Implantation of a foreign body into the peritoneal cavity is known to induce an inflammatory reaction whereby cells from peritoneal fluid attach to the surface of the implant, differentiate over time into myofibroblasts and lay down extracellular matrix. The myofibroblast-rich tissue capsule becomes covered by a single layer of mesothelial cells so that it resembles an 'inside-out' artery. In the Centre for Research in Vascular Biology (CRVB), advantage is being taken of this reaction in order to develop an artificial artery in rats, rabbits and dogs.
The first aim of this study was to create a new device which induces in-tissue-growth without adhesions and suitable for the development of an artificial artery in dogs and humans. The second aim was to determine whether other smooth muscle organs can be developed using the same technology, in particular whether peritoneal-derived tissue can be successfully grafted into the uterus ("artificial uterus").
In Chapter 3, a device was produced which consisted of a polyethylene tube covered with Dexon mesh and contained within an outer sheath with holes of different size and shape. These devices (25mm long) were implanted into the peritoneal cavity of dogs, and then harvested after 3 weeks. The results showed that the size and shape of holes in the outer sheath is important, such that holes of 2.5mm diameter and circular shape resulted in high quality tissue capsules with a low rate of adhesion. Holes of other sizes and shapes either produced no tissue capsules or had unacceptable degrees of adhesion to the omentum.
In order to produce tissue suitable for use as a uterine graft, different materials including boiled blood clot, egg white, polyethylene tubing, plastic tubing, Dexon mesh wrapped tubing and sheathed tubing were implanted into the rat peritoneal cavity. The results showed that the boiled blood clot was most effective in inducing tissue capsule formation. The capsule remained free-floating in the peritoneal cavity and produced a tissue capsule that could be molded in a variety of shapes and sizes, including a wide tube-like structure resembling the shape of a uterus. Histological analysis of the capsule tissue (Chapter 4) showed that the optimal time to harvest the tissue for transplantation to the uterus was three weeks, when the number of myofibroblasts was maximal and the wall of the tissue was thick enough for transplantation. This was confirmed by Western analysis, which showed maximal α-smooth muscle actin expression at this time.
Boiled egg white also gave a good free-floating rate and tissue capsule formation. However, the tissue was difficult to see as it is the same colour as boiled egg white, and the capsule wall is thinner than around boiled blood clot at the same time-point. Materials with smooth, non-adhesive surfaces, such as polyethylene, plastic, or sheath tubing remained free-floating in the peritoneal cavity but showed poor tissue capsule formation. Material with a rough surface, such as Dexon mesh-wrapped tubing, adhered to the omentum, connective tissue or pelvic fat bodies.
In Chapter 5, the tissue capsule was tested as a uterine graft in a rat model. In these experiments, a segment of one horn of the rat uterus was removed and replaced with the peritoneal-derived tissue capsule. The contralateral uterine horn was used as control, and the grafted tissue was harvested at different time-points. Histological analysis showed that, with time, the tissue increased in thickness and took on the morphology of uterine tissue, with endometrium overlying several layers of smooth muscle cells (myometrium?), interspersed with collagen fibrils. Vascularisation of the tissue also increased with time. Furthermore, the grafted tissues were capable of supporting a pregnancy with the elasticity required to resist the increasing pressure of the developing embryos at the later stages of pregnancy, and the capacity to supply the necessary nutrition.
In summary, the data from these studies provides valuable information in the quest to develop artificial organs from peritoneal-derived myofibroblast-rich tissue capsule.