Mesenchymal-epithelial interactions in bladder smooth muscle development: Effects of the local tissue environment

Purpose: We have previously shown that mesenchymal-epithelial interactions are necessary for the development of bladder smooth muscle. Specifically without fetal or adult urothelium embryonic rat bladder mesenchyma does not differentiate into smooth muscle. The mechanism responsible for this interaction is not known, although it is postulated that diffusable growth factors have a role. Our hypothesis is that diffusable factors within adult rat bladders influence smooth muscle differentiation. Materials and Methods: Chimeric bladders were created by surgically implanting 14-day embryonic rat bladder mesenchyma before smooth muscle differentiation into the detrusor space of adult syngeneic hosts to test whether the host urothelium would induce smooth muscle differentiation without being in direct contact with fetal bladder mesenchymal tissue. Sub-detrusor pockets were created between the serosa and smooth muscle layer, between the smooth muscle layer and lamina propria, and between the lamina propria and urothelium in direct contact with urothelium. Controls consisted of intact 14-day embryonic rat bladders with the urothelium not removed, and 14-day embryonic bladder mesenchyma recombined with urothelium (direct contact) placed within the sub-detrusor space of the bladder and under the renal capsule. Results: Immunohistochemical staining with antibodies directed against smooth muscle alpha -actin and urothelium (cytokeratin 7) revealed smooth muscle differentiation in intact embryonic bladders and bladder mesenchyma plus urothelium recombinants in contrast to bladder mesenchyma alone, which had no alpha -actin staining (morphometric smooth muscle analysis p = 0). There was no alpha -actin staining in chimeric bladders even when bladder mesenchymal grafts were placed directly in contact with host urothelium. In addition, bladder mesenchyma plus urothelial recombinants within the host bladder had less alpha -actin staining than their counterparts placed under the renal capsule (p = 0.001). Conclusions: A diffusable factor most likely exists within adult rat bladders that inhibits smooth muscle differentiation.