Ontogenetically Distinct Orofacial Muscle Exhibits Tilted Regeneration Towards Fibrosis

Objectives: Branchiomeric head muscles are ontogenetically and phylogenetically distinct from somitic limb muscles, and they exhibit different regenerative capacity. Unique satellite cell property of head muscle could explain the impaired myofiber formation, but the underlying mechanism for fibrosis is still elusive. This study was carried out to probe into the mechanism of the tilted orofacial muslcle regeneration towards fibrosis.
Methods: In this study, we first established a freezing-induced skeletal muscle regeneration model, and made comparisons between the regeneration characteristics in the tibialis anterior muscle and the masseter muscle. The process of myogenesis and fibrogenesis were investigated by a combination of histological, immunohistochemical and cellular analysis. Stem cell temporal dynamics were examined to characterize the role of muscle satellite cell and fibro-adipogenic progenitors (FAPs) in tibialis anterior and masseter muscle regeneration.
Results: Our results revealed that FAPs infiltrated the fibrotic area during masseter muscle regeneration, as evidenced by the Pdgfra positive cell staining and collagen I deposition. In contrast to the rapid rise and fall of FAPs number at the early regeneration stages in the tibialis anterior muscle, the number of masseter FAPs increased to a plateau without descending till 14 days after injury. In-vitro cellular behavior analysis revealed that satellite cells from the masseter muscle displayed an incresed proliferation capacity and delayed differentiation capability. FAPs from the masseter muscle demonstrated lower proliferation in comparison to tibalis anterior FAPs. The fibrogenic differentiation was stronger in masseter muscle FAPs but the adipogenic differentiation was weaker.
Conclusions: It is the first time that the pivotal role of FAPs in orofacial muscle regeneration was characterized. The persistence of FAPs without timely clearance in the first two weeks of regeneration could be accountable for the orofacial muscle fibrosis.