“Highlights” calls attention to exciting advances in developmental biology that have recently been reported in Developmental Dynamics. Development is a broad field encompassing many important areas. To reflect this fact, the section spotlights significant discoveries that occur across the entire spectrum of developmental events and problems: from new experimental approaches, to novel interpretations of results, to noteworthy findings utilizing different developmental organisms.

Chew on this (Asymmetric Requirement of Surface Epithelial β-Catenin During the Upper and Lower Jaw Development by Ye Sun, Ian Teng, Randi Huo, Michael G. Rosenfeld, Lorin E. Olson, Xiaokun Li, and Xue Li, Dev Dyn241:663–674) One's identity is often linked to distinguishing facial features: a pouty mouth, a strong jaw, prominent cheekbones. Here, Sun et al., show that β-catenin, an effector of canonical Wnt signaling, sculpts some of the features that make us who we are. Pitx1/Cre was used to drive expression of loss-of-function (LOF) or gain-of-function (GOF) β-catenin alleles in surface epithelium of the first pharyngeal arch. Although many features are affected, those with the most striking phenotypes are the upper and lower jaw. In β-catenin LOF mutants, the mandibles (lower jaw) are absent and mandible skeletal structures are severely deformed, while the maxilla (upper jaw) and associated skeletal structures are unaffected. By contrast, in β-catenin GOF mutants, mandibular structures are expanded or enlarged while maxillary elements are deficient. Consistent with a central role in craniofacial development, β-catenin is genetically upstream of several central players, including shh, bmp4, wnt5a, fgf8, and their downstream effectors. The work shows that epithelial β-catenin is required for lower jaw formation, while its attenuation is necessary for proper upper jawbone development. The authors suggest β-catenin as a nodal point for evolutionary craniofacial adaptations.

Turning back time (Avian Intervertebral Disc Arises From Rostral Sclerotome and Lacks a Nucleus Pulposus: Implications for Evolution of the Vertebrate Disc by Bradley J. Bruggeman, Jennifer A. Maier, Yasmin S. Mohiuddin, Rae Powers, Yinting Lo, Nuno Guimarães-Camboa, Sylvia M. Evans, and Brian D. Harfe, Dev Dyn241:675–683) Sure, chicks and mice are both vertebrates, but here Harfe and colleagues reveal that their vertebrae are quite different. At the center of mouse intervertebral discs (IVD) is the nucleus pulposa, a jelly-like structure made of proteoglycans that is important for absorbing shock and resisting spinal compression. The structure is derived from the notochord, a transient structure during mouse embryogenesis. An artistic panel of histologically stained IVD from mammals, snakes, amphibians, reptiles, fish, and birds reveals for the first time that the nucleus pulpous is a mammalian-specific structure. Do nonmammalian notochord still contribute to the IVD? Contrary to the mouse, the chick notochord persists through embryogenesis, but fate mapping studies show it does not contribute to the vertebral column. Instead, the sclerotome (rostral somite) is a major contributor to the avian disc, a finding that leads to the discovery that sclerotome also gives rise to the mouse annulus fibrosis that surrounds the nucleus pulpous. The authors suggest that the premammalian IVD was an annulus fibrosis-like structure.