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“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.

Get into the groove (Mouse Primitive Streak Forms In Situ by Initiation of Epithelial to Mesenchymal Transition Without Migration of a Cell Population by Margot Williams, Carol Burdsal, Ammasi Periasamy, Mark Lewandoski, and Ann Sutherland, Dev Dyn241:270–283) As one might expect, data would suggest that creation of the amniotic primitive streak, required for formation of the three primordial germ layers—ectoderm, mesoderm, and endoderm—takes some moving and shaking. Avian epiblast streak precursor cells first perform rotational polonaise movements, named after the elegant Polish folk dance, and after arriving at their new position at the posterior embryo, undergo convergence and extension to create the primitive streak. Mesoderm is then produced from streak cells that undergo epithelial-to-mesenchymal transition (EMT). Until now, primitive streak formation in mammals has not been well documented owing to the difficulties of culturing and imaging postimplantation embryos. Overcoming these problems, Williams et al., gloriously document cell behavior during primitive streak formation using four-dimensional live imaging of genetically labeled mouse embryos, and immunohistochemistry. Rather than dancing around the inevitable, as rabbit and chick epiblast do, mouse epiblast cells dive right in. Foregoing a specialized streak precursor population, mouse epiblast cells simply initiate EMT in situ and slip into the resulting primitive streak progressively, from posterior to anterior, thus elongating the structure. Differences in streak morphogenesis may be consequential to embryonic geometry, where cells must travel further in the flat blastodisc of the chick and rabbit than in the cup-shaped cylinder of the mouse.

A mouse is not a frog (Requirements for Jag1-Rbpj Mediated Notch Signaling During Early Mouse Lens Development by Tien T. Le, Kevin W. Conley, Timothy J. Mead, Sheldon Rowan, Katherine E. Yutzey, and Nadean L. Brown, Dev Dyn241:493–504) This study seeks to resolve a quandary that has been scuttering in lens development circles. In Xenopus, it has been established that Notch signaling hops into multiple roles in lens growth and differentiation. In the mouse, Notch pathway components are expressed early, but their contribution to early steps, lens induction and vesicle formation, have been unclear. Do the species take different approaches to lens development, or does Notch indeed regulate lens initiation in mouse? Leand colleagues attack this problem by using the AP2α-Cre driver, which initiates Cre expression in lens ectoderm earlier than the previously used Le-Cre. The AP2α-Cre driver is used to conditionally delete the only Notch ligand expressed during lens induction, Jag1, and an intracellular downstream effector, Rbpj. Their results suggest that, unlike in frog, mouse Notch scurries past lens specification and vesicle formation roles. However, removal of either component does cause defects in lens vesicle separation from the surface ectoderm, an event that occurs as the lens pit changes shape into a vesicle. Their results point toward defects in apoptotic events that control lens stalk regression that is required for this transition. The frog hops and the mouse scurries, but they both reach the same destination in the end.