FishFace

By 55 hpf, the ceratohyal (ch) consists of a condensation of chondrogenic cells in several layers that is difficult to distinguish along its dorso-posterior aspect from the ventral portion of the articulating interhyal (ih). Additionally, the articulation with the basihyal (bh) dorsally and the contralateral ceratohyal medially at the anterior midline is indistinct. Alcian blue staining of matrix is observed in these developing cartilages at this stage.

By 79 hpf, the ceratohyal (ch) has increased in size and has straightened its shape from that seen at 55 hpf. It is still difficult to distinguish the dorso-posterior aspect of the ceratohyal from the ventral portion of the articulating interhyal (ih). The separation from the basihyal (bh) dorsally and the contralateral ceratohyal medially at the anterior midline is more distinct from that seen at 55 hpf. Note in the ventral view that the middle region of the ceratohyal now consists of a single-cell layer in the mediolateral dimension, which is consistent with the idea that early cartilage elongation occurs by cell intercalation.

By 104 hpf, it is possible to distinguish the dorso-posterior aspect the ceratohyal (ch) from the ventral portion of the articulating interhyal (ih) by a slight decrease in fluorescence (*). The ventro-anterior aspect of the ceratohyal now projects further anteriorly than the dorso-anterior. The articulation with the basihyal (bh) dorsally and contralateral ceratohyal medially at the anterior midline is now clearly distinct. Branchiostegal ray 3 (bsr3) projects posteriorly from its articulation with the postero-lateral aspect of the ceratohyal.

By 150 hpf, the ceratohyal (ch) has grown considerably from that seen at 104 hpf. The region consisting of one cell layer of chondrocytes in the mediolateral dimension now extends to the posterior end, suggesting that cell intercalation continues to drive ceratohyal growth in length. Clusters of chondrocytes at both ends of the ceratohyal in the lateral view gives the overall morphology the appearance of a dumbbell. The ceratohyal bone (chb) can be seen in the ceratohyal perichondrium at the middle region, while the ventral hypohyal (hhv) can be seen in some specimens in the perichondrium at the ventro-anterior tip. The ceratohyal continues to articulate with the interhyal (ih) along the dorso-posterior edge of the ceratohyal, with the basihyal (bh) and contralateral ceratohyal at the anterior midline, and with branchiostegal ray 3 (bsr3) along the postero-lateral aspect of the ceratohyal. More anterior to branchiostegal ray 3, branchiostegal ray 2 (bsr2) projects postero-ventrally from the postero-lateral aspect of the ceratohyal.

By 199 hpf, the ceratohyal (ch) has increased in size from that seen at 150 hpf. The overall morphology of the ceratohyal increasingly appears like a dumbbell from the lateral view. The ceratohyal continues to articulate with the interhyal (ih) along the dorso-posterior edge of the ceratohyal, with the basihyal (bh) dorsally and the contralateral ceratohyal medially at the anterior midline, and with branchiostegal rays 2 and 3 (bsr2, bsr3) along the postero-lateral aspect of the ceratohyal. More anterior to branchiostegal ray 2, branchiostegal ray 1 (bsr1) projects postero-ventrally from the medial aspect of the ceratohyal bone (chb) at the middle region of the ceratohyal. The ventral hypohyal (hhv) is obvious now in the perichondrium at the ventro-anterior tip of the ceratohyal.

By 249 hpf, the overall morphology of the ceratohyal (ch) appears like a dumbbell from the lateral view, with a thicker posterior end. The ceratohyal articulates with the interhyal (ih) along the dorso-posterior edge of the ceratohyal, with the basihyal (bh) dorsally and the contralateral ceratohyal medially at the anterior midline, and with branchiostegal rays 2 and 3 (bsr2, bsr3) along the postero-lateral aspect of the ceratohyal. Also, branchiostegal ray 1 (bsr1) articulates with the medial side of the ceratohyal bone (chb), which has grown in size along the anterior-posterior aspect of the ceratohyal perichondrium. The ventral hypohyal (hhv) has spread dorsally and posteriorly in the perichondrium at the ventro-anterior tip of the ceratohyal.

By 342 hpf, the ceratohyal (ch) has grown considerably from that seen at 249 hpf, with a dramatic increase in dorsal-ventral size at the posterior end. Articulations of the ceratohyal with the interhyal (ih), the contralateral ceratohyal, and branchiostegal rays 1, 2, and 3 (bsr1, bsr2, bsr3) are visible. The ventral hypohyal (hhv) bone has now spread posteriorly in the perichondrium across a deep cleft that has formed at the ventro-anterior tip of the ceratohyal. Also visible is the epihyal (eh) bone in the dorso-posterior aspect of the ceratohyal perichondrium, which in lateral view’ is covered by the interopercle (iop).

By 516 hpf, the ceratohyal (ch) has grown in size considerably from that seen at 342 hpf, although the medio-lateral thickness at the middle region remains one cell layer. Articulations of the ceratohyal with the interhyal (ih), the contralateral ceratohyal, and branchiostegal rays 1, 2, and 3 (bsr1, bsr2, bsr3) are visible. The four bones of the ceratohyal perichondrium now almost cover the cartilage completely: the ventral hypohyal (hhv) and dorsal hypohyal (not shown) along the anterior end, the ceratohyal bone (chb) along the middle region, and the epihyal (eh) at the posterior end. Also forming in some specimens is branchiostegal ray 4 (bsr4), which articulates with the medial aspect of the ceratohyal bone anterior to branchiostegal ray 1.