We have used human Embryonic Stem cells (hESC) and human Mesenchymal Stem Cells (hMSC) in rat models of bone repair in order to assess the efficacy of these cells for treatments of trauma and skeletal diseases. Graft survival is considered to be of key importance to efficacy of these treatments. Therefore the aim of this study was to develop a technique for identifying implanted cells in histological preparations without the need for genetic engineering of the implanted cells.

Methods: In our experiments hES and hMSC were pre-differentiated during cell culture towards the osteoblast lineage, and then implanted in a Demineralised Bone Matrix (DBM) carrier into an experimentally created full thickness calvarial bone lesion. The animals were sampled seven days and fourteen days after implantation into either immune deficient (RNU-Foxn1rnu) or immune competent (wild type) Sprague Dawley rats. Fluorescent In Situ Hybridisation (FISH) using whole human genome probes identified the human cells within the host lesion site.

Results: Our results have demonstrated that hESC and hMSC derived cells survive in both immune competent (wild type) and immune compromised (nude) animals for the initial seven days post implantation. On the other hand while both the hESC and hMSC derived cells are capable of surviving for at least 14 days in immune compromised animals they do not survive for this period of time in immune competent animals.

Discussion: It appears that the cell/DBM graft is not rejected within seven days even when exposed to the wild type hosts T cell response. However longer term survival required an immune deficient model that is lacking in a T cell response. This data points to interesting future studies regarding which components of the host response are responsible for xenogenic stem cell implant rejection.

Introduction: Emerging therapies for regenerating skeletal tissues are focused on the repair of pathologically altered tissue by the transplantation of functionally competent cells and supportive matrices. Stem cells have the potential to differentiate into musculoskeletal tissue and may be the optimal cell source for such therapies. In vitro studies have demonstrated the ability of adult bone marrow stromal cells (MSC) and human embryonic stem cells (hES) to generate bone, but little is known regarding their potential to repair bone in vivo. Preclinical studies in animal models will allow investigation into the extent that regenerated tissue resembles functional and healthy tissue, and its potential clinical application.

Aim: To assess whether adult and embryonic stem cells maintained their ability to form musculoskeletal tissues in vivo using diffusion chambers implanted into the peritoneal cavity of nude mice. Currently, ongoing experiments are assessing the use of MSCs and hES cells to regenerate bone in a rodent preclinical model.

Methods: MSC cells and embryoid body-derived H9 hES cells were prepared as previously described (Haynesworth et al Bone 1992; Sottile et al Cloning Stem Cells 2003). Groups of cells were left untreated or pre-treated with osteogenic (OS) media for 5 days. Study 1: Single cell suspensions of untreated or pre-treated cells were injected into diffusion chambers which were implanted intraperitonealy into nude mice and left for 79 days. Study 2: OS pre-treated cells were implanted into an experimentally created full thickness calvarial defect in adult male Wistar rats. The defect area was left empty or filled with demineralised bone matrix (DBM: Allosource®) alone or with DBM/MSCs or DBM/hES composite. Tissues were collected 4 weeks after surgery.

Analysis: Histological and immunochemical techniques were used to evaluate cell phenotypes and the contribution of transplanted cells to tissue repair.

Results: Study 1: Both hES (in 2/3 chambers) and MSC (3/3) cells pre-treated with OS media formed only mineralised bone. No cartilage was detected in these OS pre-treated cells. Untreated hES cells formed both mineralised bone and cartilage within the chambers (2/3). In contrast, untreated MSC cells (3/3) produced no mineralised bone or cartilage. Preliminary analysis demonstrated the absence of any other tissue type in the diffusion chambers. Study 2: Active bone regeneration was observed at the edges of the calvarial defect after 4 weeks, with a high density of cells present within the MSC or hES/DBM composite. No signs of local cellular immunological response were seen.

Summary: OS pre-treatment restricted differentiation towards the osteoblast lineage in both hES and MSC cells indicating successful directed differentiation in vivo. Untreated hES and MSC cells produce a different range of cell phenotypes suggesting that the two cell sources represent cells at a different stage of commitment in a common cell lineage or cells derived from two distinct cell lineages. New bone formation was seen at the site of the calvarial defect in the presence OS pre-treated MSC and hES cells suggesting that these cells may support in vivo bone repair in a preclinical model. Funded by Geron Corporation