Fin ray branching is defined by TRAP+ osteolytic tubules

SUMMARY The shaping of bone structures relies on various cell types and signalling pathways. Here, we use the zebrafish bifurcating fin rays during regeneration to investigate bone patterning. We found that the regenerating fin rays form via two mineralization fronts that undergo an osteoblast-dependent fusion/stitching until the branchpoint, and that bifurcation is not simply the splitting of one unit into two. We identified tartrate-resistant acid phosphatase-positive (TRAP + ) osteolytic tubular structures at the branchpoints, here named osteolytic tubules (OLTs). Chemical inhibition of their bone-resorbing activity strongly impairs ray bifurcation, indicating that OLTs counteract the stitching process. Finally, by testing different osteoactive compounds, we show that the position of the branchpoint depends on the balance between bone mineralization and resorption activities. Overall, these findings provide a new perspective on fin ray formation and bifurcation, and reveal a key role for OLTs in defining the proximo-distal position of the branchpoint. Graphical summary