Urothelium Patterns Bladder Smooth Muscle Location

Abstract

PURPOSE Smooth muscle differentiation is induced in the embryonic bladder by the centrally located urothelium in the undifferentiated mesenchyme in the periphery adjacent to the serosa. We hypothesize that under the appropriate signal the entire undifferentiated bladder mesenchyme is capable of smooth muscle differentiation and that the epithelium patterns fibromuscular development. MATERIAL AND METHODS Embryonic bladders of wild-type and Green Fluorescent Protein (GFP) mice were separated into urothelial and mesenchymal components prior to smooth muscle differentiation (E12.5-E13). The urothelial layer (GFP) was recombined and grafted with the mesenchyme (wild-type) in a: 1) normal orientation (orthotopic fashion); 2) with the urothelium on the serosal surface (heterotopic); 3) with the urothelium in both orthotopic and heterotypic locations; 4) with the urothelium implanted into an ectopic location within the bladder mesenchyme; 5) embryonic bladder mesenchyme alone (controls) and 6) bladder epithelium alone (controls). Patterning experiments were also performed by combining epithelium of non-urothelial cell origin with embryonic bladder mesenchyme. After two weeks of growth under the renal capsule, the tissue recombinants were evaluated for smooth muscle patterning using immunohistochemistry. RESULTS Smooth muscle developed in its normal peripheral location adjacent to the serosa when the urothelium was placed in an orthotopic position. Smooth muscle developed in an abnormal position in the prospective submucosal zone when the urothelium was placed on the serosal surface. In all cases, a zone of smooth muscle inhibition was observed adjacent to the epithelium whether the urothelium was in an orthotopic or heterotypic position. Bladder mesenchyme grafted alone did not grow and bladder epithelium grafted alone did not grow. Epithelium of non-urothelial origin induced and patterned smooth muscle, but in a variable tissue specific manner. CONCLUSIONS The full thickness of bladder mesenchyme is capable of smooth muscle differentiation dependent on the location of urothelium.