The foreign body response to materials implanted in the peritoneal cavity has been
utilised to generate tissue suitable as grafts for smooth muscle organs such as blood
vessels, bladder, uterus and vas deferens. In order to gain insight into the regulatory
mechanisms involved in this process, this thesis used immunohistochemistry as well
as microarray technology and real-time RT-PCR to identify the temporal changes in
cellular composition and gene expression associated with the regulation of tissue
Chapter 3 defined the changes in cellular composition during development of the
tissue capsule that formed around the foreign object in the peritoneal cavity.
Immunohistochemical analysis showed that 3-7 days after implantation, the foreign
body became encapsulated by several layers of haemopoietic (CD45+) cells, mainly
macrophages (CD68+ and CCR1+). By day 14, cells of the tissue capsule no longer
expressed CD45 or CD68, but were positive for -SM actin and SM22, indicating the
presence of myofibroblasts or smooth-muscle-like cells.
Chapter 4 used gene expression profiling to elucidate key regulators of tissue capsule
development. In line with the immunohistochemical data, early capsule (days 3-7)
development was dominated by the expression of monocyte/macrophage-specific
genes (CD14, CSF-1, CSF-1R, MCP-1) and pro-inflammatory mediators (TGF-, 2
integrins, ephrin signalling). As tissue capsule formation progressed (days 14-21),
there was a significant up-regulation of myofibroblast-associated and pro-fibrotic
genes (including Wnt/-catenin signalling, TGF-inducible molecules such as CTGF,
SMADs -1, -2, -4, collagen-1 subunits). Thus this chapter highlighted two majorphases during peritoneal-derived tissue capsule formation: an inflammatory phase that
characterised early development (days 3-7) and a fibrotic phase that dominated later
stages of tissue capsule development (days 14-21).
In light of microarray data identifying vasculogenic genes associated with capsule
development, Chapter 5 investigated the ability of tissue capsule cells to
transdifferentiate into endothelial and smooth muscle cells. It was found that cells
isolated from either day 3 or day 7 tissue capsule were capable of sprouting and
formed cord- and tube-like structures (characteristics of endothelial cells) in vitro.
When cultured under conditions favouring smooth muscle differentiation (heparin and
low serum) these cells could also be induced to express smooth muscle-specific
isoforms of smooth muscle myosin - SM-1 and SM-2. Thus these experiments
demonstrated the capacity for early-stage tissue capsule cells to differentiate along
both endothelial and smooth muscle cell lineages.
Chapter 6 of this thesis investigated the role of the chemokine SDF-1 and its
receptor CXCR4 (both found to be differentially expressed in Chapter 4) in the
development of the tissue capsule. Inhibition of SDF-1 using the CXCR4-specific
antagonist, AMD3100, resulted in major alterations to the myofibroblast and
macrophage populations within the tissue capsule. Unlike in normal tissue capsule
development, AMD3100 treatment resulted in the persistence of macrophages. These
macrophages were immunohistologically different from those observed in normal
tissue capsules, suggesting the recruitment of a unique macrophage population to the
foreign object. Furthermore, immunohistochemical analysis suggested a
monocyte/macrophage origin of the myofibroblasts within AMD3100-treated tissuecapsules. Thus SDF-1 and CXCR4 are implicated as key players in regulating the
development and cellular composition of the tissue capsule.
In summary, this thesis defined the cellular composition of peritoneal-derived tissue
capsules from a pre-dominance of macrophages in the early stages to myofibroblastrich
tissue over time. In line with the cellular composition, gene profiling elucidated
key regulatory pathways, specifically those that direct the tissue response to foreign
material and the transition from inflammation to fibrosis. The fibrotic phase is
characterised by the presence of myofibroblasts, which are possibly derived from one
or more progenitors that possess the potential for differentiation to multiple cells
within the mesenchymal lineage.
The information gained in this thesis gives insight into the cellular and transcriptional
dynamics taking place during tissue capsule development, not only for tissue
engineering purposes, but may also allow the development of strategies to inhibit
inflammation and fibrosis.