Hedgehog pathway expression during zebrafish tooth development. The hedgehog ligand shha and receptors ptc1 and ptc2 are expressed in the developing zebrafish pharyngeal dentition but we do not detect tooth-related expression of the ligands shhb, dhh, ihha, or ihhb. (A, C, E, I, K, M-P) Dorsal views of 56 hpf embryos, anterior to the left, with the location of the right-side pharyngeal tooth germ and anterior/posterior position of adjacent section indicated by an arrow. (B, D, F, H, J, L) Transverse sections of 56 hpf embryos at the level of the developing pharyngeal teeth. The lumen of the pharynx is collapsed in these histological preparations, leaving no space between the upper and lower epithelial layers. The lower pharyngeal epithelium (double arrow), dental epithelium (arrowhead), dental mesenchyme (arrow), notochord (n), and yolk (y), are indicated. (A, B) dlx2b, a discrete marker of zebrafish pharyngeal tooth germs, is expressed in both dental epithelium and mesenchyme . Only epithelial expression is visible in B, however. (C, D) The hedgehog ligand shha is expressed widely in the pharyngeal epithelium including the dental epithelium, but not in the dental mesenchyme. (E, F) In contrast, at the anterior-posterior level of the developing teeth, shhb expression appears restricted to the upper pharyngeal epithelium and is absent from the lower epithelium and any part of the tooth germs. (G) A more rostral section from the same specimen as (F) exhibits expression in both pharyngeal epithelial layers. (H) Expression of shhb is also not seen in tooth germs at other stages of development such as 48 hpf when dental epithelial thickening is observed. The hedgehog receptors ptc1 (I, J) and ptc2 (K, L) are expressed widely in both the pharyngeal epithelium and in adjacent mesenchyme, encompassing both tissue layers in the tooth germs. We searched for, but were unable to detect, dental expression of the hedgehog ligands ihha (M; N, double label with dlx2b), ihhb (O; asterisk: notochord expression), and dhh (P). Scale bars: (A) 100 μm, (B) 25 μm.