Cyclically stretching developing tissue in vivo enhances mechanical strength and organization of vascular grafts
Introduction
Vascular diseases are often treated by replacing affected tissue. Synthetic grafts are successful large-diameter blood vessel replacements and for small-diameter grafts, e.g. coronary artery bypass, autologous artery or vein grafts are the major grafts. However, small-diameter synthetic grafts are prone to failure due to a lack of compliance or thrombosis [1], [2]. Synthetic materials do not possess the functional characteristics that make them viable long-term conduits in small-diameter large-flow environments such as the coronary arterial circulation, but especially in vascular shunts used for haemodialysis access in diabetic patients. Consequently, no ideal graftable material has yet been constructed, despite significant advances in polymer chemistry and tissue engineering. In contrast to these synthetic grafts, arteries respond to hemodynamic forces in the circulation by expanding and contracting. Yet they remain compliant in response to these forces because of the presence of elastic fibres and collagen fibrils. Because of these differences between the current grafts and the native tissue, and the lack of suitable small-calibre blood vessel grafts, it is necessary to search for better grafts, a major tissue engineering challenge.
Tissue derived from implantation of a foreign body in the peritoneal cavity is known to contain a population of cells that differentiate into contractile smooth muscle-like cells, producing a strong collagen matrix, and have the ability to synthesize elastin [3], [4], [5]. These cells arise from monocyte/macrophage precursors predominantly, with a mixed population of immune cells [4] and the tissue, produced in a tubular shape, can be used as a vascular graft.
The potential for the tissue derived from the peritoneal cavity to be used as an autologous graft has been explored in various animal models. The tissue capsules produced by the foreign body response in the peritoneal cavity have been grafted as an autologous interposition graft in dog femoral artery [5], rat abdominal aorta [3] and rabbit carotid artery [6]. The myofibroblasts of these grafts were almost indistinguishable from smooth muscle cells (SMCs) of the adjacent native artery into which they were grafted [7] and a 80% patency rate was observed in dogs [5]. Grafting these tubes into the arterial circulation as a vascular substitute has thus far met with limited long-term success as vascular shunts and bypass grafts, partly due to an insufficiently strong and elastic extracellular matrix (ECM) at the time of harvest.
Many groups have reported that stretching cells in vitro can induce a change in cell phenotype and stimulation of production of ECM proteins [7], [8]. A strain as small as 10% at 1 Hz for 8 days on a scaffold with resident SMCs can show a tripling of ultimate tensile strength and ECM deposition [9], [10], [11]. Stretch placed on vascular grafts is so powerful in directing differentiation and secretion of ECM that cells that otherwise do not secrete elastin in vitro will secrete elastin in response to cyclic mechanical force [12], [13]. Myofibroblast differentiation and contractile activity are dependent on mechanical stimulation [14]. It is clear that the production of a tissue-engineered vascular graft that utilizes a large proportion of live cells will need to experience mechanical force to develop a strong ECM and cells with properties of native arterial cells.
Because of the promising results obtained with foreign body derived arterial grafts and previous work done with in vitro bioreactor stimulus upon peritoneal-derived tissue (unpublished data), we hypothesize that applying cyclic stretching forces within physiological range to the developing foreign body reaction will ameliorate the strength and development of the tissue. This work aims to assess the possibility to strengthen the tissue and shorten the development time, necessary to produce adequate tissue for engraftment as a vascular graft. For this purpose an implantable device was created consisting of an inner stretchable tube, a perforated external tube and a transabdominal tube. Cyclic force was then applied by means of a ventricle pump for a period of 5–8 days. The resultant tissue was then implanted into the circulation of two sheep as a carotid artery patch.
Section snippets
Animals and devices
Lovenaar sheep (n = 8, ewes 30–40 kg) were selected and cared for in accordance with the “Guide for the Care and Use of Laboratory Animals” (NIH Publication 85-23, revised 1985). The study was approved by the local Ethics Committee.
