DIFFERENTIAL INDUCTION OF 4 MSX HOMEOBOX GENES DURING FIN DEVELOPMENTAND REGENERATION IN ZEBRAFISH

To study the genetic regulation of growth control and pattern formatio n during fin development and regeneration, we have analysed the expres sion of four homeobox genes, msxA, msxB, msxC and msxD in zebrafish fi ns. The median fin fold, which gives rise to the unpaired fins, expres ses these four msx genes during development. Transcripts of the genes are also present in cells of the presumptive pectoral fin buds. The mo st distal cells, the apical ectodermal ridge of the paired fins and th e deft and flanking cells of the median fin fold express all these msx genes with the exception of msxC. Mesenchymal cells underlying the mo st distal cells express all four genes. Expression of the msx genes in the fin fold and fin buds is transient and, by 3 days after fertiliza tion, msx expression in the median fin fold falls below levels detecta ble by in situ hybridization. Although the fins of adult zebrafish nor mally have levels of msx transcripts undetectable by in situ hybridiza tion, expression of all four genes is strongly reinduced during regene ration of both paired and unpaired fins. Induction of msx gene express ion in regenerating caudal fins occurs as early as 30 hours postamputa tion. As the blastema forms, the levels of expression increase and rea ch a maximum between the third and fifth days. Then, msx expression pr ogressively declines and disappears by day 12 when the caudal fin has grown back to its normal size. In the regenerating fin, the blastema c ells that develop at the tip of each fin ray express msxB and msxC. Ce lls of the overlying epithelium express msxA and msxD, but do not expr ess msxB or msxC. Amputations at various levels along the proximodista l axis of the fin suggest that msxB expression depends upon the positi on of the blastema, with cells of the rapidly proliferating proximal b lastema expressing higher levels than the cells of the less rapidly pr oliferating distal blastema. Expression of msxC and msxD is independen t of the position of the blastema cell along this axis. Our results su ggest distinct roles for each of the four msx genes during fin develop ment and regeneration and differential regulation of their expression.