins recovered. In 5 of 11 cases, an induction occurred, resulting in an in situ transformation of the non-glandular transitional epithelium of the adult mouse ureter into the simple columnar epithelium of the seminal vesicle (SV). Functional cytodifferentiation was examined in these neonatal rat SVM + adult mouse URE tissue recombinants using antibodies against SV-specific secretory (SVS) proteins of the mouse and rat. From the cut end of the ureter, the adult URE was induced to undergo SV morphogenesis, to express SV cytodifferentiation, and to produce the complete spectrum of major SVS proteins characteristic of the mouse. The induced seminal vesicle epithelium (SVE) also expressed androgen receptors (AR) which are not seen in urothelial tissue. Staining with Hoechst dye 33258, which can distinguish cells of mouse and rat origin, further demonstrated that the induced SVE was indeed of mouse origin and not a contaminant of the inducing rat SVM. In addition, neonatal mouse vaginal mesenchyme was grafted in situ beneath the bladder mucosa of adult male mice, and the host animals were killed after three months. The vaginal mesenchyme implanted into the bladders induced prostate-like acini, indicating that the above reprogramming of adult organs in situ is not an isolated occurrence. These results set a precedent for the “recreation” of new vital organs, such as the kidney, in situ by demonstrating that adult epithelial cells retain a developmental plasticity equivalent to their undifferentiated fetal counterparts and are capable of being reprogrammed in situ to express a completely new morphological, biochemical, and functional phenotype.