P2.27 THE MORPHOGENESIS OF SCALES IN THE ZEBRAFISH:  A MODEL FOR HAIR/TEETH FORMATION

Vu H. Nguyen, Xiaofang Zhu, and Mary C. Mullins

Department of Dermatology and Cell & Developmental Biology, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania, USA.

Objectives:  To compare the process of morphogenesis of scales in the zebrafish to that of hair and teeth.  To conduct forward mutagenesis in the zebrafish to isolate mutants with defects in scale morphogenesis to identify genes essential in the development of cutaneous appendages. 

Methods:  whole-mount in situ hybridization, antibody staining, genetic screen, transgenic zebrafish, histological examination.

Results-conclusions:  The morphogenesis of scales in the zebrafish is analogous to the formation of hair and teeth in mouse and human.  Mesenchymal-epithelial interactions are known to be essential for the development of hair and teeth.  Our recent studies in the zebrafish indicate that the formation of scales likewise involves molecular signaling between the epithelial and mesenchymal components of scale placodes.  We analyze the in vivo expression of many genes thought to be important in the morphogenesis of hair and teeth in scale placode development in the zebrafish.  These expression studies reveal many similarities between the development of scales, and of hair and teeth.  We detect the expression of various components of the Wnt, Bone Morphogenetic Protein (BMP), Sonic Hedgehog (SHH), and Notch/Delta signaling pathways in scale placodes many days prior to the appearance of scales.  We utilize a transgenic zebrafish line TOPdGFP (Dorsky RI, et al.), which consists of a destabilized GFP mutant downstream of a promoter responsive to Wnt signaling, to show that Wnt signaling occurs during the morphogenesis of scale placodes.  These lines of evidence strongly support the hypothesis that Wnt signaling is involved in the scale formation of scales in the zebrafish, similar to hairs/teeth in mouse and human.

We are analyzing the phenotype of a zebrafish mutant that lacks scales.  We confirm that this mutant does not form scales by alizarin red staining.  We have investigated the expression of various components of the Wnt, BMP, SHH, and Notch/Delta pathways in this mutant during stages of scale morphogenesis.  We report a severe reduction or absence of expression of each and all of these genes in the scale placodes of this mutant.  We conclude that the gene mutated is required at a very early stage for scale placode development in the zebrafish.