Mechanical, compositional and morphological characterisation of the human male urethra for the development of a biomimetic tissue engineered urethral scaffold
Introduction
The human urethra is a complex tubular organ that allows urine to drain from the urinary bladder. It is comprised of smooth and striated muscle that is contained within a connective tissue matrix composed primarily of collagen and elastin [1]. The muscle component functions as a sphincter by producing closure pressure to maintain urinary continence, and the extracellular connective tissue matrix plays a passive role in preventing the urethra from over distending during increases in intraluminal urethral pressure and distension [2].
Urethral trauma is a frequent and costly event that can be caused by injury, inflammation, ischaemic stricture, congenital defect or malignancy [3]. The most recent estimates place the cost of managing urethral trauma at almost $200 M [4] per annum in the US, with the annual cost of urethral repair alone at over $10 M [5,6]. Urethral trauma is often repaired using a flap or vascularised graft. However, autologous grafts are difficult to harvest, have limited availability, increase procedural costs and are associated with donor site morbidity [5,7]. Tissue engineered urethral scaffolds are not subject to such limitations and present as a viable treatment option in certain clinical cases. However, despite numerous preclinical and clinical trials [8], urethral scaffolds made of the ‘gold standard’ tissue engineered materials; small intestinal submucosa (SIS) or urinary bladder matrix (UBM), frequently fail in cases of long strictures (i.e. >2 cm), tubular grafting, grafting in unhealthy residual urethral beds or grafting of penile strictures [9].
The poor clinical outcomes associated with SIS and UBM in certain clinical indications can be linked to the disparity that exists between the scaffold material and the native tissue. SIS and UBM are fabricated from densely packed laminates of decellularised tissue and therefore do not mimic the composition, structure or mechanical properties of the native tissue. Developing a urethral scaffold that is more mimetic of the native tissue would enable improved physiological function, provide more appropriate biomechanical cues to endogenous cells, and allow for improved integration of the scaffold by the native host tissue, thereby improving clinical outcomes [10,11].
Data characterising the relationship between the mechanics, composition and structure of human urethral tissue is therefore paramount to providing an accurate baseline for biomimetic tissue engineered urethral scaffolds. However, such data has not yet been made available to the scientific community. Therefore, a crucial gap in the current literature exists that restricts the development of biomimetic tissue engineered scaffolds for urethral grafting.
For the first time, this study comprehensively characterises the baseline mechanical properties of human male anterior urethral tissue and relates these properties to the composition and gross morphology of the tissue. Our results reveal that urethral tissue exhibits viscoelastic mechanical properties, with no directional or regional variance, that correlate with elastin and collagen content. These novel data are used to inform the design of the first biomimetic tissue engineered urethral scaffold. Through a combination of in vitro and in vivo tests, our novel bilayered scaffold is shown to be more mimetic of the native urethral tissue in terms of mechanical properties, composition and gross structure compared to existing gold standard tissue engineered materials, and also exhibits improved cellular infiltration.
Section snippets
Acquisition of donor tissue
Following hospital ethical research committee approval, human anterior urethras were obtained from 9 consenting patients undergoing male to female gender reassignment surgery at the University Hospital Essen, Essen, Germany. The mean age of the patients was 40 ± 13.13 years (Range: 18–58 years). Samples included the anterior portion of the urethra which can be further divided into the bulbar portion of the urethra at the proximal end, penile (pendulous) urethra and the urethra bearing potion of
Pressure-diameter testing of urethra samples
Pressure-diameter testing of the urethra samples was performed to characterise the baseline mechanical properties of the tissue. Fig. 1 shows the mechanical results obtained from static and dynamic pressure-diameter testing of intact human male urethras (n = 9). The results reveal a nonlinear response typical of biological tissue whereby the urethral tissue becomes stiffer at higher intraluminal pressures during both static and dynamic loading in all samples, Fig. 1B and C. The stiffening
Discussion
This study addresses a crucial gap in the literature, that previously restricted the development of biomimetic urethral scaffolds, by comprehensively investigating the relationship between urethral tissue mechanics, composition and gross structure. We then utilised these data to develop the first biomimetic urethral scaffold and demonstrate that it’s physical properties more accurately mimic the native tissue than the existing gold standard tissue engineered options; SIS and UBM. Urethral
Conclusion
This study is the first to comprehensively characterise the passive mechanical properties of explanted human male urethra tissue. We then utilised these data to develop a biomimetic urethral scaffold. Urethral tissue exhibits a nonlinear pressure stiffening response that is distinct during static and dynamic loading. Furthermore, both the elastic and viscous responses of the tissue to extension are free from directional and regional variance. Elastin and collagen significantly influence tissue
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors would like to acknowledge the work of Dr Emily Ryan and Dr Alan Hibbits at RCSI for their assistance in fabricating bilayered collagen-elastin scaffolds, the work of Prerak Gupta of the Indian Institute of Technology Guwahati for his assistance in acquiring SEM images, and Dr John Mulvihill for providing the chemicals required to prepare tissue samples for SEM analysis. The collagen used in this study was supplied under a Materials Transfer Agreement by Integra Life Sciences. The
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