Schwann Cell Mapping and Characterization in Bone of Different Embryonic Origins.

Schwann cells (SCs) provide support for nerves throughout the body. Despite importance for nerve function and repair, the morphology and distribution of SCs in bone remains largely undefined. In this study we used a "Schwann Cell Mapper" mouse (Mpz-Cre+/-;TdT+/+;Ngfr-eGFP+/+) and a "p75 Lineage Tracer" mouse (Ngfr-CreERT2+/-;ZsGreen1+/+) to study SC localization and morphology within the adult mouse calvaria, limb, and vertebrae. We found that all nerves in bone were covered by mature myelinating or non-myelinating SCs, labeled by MPZ and p75-NGFR, respectively. Mature SCs populated the periosteum and entered the bone marrow through transcortical canals. Non-myelinating SCs outnumbered myelinating SCs in bone, with a ratio of ~2:1 by length density. Non-myelinating SCs in periosteum had more branching and increased size relative to myelinating SCs. In addition, we identified two candidate populations of MPZ lineage+ and p75-NGFR+ immature SCs (iSCs) that were distributed throughout the calvarial periosteum. Similar to mature SCs, p75-NGFR+ candidate iSCs were more prevalent at a ratio of ~3:1. Overall, neural crest-derived calvarial bone had evidence of increased SC maturity relative to mesoderm-derived sites, identifying niche-level differences in SC maturation. Lastly, lineage tracing revealed that mesenchymal lineages in bone were largely negative for both MPZ and p75-NGFR (<0.1% labeling). These findings provide a framework of SC organization in bone and highlight previously unrecognized diversity across skeletal compartments. By defining distribution and morphology, this work lays the foundation for future studies investigating how SCs contribute to bone biology, including roles in repair, pain, and homeostasis.