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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.

Stem cells on demand (Facultative Stem Cells in Liver and Pancreas: Fact and Fancy by Kilangsungla Yanger and Ben Z. Stanger, Dev Dyn240:521–529) Video on demand enables the viewer to watch what he wants (feel-good movies), when he wants (while sick in bed). Similarly facultative stem cells (FSCs)—differentiated cells that become multipotent—enable an organ to generate what it needs (stem cells) when it needs them (following injury). In a famous example, salamander limb loss triggers mature mesenchymal cells to de-differentiate and then re-differentiate into the myriad cell types needed to reconstitute a new limb. In this provocative review, Yanger and Stanger discuss the controversy surrounding FSC-based regeneration in mammals. Presented in detail are two putative FSC populations, oval cells that appear following hepatotoxin-based injury of the liver, and ductal cells in the regenerating pancreas. The authors discuss what is known about the existence, origins, and potential of these cell populations, and do not shy away from pointing out potential flaws in published results. What becomes clear is thatsettling the controversy is paramount. Defining what allows a differentiated cell to become a stem cell “on demand” could profoundly impact regeneration research and our understanding of the multipotent state.

GATA get it right (Different Requirements for GATA Factors in Cardiogenesis Are Mediated by Non-Canonical Wnt Signaling by Boni A. Afouda and Stefan Hoppler, Dev Dyn240:649–662) Most people hate to be mistaken for their sibling. After all, everyone deserves to be recognized as an individual. The same is true for the GATA family of transcription factors, collectively known for their roles in endoderm and cardiac development. To distinguish functions of GATA 4, 5, and 6 in cardiac development—a task that has proven to be difficult in vivo—Afouda and Hoppler employ animal cap explants that have been induced to undergo cardiogenesis. Expression of early and late cardiogenic markers were used to assess effects of morpholino depletion and ectopic induction of GATA 4-6 in such cultured explants. The combined results point to overlapping roles for GATA4 and 6 in early cardiogenesis and cardiomyocyte differentiation, and for GATA5 in differentiation. Their differential roles are further supported by the finding that ectopic expression of Wnt 11b, or the downstream non-canonical Wnt component Disheveled, rescues GATA4 and 6, but not GATA5 morphants. These results suggest that GATA4 and 6 uniquely function through the non-canonical Wnt11b pathway. Parsing the specific roles of GATAs, and their pathways may help to devise effective stem cell therapies. It is only when an individual's unique abilities are understood that they can be used to their full potential.

Live action BMP signaling (Dynamic Analysis of BMP-Responsive Smad Activity in Live Zebrafish Embryos by Derek W. Laux, Jennifer A. Febbo, and Beth L. Roman, Dev Dyn240:682–694; Dynamic Smad-Mediated BMP Signaling Revealed Through Transgenic Zebrafish by Ross F. Collery and Brian A. Link, Dev Dyn 240:712–722) Those investigating sites of bone morphogenetic protein (BMP) activity, an important regulator of development and disease, have traditionally been relegated to piecing together static images of fixed, stained tissues. Knowing that live action beats stop motion, two groups have generated fluorescent transgenic reporter lines that display live BMP signaling in the optically clear zebrafish. Their reporters are based on conserved BMP-responsive elements (BRE) from the mouse inhibitor of differentiation-1 (Id1) promoter, that are responsive to the BMP effectors, Smad 1/5/8. The reporters effectively replicate many well-known sites of Smad 1/5/8 activity, respond to genetic and chemical manipulations of the BMP pathway, and potentially reveal previously uncharacterized sites of BMP action. However, they also have some limitations; the authors discuss why the reporters fail to detect some known sites of BMP activity. Regardless, the dynamic reporter lines will undoubtedly prove useful in screening for small molecules that affect BMP signaling and identifying genetic modulators of the pathway.