The peritoneal implant devices (Fig. 1A) were designed to recruit cells around a pulsating inner tubular scaffold of 100 mm length and 6 mm external diameter and providing protection against adhering to the omentum or visceral organs. The outer tubes had 2 mm diameter
Pulsation and macroscopy
In all implantations, both pulsed devices and unpulsed controls, avascularized unadhered free-floating tissue was produced. The macroscopic integrity of tissue was not different between devices implanted on the left and the right side of the animal. Four animals developed device leaks approximately 4–5 days into the pulsing procedure. These device incubations had to be terminated at this time. However, the sheep’s health and the tissue quality that was produced in unpulsed or pulsed samples
Discussion
These results have shown how pulsatile mechanical stretch can initiate changes in the development of tissue in the peritoneal cavity. At the macroscopic level, the differences between pulsed and unpulsed tissue were remarkable. Pulsed tissue was much paler in colour, more homogenous, dense, less translucent and of a consistent colour that resembled native tissue. In contrast, unpulsed tissue expressed a yellow tinge that was slightly opaque and jelly-like. The pulsed tissue also displayed tone
Conclusions
This study clearly shows that applying cyclic and physiological loading (20% diameter increase at 1 Hz for 8 days) to a tissue that develops in vivo alters the mechanical properties towards a more than twofold increase in tissue strength, which clearly proves our initial hypothesis. This stretch is even sufficient to produce changes in ECM organization and some contractile protein expression. However, mechanical properties were still inferior to native tissue properties and higher-order
Author disclosure statement
No competing financial interests exist.
Acknowledgements
The authors thank Ruth Plusquin, Kristof Reyniers, Veerle Leunens and Grant Edwards for their excellent technical assistance. This project was made possible with the assistance of the University of Queensland Postgraduate School Research Travel Grant scheme and partially funded by FWO (G.0549.06).
References (33)
- et al.
Pathophysiology of vein graft failure: a review
Eur J Vasc Endovasc Surg
(1995) Dog peritoneal and pleural cavities as bioreactors to grow autologous vascular grafts
J Vasc Surg
(2004)- et al.
Mechanical tension controls granulation tissue contractile activity and myofibroblast differentiation
Am J Pathol
(2001) Functional and biomechanical evaluation of a completely recellularized stentless pulmonary bioprosthesis in sheep
J Thorac Cardiovasc Surg
(2008)The remodeling of cardiovascular bioprostheses under influence of stem cell homing signal pathways
Biomaterials
(2010)Apparent blood stream origin of endothelial and smooth muscle cells in the neointima of long, impervious carotid-femoral grafts in the dog
Ann Vasc Surg
(1998)- et al.
Developments towards tissue-engineered, small-diameter arterial substitutes
Expert Rev Med Devices
(2008) - et al.
Novel vascular graft grown within recipient’s own peritoneal cavity
Circ Res
(1999) - et al.
Haemopoietic origin of myofibroblasts formed in the peritoneal cavity in response to a foreign body
J Vasc Res
(2000) - et al.
Blood vessels from bone marrow
Ann NY Acad Sci
(2000)
The effect of environmental cues on the differentiation of myofibroblasts in peritoneal granulation tissue
J Pathol
Fluid shear stress upregulates vascular endothelial growth factor gene expression in osteoblasts
Ann NY Acad Sci
Dynamic mechanical conditioning of collagen-gel blood vessel constructs induces remodeling in vitro
Ann Biomed Eng
The role of matrix metalloproteinase-2 in the remodeling of cell-seeded vascular constructs subjected to cyclic strain
Ann Biomed Eng
Long-term cyclic distention enhances the mechanical properties of collagen-based media-equivalents
Ann Biomed Eng
Decreased elastin synthesis in normal development and in long-term aortic organ and cell cultures is related to rapid and selective destabilization of mRNA for elastin
Circ Res
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These authors contributed equally to this work.