ESPE Yearbook of Paediatric Endocrinology

To read the full abstract: Stem Cells, 2020;38(6):769–781.

In brief: A novel population of osteoprogenitor cells in proximity to transcortical channels is found to persist during skeletal maturation. These migrate, expand and contribute to bone formation.

Commentary: Bone formation and regeneration requires multiple distinct populations of progenitor cells, contributing to a complex orchestration of cytokine production, cell migration and differentiation. Lineage tracing experiments revealed multiple cellular niches, including perivascular osteoprogenitors in the periosteum and bone marrow. Here, Root et al identified and characterized a perivascular stem cell niche which, in contrast to previously identified niches, is specific to cortical bone.

By combining inducible DMP1-dependent labelling with Col1-coupled GFP expression in a murine model system, the Authors localized a population of osteoprogenitors in proximity to transcortical channels residing independently of skeletal maturation. They proved in a cortical bone transplantation model that these perivascular progenitor cells contribute to bone formation and remodelling but have a rather marginal role in fracture repair. Given that this novel cell type is specific to cortical bone, the ability of migration, expansion and differentiation suggests a distinct role in intracortical remodelling. Since this still poorly understood process is of high clinical importance, the characterization of the involved cellular populations is particularly important to decipher the regulation of cortical bone mass and cortical porosity. However, the architecture of cortical vascular canals and osteons is highly species-specific and the lack of Harvesian canals in mice may hamper direct translation from murine data to human physiology. Future studies will be needed to investigate the existence and relevance of the identified cortical niche of osteoprogenitors, not at least as a potential target for the development of novel therapeutic approaches.