Urethral stricture is a common urological disease that seriously affects quality of life. Urethroplasty with grafts is the primary treatment, but the autografts used in clinical practice have unavoidable disadvantages, which have contributed to the development of urethral tissue engineering. Using various types of seed cells in combination with biomaterials to construct a tissue-engineered urethra provides a new treatment method to repair long-segment urethral strictures. To date, various cell types have been explored and applied in the field of urethral regeneration. However, no optimal strategy for the source, selection, and application conditions of the cells is available. This review systematically summarizes the use of various cell types in urethral regeneration and their characteristics in recent years and discusses possible future directions of cell-based therapies.

The regeneration of biological tissues in medicine is challenging, and 3D bioprinting offers an innovative way to create functional multicellular tissues. One common way in bioprinting is bioink, which is one type of the cell-loaded hydrogel. For clinical application, however, the bioprinting still suffers from satisfactory performance, e.g., in vascularization, effective antibacterial, immunomodulation, and regulation of collagen deposition. Many studies incorporated different bioactive materials into the 3D-printed scaffolds to optimize the bioprinting. Here, we reviewed a variety of additives added to the 3D bioprinting hydrogel. The underlying mechanisms and methodology for biological regeneration are important and will provide a useful basis for future research.

In recent years, decellularized extracellular matrices (dECM) derived from organs have attracted much attention from researchers due to their excellent biocompatibility, capacity to promote cell proliferation and migration, as well as pro-vascularization. However, their inferior mechanical properties, slow cross-linking, weak strengths, and poor supporting properties remain their inevitable challenges. In our study, we fabricated a novel dECM hydrogel with better crosslinking strength and speed, stronger support properties, and better mechanical properties. The hydrogel, which we named gelatin-based dECM powder hydrogel (gelatin-dECM hydrogel), was prepared by mixing dECM microparticles in gelatin solution and adding certain amount of 1-Ethyl-3-(3-dimethyl-aminopropyl-1-carbodiimide) (EDC) and N-hydroxysuccinimide (NHS). We evaluated the utility of this hydrogel by assessing the degradation rate, mechanical properties, and biocompatibility. The results showed that the gelatin-dECM hydrogel has high mechanical properties and biocompatibility and also has the ability to promote cell proliferation and migration. After injection of this hydrogel around the surgical sites of urethras in rabbits, the incorporation of dECM powder was demonstrated to promote angiogenesis as well as scarless repair by histological sections after surgery. The application of this novel hydrogel provides a new perspective for the treatment of post-traumatic urethral stricture.

The dynamic pathological microenvironment formed post-urethral injury drives an inflammatory-fibrotic cascade, leading to urethral stricture. Herein, we designed a novel hydrogel dressing (CPT-Cel) to modulate this dynamic urethral microenvironment for scarless reconstruction. The hydrogel responsively degrades and releases its contents upon encountering acidic environments and ROS bursts, interrupting the severe oxidative stress-inflammation cycle and creating a favorable regenerative microenvironment in the early wound phase. The incorporated celastrol continuously releases into the late repair phase, inhibiting the hyperactivation of urethral fibroblasts via the TGF-β/NF-κB pathway in urethral microenviroment, thereby preventing excessive collagen secretion. Furthermore, the dressing, featuring an ECM-mimicking microstructure, effectively adheres to and integrates with the injury site, providing hemostatic and antibacterial functions. In animal models of urethral injury, CPT-Cel accelerated urethral repair within the inflammatory milieu, significantly reduced collagen deposition and ultimately improved stricture outcomes. This study not only validates the core mechanism by which an intelligent drug-loaded hydrogel modulates the dynamic urethral microenvironment but also proposes a new paradigm for urethral tissue engineering.