A gene expression map of the larval Xenopus laevis head reveals developmental changes underlying the evolution of new skeletal elements.

The morphology of the vertebrate head skeleton is highly plastic, with the number, size, shape, and position of its components varying dramatically between groups. While this evolutionary flexibility has been key to vertebrate success, its developmental and genetic bases are poorly understood. The larval head skeleton of the frog Xenopus laevis possesses a unique combination of ancestral tetrapod features and anuran-specific novelties. We built a detailed gene expression map of the head mesenchyme in X. laevis during early larval development, focusing on transcription factor families with known functions in vertebrate head skeleton development. This map was then compared to homologous gene expression in zebrafish, mouse, and shark embryos to identify conserved and evolutionarily flexible aspects of vertebrate head skeleton development. While we observed broad conservation of gene expression between X. laevis and other gnathostomes, we also identified several divergent features that correlate to lineage-specific novelties. We noted a conspicuous change in dlx1/2 and emx2 expression in the second pharyngeal arch, presaging the differentiation of the reduced dorsal hyoid arch skeletal element typical of modern anamniote tetrapods. In the first pharyngeal arch we observed a shift in the expression of the joint inhibitor barx1, and new expression of the joint marker gdf5, shortly before skeletal differentiation. This suggests that the anuran-specific infrarostral cartilage evolved by partitioning of Meckel's cartilage with a new paired joint. Taken together, these comparisons support a model in which early patterning mechanisms divide the vertebrate head mesenchyme into a highly conserved set of skeletal precursor populations. While subtle changes in this early patterning system can affect skeletal element size, they do not appear to underlie the evolution of new joints or cartilages. In contrast, later expression of the genes that regulate skeletal element differentiation can be clearly linked to the evolution of novel skeletal elements. We posit that changes in the expression of downstream regulators of skeletal differentiation, like barx1 and gdf5, is one mechanism by which head skeletal element number and articulation are altered during evolution.

Redo pull-through in Hirschsprung's [corrected] disease for obstructive symptoms due to residual aganglionosis and transition zone bowel.

BACKGROUND: Reoperations in Hirschsprung disease may be required for residual aganglionosis or transition-zone bowel found at the distal pull-through. We aimed to review the management of patients who had this complication and offer suggestions on how to avoid it. METHODS: Ninety-three patients with Hirschsprung disease were referred to our institution with recurrent problems after a pull-through done elsewhere. All required reoperations with a variety of indications, and of these, 25 had residual aganglionosis/transition-zone histology. This was the only indication for redo in 16 children. RESULTS: Children (range, 2-17 years) presented 6 to 66 months after the initial pull-through. The predominant symptoms were enterocolitis (n = 9 [56%]), constipation (n = 7 [44%]), failure to thrive (n = 5 [31%]), and impaction (n = 4 [25%]). The rectal biopsy performed as part of their post pull-through work up showed hypertrophic nerves (n = 16), absent ganglion cells (n = 6), and normal ganglion cells (n = 10). The original frozen-section biopsy, determining the level of the pull-through, only sampled the seromuscular layer in 3 children, leading to misdiagnosis. Reoperations involved a transanal resection (n = 15) and a posterior sagittal approach (n = 1). In all cases, obstructive symptoms were resolved, and no patient has had recurrent enterocolitis. CONCLUSION: Patients' post pull-through with recurrent obstructive symptoms may have residual aganglionosis or transition-zone bowel. Reoperation can result in the resolution of these symptoms. A full-thickness biopsy at the time of the initial pull-through to include the mucosa and submucosa may increase the possibility of identifying hypertrophic nerves.