Tissue regeneration during lower jaw restoration in zebrafish shows some features of epimorphic regeneration

Zebrafish have the ability to regenerate skeletal structures, including the fin, skull roof, and jaw. Although fin regeneration proceeds by epimorphic regeneration, it remains unclear whether this process is involved in other skeletal regeneration in zebrafish. Initially in epimorphic regeneration, the wound epidermis covers the wound surface. Subsequently, the blastema, an undifferentiated mesenchymal mass, forms beneath the epidermis. In the present study, we re‐examined the regeneration of the zebrafish lower jaw in detail, and investigated whether epimorphic regeneration is involved in this process. We performed amputation of the lower jaw at two different positions; the proximal level (presence of Meckel's cartilage) and the distal level (absence of Meckel's cartilage). In both manipulations, a blastema‐like cellular mass was initially formed. Subsequently, cartilaginous aggregates were formed in this mass. In the proximal amputation, the cartilaginous aggregates were then fused with Meckel's cartilage and remained as a skeletal component of the regenerated jaw, whereas in the distal amputation, the cartilaginous aggregates disappeared as regeneration progressed. Two molecules that were observed during epimorphic regeneration, Laminin and msxb, were expressed in the regenerating lower jaw, although the domain of msxb expression was out of the main plain of the aggregate formation. Administration of an inhibitor of Wnt/β‐catenin signaling, a pathway associated with epimorphic regeneration, showed few effects on lower jaw regeneration. Our finding suggests that skeletal regeneration of the lower jaw mainly progresses through tissue regeneration that is dependent on the position in the jaw, and epimorphic regeneration plays an adjunctive role in this regeneration. In this study, we showed cartilaginous aggregates, named regenerated cartilage, were formed at the early stage of lower jaw regeneration in zebrafish, although the cartilage showed distinct tissue property depending on the amputation position in the jaw. Furthermore, we found that although blastemal markers were expressed during lower jaw regeneration, their expressions were restricted in the distal region of the jaw, and inhibition of Wnt/β‐catenin signaling pathway, which is associated with epimorphic regeneration of the caudal fin, had little effect on jaw regeneration.