Skeletal site-specific variations in myeloid cells: Insights from single-cell RNA sequencing of the mandible and femur

To understand differences that exist between the cell populations in different skeletal sites, we performed an unbiased genetic survey via single-cell RNA sequencing of CD11b+ myeloid cells from mandibular- and femur-derived bone marrow of 2-mo-old C57BL/6 mice. Our results reveal transcriptomic evidence that suggests a uniquely inflammatory genetic profile of the mandibular-derived CD11b+ cells. The monocyte cell population found within the CD11b+ cells analyzed by scRNA-seq expressed Csf1r and Tnfrsf11a suggesting that this population contained osteoclast precursors. Osteoclasts of the craniofacial region facilitate processes such as tooth eruption and jawbone development. Evidence from multiple researchers suggests that craniofacial osteoclasts exhibit differences in size, gene expression, and activity compared to their counterparts within the appendicular skeleton. A biological mechanism to explain the observable difference between craniofacial osteoclasts and osteoclasts in the long bones has not been previously explored. This monocyte population had enhanced inflammatory gene expression by qRT-PCR which correlated with an increase in select areas of open chromatin by assay for transposase-accessible chromatin using sequencing. Further exploration into a specific upregulated gene determined KLF4 was both necessary and important for proper differentiation in mandibular- but not femur-derived cells.