Title

Investigating the Cellular Origins of Heterotopic Ossification and Assessing the Plasticity of Tissue Resident Progenitor Cells

Date of Completion

January 2011

Keywords

Biology, Molecular|Biology, Cell|Health Sciences, Human Development

Degree

Ph.D.

Abstract

In regenerative biology, the overarching goal is to rebuild degenerating or absent tissue with a patient's own cells. While this ultimate aim is rather ambitious, the motivation to achieve these ideal clinical tissues has driven the field of stem and progenitor cell biology to unprecedented levels of scientific and public involvement. Additionally, many pathological ailments, such as cancer and heterotopic ossification, are believed to be the consequence of abnormal progenitor cell behavior. Here, we focus on identifying and characterizing tissue resident progenitors. These primitive-like cells hold promise as an autologous cell source for regenerative treatments and importantly, afford us the opportunity to begin to understand seemingly normal cellular processes in pathological contexts. In this regard, we have concentrated on the tissue resident adult progenitors in skeletal muscle and specifically, their role in soft tissue associated heterotopic ossification. We use extensively, the Cre/loxP lineage tracing system to label putative plastic cell types and assess the inherent plasticity of these cells in biologically relevant mouse-chick chimeric models of developmental potency and bioassays of heterotopic ossification. ^ Skeletal muscle, a known and plentiful source of progenitor cells is commonly associated with extraskeletal pathologies such as muscular dystrophy and heterotopic ossification. Two groups of resident progenitors, the skeletal muscle satellite cells (SCs) and vascular endothelium (VE)/vascular endothelial progenitors (VEPs), have been considered to be putative cell sources for ectopic lesions of heterotopic ossification due to their reported multipotent characteristics. However, we find that SCs and VE/VEPs remain committed to their respective lineages in biologically relevant tests of osteogenicity, contradicting much of the published work in regards to their alleged skeletogenic potential. Instead, we identify a novel mesenchymal cell type located in the interstitial space of skeletal muscle that harnesses robust in vivo chondrogenic, osteogenic and adipogenic capabilities. Furthermore, we decisively prove through in vitro clonal assays that the majority of these mesenchymal progenitors are capable of multilineage differentiation, contributing to spontaneously forming adipogenic and fibrocyte-like cells, and to BMP-induced osteogenic cells. We use in parallel many technologies that have been considerably refined to identify, isolate, and characterize, specific and practically pure cell populations, critically allowing us to attribute functional outcomes of potency tests to unique cell types. In conclusion, these data decisively establish degrees of potency for skeletal muscle resident progenitors and identify the putative cell-of-origin for the ectopic skeletal anlagen of heterotopic ossification. ^