“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.

Strange bedfellows (Mapping Differentiation Kinetics in the Mouse Retina Reveals an Extensive Period of Cell Cycle Protein Expression in Postmitotic Newborn Neurons by Marek Pacal and Rod Bremner, Dev Dyn 241:1525–1544) Pacal and Bremner's careful analysis of expression patterns of cell cycle and differentiation markers in the developing retina, reveal oddities within. The neuroblastic layer (NBL) harbors mitotic progenitors at its apical surface that initiate cell cycle exit as they migrate, eventually terminally differentiating at the basal surface. The authors find that a subpopulation of cells labeled with pan cell cycle markers – Pcna, Mcm6, and Ki67 – co-localizes with neuronal differentiation proteins. This finding is in contrast to expression of a number of other cell cycle and progenitor cell markers (Cyclins A, B, D, Vsx2) whose expression is mutually exclusive with differentiation markers. A particularly intriguing finding is that Ki67 expression is extinguished in the subpopulation of post-mitotic Ki67+ cells (6-7%) upon reaching the terminally differentiated ganglion cell layer (GCL). One explanation is that Ki67 is not functionally relevant in this population. Another, more provocative possibility is that postmitotic Ki67+ cells are a previously described subpopulation that can re-enter the cell cycle in the absence of Rb, a gene required for cell cycle exit, representing an intermediate state at the cusp of terminal differentiation. Further experiments will reveal whether the cells are simply an anomaly, or whether they reveal insights into the progression from proliferation to differentiation.

It's not witchcraft (Lens Regenerates by Means of Similar Processes and Timeline in Adults and Larvae of the Newt Cynops pyrrhogaster by Takeshi Inoue, Ryo Inoue, Rio Tsutsumi, Kikuo Tada, Yuko Urata, Chiaki Michibayashi, Shota Takemura, and Kiyokazu Agata, Dev Dyn 241:1575–1583). There may be a good reason why witches' boiling cauldrons contain eye of newt. Urodeles such as the newt Cynops pyrrhogaster wield the seemingly supernatural power to regenerate lost body parts throughout their lifetime. The observation that juveniles replace lost body parts faster than adults have led to the hypotheses that younger cells have a greater regenerative ability, or that mechanisms for regeneration differ over time. Based on careful examination of lens regeneration in embryos and adults, Inoue and colleagues have reached an alternative conclusion. The authors performed lentectomies in embryonic, larval, and adult transient transgenic newts (F0 generation) that display red fluorescence under control of the γ-crystallin promoter, a marker of differentiation. Using immunohistochemistry, morphology, and reporter expression they find very similar rates of regeneration, and regeneration stages, suggesting conservation of cellular and molecular regeneration mechanisms at different life stages. They conclude that lens completion takes longer in adults because the adult lens must expand to a 1,000 μm in diameter vs. 300 μm in diameter in juveniles. This strikingly simple explanation raises an obvious question, does this finding hold true in other regenerative systems? Some aspects of regeneration may not be witchcraft after all.