| Summary: | Human mesenchymal stromal/stem cells (MSC) isolated from aborted first trimester fetal bone marrow (BM) hold promise for use in tissue engineering applications and cell-based therapies due to their advantageous characteristics compared to their adult BM-MSC counterparts; faster growth kinetics, active telomerase, smaller size and higher differentiation potency. However, their isolation is restricted ethically and technically and therefore there is a need to identify a cell source with high therapeutic potential that is easily accessible in the clinic without ethical restrictions. The placenta is a potential source of readily-obtainable chorionic stem cells (CSC) throughout pregnancy. The aim of my thesis was to study the evolution of the CSC phenotype (i.e. change of stem cell characteristics) during gestation and to assess their capacity for bone repair in a mouse model of osteogenesis imperfecta (oim). I hypothesised that early fetal placental chorionic stem cells (e-CSC) were physiologically superior to their term counterparts, late chorionic stem cells (l-CSC), with advantages for use in fetal stem cell therapy of osteogenesis imperfecta (OI). In the first chapter I showed that e-CSC and l-CSC shared a common phenotype, which was intermediate between adult BM-MSC and human embryonic stem cells, with characteristics of both. I also showed the phenotype of CSC evolves during gestation with e-CSC displaying characteristics of an earlier state of stemness compared to l-CSC, such as smaller size, faster kinetics, unique expression of OCT4A variant 1 and higher levels of Nanog, Sox2, c-Myc and Klf4 expression, as well as the capacity to differentiate into lineages of the three germ layers through embryoid body formation. In the second chapter I showed the more primitive in vitro characteristics of e-CSC translated to higher tissue repair in vivo compared to l-CSC; accelerating healing when applied to a skin wound and increasing bone quality and plasticity following neonatal transplantation into the oim model. I subsequently used the oim model to assess the therapeutic potential of e-CSC for use in fetal cell therapy for OI. I showed compared to non-transplanted mice, oim transplanted with e-CSC had a two third reduction in fracture incidence, more ductile bones and increased trabecular bone volume. Prevention of fractures was attributed to the differentiation of exogenous cells to osteoblasts that expressed mature osteoblast genes and synthesised human type 1 collagen (COL1A2). However, this beneficial effect may also have resulted from an indirect effect of the transplanted cells on the endogenous cells of the host mouse, since transplanted mice had upregulation of endogenous genes involved in endochondral ossification and osteoblast differentiation. Altogether, my thesis characterises early and late human fetal chorionic stem cells, providing insight into the ontogenesis of stemness phenotype during fetal development and shows the first trimester placenta is a practical source of stem cells that can be used to treat OI during bone development.
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