Osteogenesis imperfecta (OI) is a congenital disorder characterized by brittle bones prone to fracture. Intrauterine transplantation (IUT) of mesenchymal stem cells (MSC) into a murine OI model (oim) yields up to 5% donor cell chimerism in bone with concomitant phenotypic improvement1 . However, bone marrow transplantation has recently been suggested to give rise to arguably greater donor cell-derived osteopoeisis in OI mouse models2,3 .
Here we profile the identity of donor-derived bone cells in oim mice following IUT of murine bone marrow from adult CAG-EGFP C57Bl/6 donors. At e14.5 fetuses underwent intraperitoneal injections to deliver 5 x 106 cells. Fracture occurrence was monitored by X-ray and tissue analysis conducted at 6wks. Quantitation of GFP sequence within genomic DNA demonstrated that 8 of 11 (72%) transplanted mice had chimeric bone marrow (median: 0.06%, range: ~0.01%-1.0%). Chimeric mice had a median of one long bone fracture per mouse, comparable to vehicle-injected controls. Histology of humeri confirmed the presence of GFP+ cells both within the marrow and on/near the endosteal bone surfaces. Immunofluorescent staining revealed that none of the GFP+ cells were osteoblasts (uniformly negative for osterix, osteocalcin and collagen type I), but were often located adjacent to osteoblasts. GFP+ cells on the bone surface expressed galectin-3, consistent with an osteoclast identity, whereas GFP+ cells near the bone surface, often forming a canopy over osteoblasts, expressed the macrophage marker, F4/80, confirming their identity as osteomacs.
Our data demonstrate that donor cells following bone marrow IUT contribute to haematopoietic lineage bone cells (osteoclasts and osteomacs) but not mesenchymal lineage bone cells (osteoblasts). This is consistent with the presence of robustly-transplantable HSC within bone marrow and a scarcity of robustly transplantable MSC or common HSC/MSC progenitor.