Whole-mount observation of pharyngeal and trabecular cartilage development in lampreys

Because the jaw is thought to have evolved as a dorsal-ventral articulation of the anterior pharyngeal arch, knowledge of developmental patterning in the pharyngeal arch is critical to understanding the origin and evolution of the jaw in gnathostomes. It is particularly important to determine whether Agnatha already possessed developmental polarity along the dorsal-ventral axis of the pharyngeal arch. We used the Weigert staining method to examine the development of cartilage in whole-mount lamprey specimens. We found that although the transverse rods showed symmetrical patterning along the dorsal-ventral axis, the hypobranchial bar and subchordal rod showed distinct developmental patterning. Thus, our observations suggest that pharyngeal cartilage also differentiates along the dorsal-ventral axis. In addition, the parachordal rods were shifted dorsally compared to the subchordal rods. Although the development of cartilage occurred earlier in the anterior arches, the fusion of the subchordal rods occurred earlier in the posterior arches. We also noted a unique morphology of cartilage in the ninth pharyngeal arch. Our descriptions and the methods used here facilitate the observation of pharyngeal cartilage in whole-mount specimens, and will aid in the study of developmental patterning in the lamprey pharyngeal arch.