Effects of decreased masseter muscle function and its recovery on mandibular morphology in growing mice: condylar structural properties and transcriptome changes

Reduced masticatory muscle function during the growth period is implicated in the development of craniofacial deformities; however, its effects on skeletal development remain unclear. This study investigated systemic and local skeletal responses to reduced masticatory muscle function using botulinum neurotoxin type A (BTX) in growing mice. Four-week-old male mice were divided into three groups: a saline-injected Sham group; a BTX-injected group (both injected into the right masseter muscle); and an untreated control group. Blood samples, body weight, and mandibular deviation were assessed 2, 4, and 8 weeks after injection or control treatment. Mandibular morphology was evaluated using micro-computed tomography and histology. Masseter muscle weight was measured, and bulk RNA sequencing of the right mandibular condyle was performed. Body weight and masseter muscle weight were significantly lower in the BTX group throughout the experimental period. BTX treatment induced mandibular deviation and transient changes in condylar cartilage thickness and bone morphology, which became comparable to those of the Sham group at week 8. No significant differences in circulating bone metabolism markers were observed between the BTX and Sham groups. Transcriptome analysis indicated that genes associated with the canonical Wnt signaling pathway (GO:0044338), which is involved in mesenchymal stem cell differentiation, exhibited dynamic expression changes during recovery. These findings demonstrate that unilateral reduction of masseter muscle function during the growth phase induces transient mandibular deformation accompanied by histological, morphological, and gene expression changes, and they suggest that specific gene regulatory pathways may contribute to the regulation and recovery of jaw deformities.