Two origins of blastemal progenitors define blastemal regeneration of zebrafish lower jaw

Zebrafish possess a remarkable ability to regenerate complicated structures by formation of a mass of undifferentiated mesenchymal cells called blastema. To understand how the blastema retains the original structural form, we investigate cellular transitions and transcriptional characteristics of cell identity genes during all stages of regeneration of an amputated lower jaw. We find that mesenchymal blastema originates from multiple sources including nucleated blood cells, fibroblasts, damaged muscle cells and pigment cells. These cells are transformed into two populations of blastemal progenitors: foxi1-expression and isl1-expression, before giving rise to cartilage, bone, and muscle. Time point- based transcriptomal analysis of 45 annotated Hox genes reveal that five 3'-end Hox genes and an equal number of 5'-end Hox genes are activated largely at the stage of blastema reformation. RNA in situ hybridization shows that foxi1 and pax3a are respectively expressed in the presumptive mandible skeletal region and regenerating muscle at 5 dpa. In contrast, hoxa2b and hoxa11b are widely expressed with different domain in chondrogenic blastema and blastema mesenchyme. Knockdown foxi1 changes the expression patterns of sox9a and hoxa2b in chondrogenic blastema. From these results we propose that two origins of blastemal progenitors define blastema skeleton and muscle respecifications through distinct signaling pathways. Meanwhile, the positional identity of blastema reformation is implicated in mesenchymal segmentation and characteristic expression pattern of Hox genes.