When a foreign body is implanted in the peritoneal cavity, it becomes encapsulated by cells within a few days. Over the next 2 weeks, these cells undergo
differentiation to myofibroblasts and produce an organized matrix. When tubes of this tissue are grafted into a high pressure arterial site, the cells differentiate further towards smooth muscle and form a transplantable artificial artery, or vascular graft. The aims of the present study were to determine the optimal material around which this tissue capsule develops, identify the cells that contribute to the tissue capsule and finally, test the cells at different stages of capsule development for response to mesenchymal growth factors.
Five different types of objects were chosen for implantation into the peritoneal cavity of rats: natural materials— boiled blood clot and boiled egg white, and synthetic materials— Norton Tygon® R-1000 ultra soft tubing, Cole-Parmer C-flex® tubing, latex balloon and biodegradable polymer. It was found that boiled blood clot induced
the best response, with most tissue capsules free-floating (81.2% free-floating rate), and a thick tissue layer formed by 2-3 weeks after implantation. These tissue capsules could be produced in desired shapes according to how blood clots were trimmed. Boiled egg white also induced a high percentage (up to 72.5%) of free-floating capsules. However, it was more difficult to observe the tissue capsule around egg white since it is the same colour, and the capsule wall was thinner than that formed around a boiled blood clot at the same time-point.
Synthetic materials with a smooth, non-adhesive surface (e.g. Norton Tygon® R-1000 ultra soft tubing and latex balloon) showed poor tissue capsule formation. However, synthetic materials with a rough surface (e.g. biodegradable polymer, dexon mesh) adhered to omentum, connective tissue or pelvic fat bodies. Consequently, the majority of experiments performed in this project to
determine other parameters utilized boiled blood clots as foreign bodies to form the tissue capsules.
Immunofluorescence staining for the common leukocyte antigen (CD45) and the macrophage marker (EDI) showed that in the early stage of tissue formation, cells were primarily of haemopoietic origin. Examination of cell morphology with transmission electron microscope (TEM) identified most of these cells as macrophages/monocytes. With further development of the capsule, most cells took on a myofibroblast appearance and expressed a-smooth muscle actin (α-SMA) but no longer expressed CD45 or EDI. TEM showed many of these cells contained myofilaments, suggesting either the transdifferentiation of monocyte/macrophages or repopulation of the tissue capsule with smooth muscle-like cells.
When capsules were enzyme-dispersed and plated in culture, cells from the 'early' (day 3)
capsule had the morphology of macrophages and did not respond to the mesenchymal growth factors PDGF-BB and FGF-2. However, at a more mature stage (day 11) of capsule development, cells responded to PDGF-BB, FGF-2 and TGF-pi in a similar manner to vascular SMC. Treatment with PDGF-BB (1-100ng/ml) or FGF-2 (1-10ng/ml) induced the proliferation of cells from 11-day tissue capsules; treatment with TGF-β1 (0.2-20ng/ml) inhibited the cells' proliferative response. Treatment of cells from the day 3 tissue capsule with either FGF-2, PDGF-BB or TGF-β1 did not induce the myofibroblast phenotype. However TGF-β1 treatment (along with serum-deprivation) of cells from the day 11 capsule induced their differentiation to the myofibroblast phenotype, indicated by expression of α-SM actin. Cells did not express the SM-specific
marker, SM-MHC, in any of these treatments with growth factors.
These studies provide information concerning the biological processes involved in the response to a foreign body implanted in the peritoneal cavity, the cells that form the capsule around this foreign body, and the growth factors that regulate their growth and differentiation. The data provide valuable information in the quest to develop "grow-your-own" vascular grafts in the peritoneal cavity.