“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.

Discovering quandries (Rab11 Regulates Planar Polarity and Migratory Behavior of Multiciliated Cells in Xenopus Embryonic Epidermis by Kyeongmi Kim, Blue B. Lake, Tomomi Haremaki, Daniel C. Weinstein, and Sergei Y. Sokol, Dev Dyn 241:1385–1395) He just stands there and worries. He simply can't stop . . . Is his top-side his bottom? Or bottom-side top? – Dr. Seuss, On Beyond Zebra! Although the venerable Dr. Seuss was articulating the concerns of a globular creature named Quandry, his words also describe the predicament of Xenopus ectodermal Multiciliated Cells (MCCs) in which Rab11 function has been inhibited. MCCs derive from the inner layer of the epidermal ectoderm, acquire apicobasal polarity, and in a process that is not well understood, then migrate and intercalate into the outer, superficial layer. Given that the Rab11 GTPase regulates cell polarity and migration in other systems, Kim and colleagues reasoned that understanding MCC Rab11 function might yield insights into how MCCs journey into the outer layer. Their work suggests that Rab11 is polarized in MCC precursors and localized to the apical surface in differentiated cells. Inhibition of Rab11 function interferes with MCC intercalation, and causes randomization of cell orientation within the tissue plane (i. e. planar polarity), despite the fact that the cells maintain their individual apicobasal polarity.These findings suggest that, although these perturbed cells have a top and bottom side, they cannot distinguish directionality, and are unable to integrate with the outer layer. Whether polarity is required for proper intercalation, or intercalation helps establish planar polarity by means of contact with new epithelial neighbors, is another quandary that remains.

Double life (Time-Lapse Microscopy of Macrophages During Embryonic Vascular Development by Sarah Al-Roubaie, Jasmine H. Hughes, Michael B. Filla, Rusty Lansford, Stephanie Lehoux, and Elizabeth A.V. Jones, Dev Dyn 241:1423–1431). Best known for their insatiable appetite for cellular debris and pathogens, macrophages also play a role in adult angiogenesis. Although the natural assumption is that the cell type also participates in embryonic vascular development, genetic manipulations and knock-out strategies to test the idea are hampered by the complex nature of macrophage development. Here, Al-Roubaie et al., develop time-lapse technology to view potential interactions between macrophages and endothelia, cells that line vasculature, in the living embryo. The cell types were labeled by injecting chick embryos with fluorescently labeled AcLDL, which labels macrophages and endothelial cells, and with the red fluorescent phagocyte labeling dye PKH26-PCL, which labels macrophages. Their method, which is much quicker and less expensive than genetic techniques, illuminates that double-labeled cells can adopt an endothelial-like morphology and incorporate into the vessel wall. Even more tantalizing, they see that double-labeled cells are recruited to a site of induced vascular remodeling. These findings certainly merit further investigation of a role for macrophages in vascular development and remodeling.