The Zebrafish Science Monitor Vol 3(4)
DIFFERENTIAL INDUCTION OF FOUR MSX HOMEOBOX GENES DURING FIN DEVELOPMENT AND REGENERATION IN ZEBRAFISH(in press, Development)
M.- A. Akimenko1,2, S.L. Johnson3, M. Westerfield3 and M. Ekker1,2 1Loeb Institute for Medical Research, Ottawa Civic Hospital; and 2 Departments of Medicine and Anatomy & Neurobiology, University of Ottawa, 725 Parkdale Avenue, Ottawa, Ont., Canada, K1Y 4E9; 3Institute of Neuroscience, University of Oregon, Eugene, OR 97403, USA
To study the genetic regulation of growth control and pattern formation during fin development and regeneration, we have analysed the expression of four homeobox genes, msxA, msxB, msxC, and msxD in zebrafish fins. The median fin fold, which gives rise to the unpaired fins, expresses these four msx genes during development. Transcripts of the genes are also present in cells of the presumptive pectoral fin buds. The most distal cells, the apical ectodermal ridge of the paired fins and the cleft and flanking cells of the median fin fold express all these msx genes with the exception of msxC. Mesenchymal cells underlying the most 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 fertilization, msx expression in the median fin fold falls below levels detectable by in situ hybridization. Although the fins of adult zebrafish normally have levels of msx transcripts undetectable by in situ hybridization, expression of all four genes is strongly reinduced during regeneration of both paired and unpaired fins. Induction of msx gene expression in regenerating caudal fins occurs as early as 30 hours post- amputation. As the blastema forms, the levels of expression increase and reach a maximum between the third and fifth days. Then, msx expression progressively declines and disappears by day 12 when the caudal fin has grown back to its normal size. In the regenerating fin, the blastema cells that develop at the tip of each fin ray express msxB and msxC. Cells of the overlying epithelium express msxA and msxD, but do not express msxB or msxC. Amputations at various levels along the proximo- distal axis of the fin suggest that msxB expression depends upon the position of the blastema, with cells of the rapidly proliferating proximal blastema expressing higher levels than the cells of the less rapidly proliferating distal blastema. Expression of msxC and msxD is independent of the position of the blastema cell along this axis. Our results suggest distinct roles for each of the four msx genes during fin development and regeneration and differential regulation of their expression.
Zebrafish Science Monitor Vol 3(4)
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