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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.

Joining forces (Fusion of Uniluminal Vascular Spheroids: A Model for Assembly of Blood Vessels by Paul A. Fleming, W. Scott Argraves, Carmine Gentile, Adrian Neagu, Gabor Forgacs, and Christopher J. Drake, Dev Dyn 239:398–406). In a process common among large caliber blood vessels, the descending aorta is formed upon fusion of two smaller vessels, in this case, the bilateral dorsal aortae. Fleming et al. use an in vitro system they previously developed, uniluminar vascular spheroids, to understand physical aspects of vessel fusion. Similar to blood vessels, spheroids have an outer layer of smooth muscle cells (SMCs) and an inner endothelial cell (EC) layer. Here, the authors show that when two spheroids are juxtaposed in hydrogel culture, where they can also engage in extra-spheroid adhesive interactions, they fuse such that they first form a peanut shape before making a larger, uniluminar ovoid. Aspects of these properties were also observed in sections of developing mouse descending aortas, which fuse in a cranial to caudal direction, suggesting spheroid fusion approximates in vivo vessel fusion. They also show that spheroid fusion in hanging drop culture displays properties found in a tissue liquidity model, demonstrating that the multicellular system has liquid-like behaviors. This bilateral approach—joining forces of in vitro and in vivo systems—promises to continue to reveal mechanisms of vessel fusion, and will help in devising designs for tissue-engineered blood vessels.

Wrapping it up (Visualization of Myelination in GFP-Transgenic Zebrafish by Seung-Hyun Jung, Suhyun Kim, Ah-Young Chung, Hyun-Taek Kim, Ju-Hoon So, Jaeho Ryu, Hae-Chul Park, and Cheol-Hee Kim, Dev Dyn 239:592–597). One of the great advantages of zebrafish is the ability to spotlight biological processes in real time by means of green fluorescent protein (gfp) -labeled transgenes. To further understand myelanation—the process by which glia-derived myelin enwraps axons in the central nervous system—Jung and colleagues use the technique. The insulating properties of the multilayered sheath enable rapid propagation of action potentials. The authors show that when gfp is placed under control of the promoter of an abundant protein in myelin, myelin basic protein (mbp), expression is limited to oligodendrocytes and Schwann cells. Using the tool, they show that oligodendrocytes myelinate spinal cord axons not only during embryogenesis but also in the postembryonic fish until at least 6 months of age. They also reveal that spinal cords in adult fish bear unmyelinated axons with a morphology distinct from myelinated axons. Hiding in plain site until the new tool exposed them, the adult unmyelinated axons are no longer under wraps.

Missed by a whisker (EphA4 Is Necessary for Spatially Selective Peripheral Somatosensory Topography by H.A. North, A. Karim, M.F. Jacquin, and M.J. Donoghue, Dev Dyn 239:630–638). In the veil of night, a mouse dexterously dodges a cat's swat and escapes into a small hole in the wall—feats made possible by a whisker-based awareness of its environment. Whiskers of the snout, or mystacial vibrissae, are arranged in five rows (A–E), a configuration shared by trigeminal neurons that innervate them. Despite the importance of the sematosensory system, little is known about how proper connections between vibrissae and innervating neurons are made. Here, North and colleagues test whether EphA4, an Eph receptor involved in axon guidance in other contexts, participates in this process. They show that EphA4 is expressed most highly in the ventral whisker pad during vibrissae specification and innvervation, while several EphA4 ligands are expressed on innvervating trigeminal axons. In EphA4−/− mice, development and innervation of A–D row vibrissae are mostly normal. By contrast, E row ventroposterior vibrissae fail to develop, and corresponding innervating neurons first display abnormal whisker pad invasion, and later, premature defasciculation. The results show that precise topography of the somatosensory system is specified by region-specific cues expressed on neuron targets. The findings also suggest a model whereby vibrissae formation is dependent on innervation by a threshold number of nerve endings. In the case of EphA4−/− ventroposterior E row vibrissae, the threshold was missed by a whisker.