# Identification of Smmhc-expressing mesenchymal cells in orofacial bone at single-cell resolution

## TL;DR

This study identifies a new type of stem cell in mouse orofacial bone that is crucial for bone development and tissue balance.

## Contribution

The discovery of Smmhc-expressing mesenchymal stem/stromal cells as a novel and functionally essential subset for craniofacial bone homeostasis.

## Key findings

- Smmhc+ MSCs are multipotent and give rise to osteoblasts, osteocytes, periodontal ligament cells, and dental pulp cells.
- Ablation of Smmhc+ MSCs impairs orofacial bone development and disrupts tissue homeostasis.
- Smmhc+ MSCs regulate the balance between osteogenesis and bone resorption in the orofacial skeletal niche.

## Abstract

Craniofacial bone regeneration remains a major clinical challenge, yet the identity of orofacial mesenchymal stem/stromal cells (OMSCs) has not been fully elucidated. Here, we performed single-cell RNA sequencing (scRNA-seq) on mouse orofacial bone and identified multiple stromal cell clusters. Cell-cell communication mapping and trajectory inference uncovered the heterogeneity of OMSCs and functional divergence among subpopulations. We identified a previously unrecognized population, Smmhc-expressing mesenchymal stem/stromal cells (MSCs), at the earliest stage of the progenitor lineage trajectory. In vivo lineage tracing demonstrated that Smmhc+ MSCs are multipotent, giving rise to osteoblasts, osteocytes, periodontal ligament (PDL) cells, and dental pulp cells. Targeted ablation of Smmhc+ MSCs using SmmhcCreER;iDTR mouse model led to impaired orofacial bone development and disrupted orofacial tissue homeostasis, characterized by reduced osteogenic differentiation and non-cell autonomous reduction of bone resorption. Collectively, this study establishes a cellular atlas of OMSCs and identifies Smmhc+ MSCs as a functionally indispensable subset for craniofacial bone homeostasis, orchestrating the dynamic balance between osteogenesis and bone resorption within the orofacial skeletal niche.