It has been proposed that cells are regulated to form specific morphologies and sizes according to positional information. However, the entity and nature of positional information have not been fully understood yet. The zebrafish caudal fin has a characteristic V-shape; dorsal and ventral fin rays are longer than the central ones. This fin shape regenerates irrespective of the sites or shape of fin amputation. It is thought that reformation of tissue occurs according to positional information. In this study, we developed a novel transplantation procedure for grafting a whole fin ray to an ectopic position and examined whether the information that specifies fin length exists within each fin ray. Intriguingly, when long and short fin rays were swapped, they regenerated to form longer or shorter fin rays than the adjacent host fin rays, respectively. Further, the abnormal fin ray lengths were maintained for a long time, more than 5 months, and after further re-amputation. In contrast to intra-fin grafting, when fin ray grafting was performed between fish, cells in the grafts disappeared due to immune rejection, and the grafted fin rays adapted to the host position to form a normal fin. Together, our data suggest that the information that directs fin length does exist in cells within a single fin ray and that it has a robust property-it is stable for a long time and is hard to rewrite. Our study highlighted a novel positional information mechanism for directing regenerating fin length.

The interaction between immune cells and injured tissues is crucial for regeneration.Previous studies have shown that macrophages attenuate inflammation caused by injuries to support the survival of primed regenerative cells. Macrophage loss in zebrafish mutants like cloche (clo) causes extensive apoptosis in the regenerative cells of the amputated larval fin fold. However, the mechanism of interaction between macrophage and injured tissue is poorly understood. Here, we show that a phosphoinositide 3-kinase gamma (PI3Kγ)-mediated signal is essential for recruiting macrophages to the injured tissue. PI3Kγ inhibition by the PI3Kγ-specific inhibitor, 5-quinoxalin-6-ylmethylene-thiazolidine-2,4-dione (AS605240 or AS), displayed a similar apoptosis phenotype with that observed in clo mutants. We further show that PI3Kγ function during the early regenerative stage is necessary for macrophage recruitment to the injured site. Additionally, protein kinase B (Akt) overexpression in the AS-treated larvae suggested that Akt is not the direct downstream mediator of PI3Kγ for macrophage recruitment, while it independently plays a role for the survival of regenerative cells. Together, our study reveals that PI3Kγ plays a role for recruiting macrophages in response to regeneration.