Ossifications in the young wild-type (WT) zebrafish (A) and homozygous edn1 mutant (B). Ventral views, anterior to the top, of negative images of bones fluorescently labeled with Calcein in larvae at 7-days postfertilization. Ventral bones of the pharyngeal arches are identified (Cubbage and Mabee, 1996) by their labels on the left side, and dorsal bones are labeled on the right side in both panels, all of them are present as bilateral pairs. (A) The wild type first or mandibular arch includes a dorsal and a ventral dermal bone, the maxilla (max) and dentary (den), and a dorsal and a ventral cartilage-replacement bone, the quadrate (qu) and retroarticular (ra). The second or hyoid arch includes a dorsal and two ventral dermal bones, opercle (op), and two branchiostegal rays (bsrp and bsrm), and dorsal and ventral cartilage replacement bones (very incompletely ossified at this stage), the hyomandibula (hm) and ceratohyal (ch). The most posterior arch includes a cartilage-replacement bone, ceratobranchial 5 (cb5). Overlaying ceratobranchial 5 is the cleithrum (cl), a long dermal bone connecting the posterior skull and the pectoral girdle. Two other craniofacial bones present at this stage lie deeper in the tissue and are not labeled, the parasphenoid and the endopterygoid. (B) Many of the anterior ossifications (in the first two arches) are missing in the edn1 mutant. Ceratobranchial 5 and the cleithrum are present, shortened and somewhat malformed. In the mandibular arch dermal bones (max/den) are present but severely malformed, an example of the `wicket′ phenotype discussed in the text (see also Fig. 3). In the hyoid arch the opercle is present and its joint region (upper part of the bone) is markedly expanded, a mild example of the `opercle-gain′ phenotype described in the text and other Figures. Scale bar: 100 μm.