Various urethral conditions often require substitute urethral reconstruction including long-segment urethral strictures, traumatic defects, complicated hypospadias and previous failed urethroplasty[8]. Serveral substituted materials have been adopted for clinical urethroplasty: full-thickness skin graft, bladder mucosa, oral mucosa( buccal or lingual mucosa) and colonic mucosa[9–11]. Buccal mucosa and preputial skin graft remain the widely used tissues for urethral replacement, especially in cases of complex urethral reconstruction. However, there are risk of complications, such as graft necrosis, hair growth, stricture recurrence, and fistula formation[12, 13]. In addition, harvesting buccal mucosa may also lead to possible morbidities such as intraoperative hemorrhage, submucosal scarring, pain, postoperative infection, and injury to salivary ducts[3][14]. Wood et al[15] reported the morbidity of buccal mucosa grafts harvested for urethroplasty in 57 patients. Of these patients, 68% had perioral numbness that persisted after 6 months in 26% of the patients, 83% developed postoperative pain, 67% initially had diffificulty with mouth opening that persisted after 6 months in 9% of the patients, and 2% had a mucous retention cyst. Dublin et al [6]reported that 57% patients had oral numbness after surgery (during the first 48 hours) that persisted for a year in 16% of patients and that 75% of patients complained of tightness of the mouth after surgery (during the first 48 hours) that persisted for nearly 2 years in 32% of cases. When urethral defect is long, donor tissue from oral cavity or prepuce may be insufficient. For these reasons, many attempts have been made to select alternative tissues that would serve as adequate urethral substitutes. Tissue engineering may be a promising option for the creation of artificial urethral tissues. But accompanied techniques are complex with multi-steps and the results were dissatisfactory in some preliminary clinical applications[16, 17]. Till now, tissue engineered urethra mainly at the stage of animal experiments so that it has not been widespread accepted in the clinic[18].
For anterior urethral strictures or hypospadias, a multi-stage urethroplasty in onlay fashion is generally recommended[4][19]. However, there is a lack of exploration of one-stage reconstruction using tubularized graft. In some cases, onlay grafts may not be suitable and tubularized urethroplasty is necessary[20, 21]. We previously performed urethroplasty in a rabbit model with saphenous vein patch as onlay graft and gained encouraging results[7]. Saphenous vein is a hollow tubular structure with a similar diameter of urethra. It has advantages of easy to be harvested and improved cosmetic outcomes, almost involves no injury to its donor site.
Tuffer performed urethral reconstruction using vein graft for the first time in 1910[13]. Subsequently, several animal trials have been reported using vein grafts for urethral reconstruction[22–24]. Hubner et al[24]and Foroutan et al[25]used everted vein grafts and their outcomes were improved. They thought their modified technique eliminated negative effects on urine stream from the valve inside the venous lumen. To the best of our knowledge, one of the key factors in reconstructing urethra is adequate blood supply and fast angiogenesis in implanted graft. Vascular density in the urethral cavity plays a central role in determining the fate of tissue repair[26, 27]. Insufficient blood supply inevitably results in shrinkage of grafts and fibrosis formation. In the study, the saphenous vein was everted and the vascular endothelium became outside. We hypothesize that the vascular endothelium could gain blood supply from periurethral muscles. We previously published our hypothesis that everted vein graft could survive without graft necrosis and slough off [7]. In this study, IHC staining with CD31 showed dense vascular capillaries in the experimental group. We demonstrate that everting the vein graft improved its ability of angiogenesis.
Adult male beagles were selected as our experimental model because the urinary tract in beagle resemble that of the human. Foroutan et al[25]performed histological study after euthanizing rabbits at days 7, 10, 14, 22, and 30 after operation, they found that gradual uroepithelialization occurred within one month. They demonstrated that the vein graft functioned as a guide for migration of uroepithelium. The histological examination in our previous experiment revealed similar progress[7]. The everted vein graft showed full integration to the neourethra. Urethral lumen was completely covered by regenerative uroepithelium 6 month postoperatively. Fistula formation, stenosis at the anastomosis were noted in the animals of control group. The experimental group showed improvement over the control group both in retrograde urethrography and histologic analysis. The MT staining showed that well-formed collagen and muscle fibers in experimental group. However, abundant collagen fiber, narrow urethral lumen and urothelium defect were found in control group. These results confirmed and supplemented our hypothesis. The rapid survival of the implant graft contribute to its function serving as a barrier of urine extravasation, facilitating urothelium cell migration and proliferation in the newly formed urethral tissue[28].
We create urethral defect models in a healthy urethra of normal animals, which cannot fully resemble the exact clinical situation of urethral stricture, which is characterized by the fibrotic urethra bed. Other limitations include a small sample size and a short follow-up time. The number of animals used in an experimental study is always a concern of the Ethics Committee, so the minimum number was used to answer the questions that were proposed. Further investigations with longer follow-up are necessary to assess its technical applicability and to translate this technology into clinic.