A regulatory pathway involving retinoic acid and calcineurin demarcates and maintains joint cells and osteoblasts in regenerating fin

During zebrafish fin regeneration, blastema cells lining the epidermis differentiate into osteoblasts and joint cells to reconstruct the segmented bony rays. We show that osteoblasts and joint cells originate from a common cell lineage, but are committed to different cell fates. Pre-osteoblasts expressing commit to the osteoblast lineage upon expressing , whereas the strong upregulation of correlates with a commitment to a joint cell type. In the distal regenerate, , and are sequentially upregulated at regular intervals to define the newly identified presumptive joint cells. Presumptive joint cells mature into joint-forming cells, a distinct cell cluster that maintains the expression of these factors. Analysis of null mutants reveals that is acting upstream of and downstream of or in parallel with Calcineurin activity, potentially through the inhibition of retinoic acid signaling, regulates , and expression during joint formation. Furthermore, retinoic acid treatment induces osteoblast differentiation in mature joint cells, leading to ectopic bone deposition in joint regions. Overall, our data reveal a novel regulatory pathway essential for joint formation in the regenerating fin.