“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.

Limbs made to order (Role of Paraxial Mesoderm in Limb/Flank Regionalization of the Trunk Lateral Plate by Miyuki Noro, Hiroki Yuguchi, Taeko Sato, Takanobu Tsuihiji, Sayuri Yonei-Tamura, Hitoshi Yokoyama, Yoshio Wakamatsu, and Koji Tamura, Dev Dyn240:1639–1649) Gnathostomes (jawed vertebrates) invariably have two sets of paired appendages, but their placement and size vary by species. Noro et al. test the hypothesis that the presomitic mesoderm (PSM) regulates regional specification of the limb field within the subjacent lateral plate mesoderm (LPM). Here, they find that PSM from different positions along the anterior–posterior (A–P) axis specify different aspects of limb growth. Ablation of forelimb-level PSM results in smaller limb width along the A–P axis. By contrast ablation of PSM posterior to the forelimb, called flank PSM, results in limbs that do not protrude as far distally. In addition, flank PSM triggers apoptosis in the flank LPM at the beginning stages of limb bud outgrowth, a process that can be inhibited by implantation of an FGF8-soaked bead into the LPM. This finding is significant in light of the fact that FGF8 signaling from the apical ectodermal ridge (AER) is an initiating event in limb outgrowth. Based on these and other data, the authors present a model in which FGF-8 responsive LPM cells are eliminated by apoptosis, a mechanism that may help control limb spacing, and also limit the number of limbs formed. This work suggests that species variation in limb size and position may be controlled by differential allocation of PSM to forelimb and flank regions.

Stress response (Tgfβ/Alk5 Signaling Is Required for Shear Stress Induced Klf2 Expression in Embryonic Endothelial Cells by Anastasia D. Egorova, Kim Van der Heiden, Simone Van de Pas, Peter Vennemann, Christian Poelma, Marco C. DeRuiter, Marie-Jose T.H. Goumans, Adriana C. Gittenberger-de Groot, Peter ten Dijke, Robert E. Poelmann, and Beerend P. Hierck, Dev Dyn240:1670–1680) Snuggled within the womb, or protected by a shell, you may not think that an embryo would experience stress. But within them, an emerging bloodstream heaves biomechanical forces against pristine vessel walls. The finding that shear stress activates expression of a gene critical for endothelial function, Krüppel like factor 2 (KLF2), has helped show the importance of stress on normal development. Here, Egorova et al. dissect the molecular pathway that regulates stress-activated KLF2. After discovering that the TGFβ/Alk5 pathway, a well-studied developmental regulator, is activated in endothelial cells (ECs) lining the cardiac cushion in a chick shear stress model in vivo, they explore the pathway further in vitro. In cultured mouse embryonic endothelial cells (MEEC), they very thoroughly show that shear stress induction of KLF2 is dependent upon dose-response signaling through TGFβ/Alk5/Smad2 and then a Mek/Erk5 pathway. Of interest, the pathway is not activated by shear stress in two postnatal EC cell lines. Determining whether the embryonic shear stress pathway is activated under disease conditions that create abnormal shear stress, like hypertension, could lead to novel therapies. Understanding embryonic stress may, in the end, reduce ours.

Untapped potential (Multipotential Differentiation Even After Lineage-Restricted Stages by Tsutomu Motohashi, Katsumasa Yamanaka, Kairi Chiba, Kentaro Miyajima, Hitomi Aoki, Tomohisa Hirobe, and Takahiro Kunisada, Dev Dyn240:1681–1693) This work reveals the hidden potential in in vivo, lineage-restrictedneural crest (NC). Embryonic stem cells (ESCs) were generated by homologous recombination in which expression of the NC marker Sox10 is labeled with fluorescent protein. Following a protocol established previously, cells with NC-like characteristics were derived from the ESCs. Surprisingly, when cultured in vitro, the NC-like cells, which were expected to be lineage restricted, could differentiate as melanocytes (M), neurons (N), and glia (G). This included Sox10+/Kit+ cells, thought to represent dorosolaterally migrating, M restricted cells, and Sox10+/Kit− cells, representing ventrolaterally migrating, N and G restricted cells. The results were repeated with cultured Sox10+/Kit+ and Sox10+/Kit− NCs isolated from mice that were derived from the transgenic ESC line. It remains to be determined whether the repressed multipotentiality of in vivo lineage-restricted NCs is tapped later in development, or during homeostasis or regeneration.