The role of stem cells in skeletal health and disease

National Institute Of Dental & Craniofacial Research

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Abstract

Biological Activity Post-natal skeletal stem cells The bone/marrow organ is a complex system that includes not only hard tissue proper, but also a population of bona fide SSCs and more committed BMSC progenitor cells. SSCs arise from different specifications of neural crest and mesoderm, and during development, they are formed by the incorporation of osteogenically committed cells onto blood vessel walls as pericytes. BMSCs/SSCs remain quiescent until stimulated by factors that signal for a bone turnover event, in which BMSCs/SSCs control, at least in part, the formation and activity of osteoclasts, and replace resorbed bone. BMSCs/SSCs are also “flexible,” and respond to changes in the microenvironment to change their phenotype: they can adopt an osteogenic, chondrogenic, adipogenic, or stromal cell fate. BMSC/SSC cell fate is controlled by many extrinsic factors (hormones, growth factors, cytokines), mediated by a multitude of signaling pathways, but also intrinsic factors (mutations, epigenetics) that alter the biological activity of BMSCs/SSCs (reviewed in Robey, 2025). These cells are quite unique in their ability to not only form the specialized mineralized matrix of bone, but also in their support of hematopoiesis, a property that is not found anywhere else in the normal postnatal organism. These cells are not like other types of “MSCs” based on these exclusive properties. Likewise, nonskeletal “MSCs” are not like BMSCs/SSCs, and for these reasons, the terminology should be abandoned in favor of nomenclature that reflects their tissue of origin and differentiation capacity, and more accurately describes their status as a stem or progenitor cells, based on rigorous assays (reviewed in Robey, 2025). Differences between BMSCs/SSCs derived from bones and cartilage that develop from different embryonic origins (that is, of the axial and appendicular skeleton that develop from paraxial and lateral plate mesoderm, respectively), and of the facial skeleton that develop from neuroectoderm are not well known. In consideration of the use of BMSCs/SSCs for human craniofacial reconstruction, studies addressing this issue are of importance. Studies are also aimed at determining the ability of hiPSCs to form germ layer-specific bone and cartilage. The different characteristics of bone and cartilage based on different embryonic sources may explain differences in the phenotypic display of certain genetic diseases depending on location, and how they can be used successfully in TE/RM (reviewed in Robey, 2025). Pluripotent stem cells It is now known that one important property of a type of pluripotent stem cell, human embryonic stem cells (hESCs), is their ability to exist in primed and naïve pluripotent states. A previous meta-analysis indicated the existence of heterogeneous pluripotent states derived from diverse naïve protocols using medium designed to “reset” the cells to a naïve state. In a recent study, we characterized a commercial, RSeT-based pluripotent state under various growth conditions. Notably, RSeT hESCs can circumvent the hypoxic growth conditions required by naïve hESCs, although some RSeT cells (e.g., H1 cells) exhibit much lower single-cell plating efficiency and display altered or significantly retarded cell growth under both normoxia and hypoxia. Importantly, RSeT hPSCs lack many transcriptomic hallmarks of naïve and formative pluripotency (the phase between naïve and primed states). Integrative transcriptome analysis suggests that our primed and RSeT hESCs are similar to the early stage of post-implantation embryos, in line with previously reported primary hESCs and early hESC cultures. Moreover, RSeT hESCs do not express naïve surface markers such as SUSD2 and CD75 at significant levels. At the biochemical level, RSeT hESCs show differential dependence on FGF2 and co-independency on both Janus kinase (JAK) and TGFβ signaling in a cell line–specific manner. Thus, RSeT hESCs represent a previously unrecognized pluripotent state downstream of naïve pluripotency. Our data suggest that human naïve pluripotent potentials may be restricted in RSeT medium, which sustains FGF2 activity. Hence, this study provides new insights into pluripotent state transitions in vitro (Chen et al, Stem Cells, in press). Disease In current studies, comparisons are being made between normal IPSC-derived cartilage progenitors to those derived from patients with Jansen metaphyseal chondrodysplasia. These patients have activating mutations of the PTH/PTHrP Receptor 1R (PTH1R), and present with not only axial and appendicular dysmorphology (mesoderm-derived) but also a severe temporomandibular joint (neural crest) disorder as well. Studies are aimed at determining signaling pathways that are disrupted downstream of the mutant receptor that could represent targets for novel treatments. These studies also shed light on the pathways that are operative during normal skeletal development. Tissue engineering In addition to serving as a model system for study of skeletal homeostasis and disease, the remarkable ability of BMSCs/SSCs to recreate a bone/marrow organ themselves (when transplanted with an appropriate scaffold) makes them a leading candidate for bona fide tissue engineering. However, there are many hurdles yet ahead in terms of appropriate delivery and stabilization strategies, based on the time required to generate functional weight-bearing bone. While many have hoped that these cells would be useful for treatment of systemic skeletal diseases and disorders by systemic infusion or direct injection due to paracrine, immunomodulatory, and immunosuppressive effects of the cells, this is not a stem cell therapy. Nor are there clear mechanisms of action based on the fact that the cells rapidly disappear. More preclinical studies that identify rigorous outcome parameters to be measured and establishment of efficacy are needed (reviewed in Robey, 2025). A major goal is to develop techniques for bone and cartilage regeneration using stem cells in pre-clinical studies for translation into clinical trials in human patients. In previous studies, we identified a scaffold (hyaluronic acid-coated fibrin microbeads) that supported stable, non-hypertrophic cartilage by BMSCs/SSCs up to 28 wks in a subcutaneous pouch in immunocompromised mice and rats. However, when these cells and scaffolding were transplanted into cartilage defects in immunocompromised rodents, they initially made cartilage at 1-2 months, but the cartilage disappeared. It was noted that the scaffold seemed to disappear more rapidly in the injury site than in the subcutaneous site. These results suggest that improvements are needed in either the chondrogenic differentiation of the BMSC/SSCs, or a change in the scaffold to make it more durable. Subsequently, a method was developed to differentiate induced pluripotent stem cells into stable cartilage of paraxial mesodermal origin, which gives rise to the axial skeleton. Unlike the cartilage made by BMSC/SSCs, this cartilage was stable beyond 5 months in an in vivo cartilage injury model in rodents. Current studies are focused on devising a similar protocol for differentiation of iPSCs into lateral plate mesoderm for repair of cartilage in the appendicular skeleton.