“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 will spotlight 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.
Changing Tide (Dev Dyn237:545–553) Like ridges in the sand carved by ocean waves receding toward low tide, the favored Clock and Wavefront model predicts that iterated somite boundary formation results from molecular juxtaposition of two inputs: a posterior moving determination wavefront, and a genetic oscillator, or somitogenesis clock. To enable quantitative study of the process, Schröter and colleagues document somitogenesis period in wild-type zebrafish (time between boundary formation) and somite length (length between boundaries) in wild-type zebrafish. Gauging these determinants is imprecise; therefore, groups of live embryos were imaged simultaneously to obtain mean population measurements. The authors find that somitogenesis period is linearly temperature-dependent, while somite length is temperature-compensated. In addition, both somite period and length varied from trunk to tail, but changes in the two measurements did not track together as expected, challenging the text-book version of the Clock and Wavefront model. The authors propose instead that wavefront velocity changes during somitogenesis, independent of clock periodicity. In other words, neither the rhythm of the lapping waves nor the rate at which the tide falls is constant. Theories ebb and flow, but the best adapt to accommodate new discoveries.
The Great Uncoupler (Dev Dyn237:713–724) To form a proportioned, mature embryo, growth and development must work hand-in-hand. But are these processes necessarily linked, or can they be genetically distinct? Supporting the former idea, Lavine and colleagues previously showed that development of coronary vasculature and myocardial growth were both outputs of fibroblast growth factor (FGF) signaling to the cardiomyoblast. Here, they test this concept further by teasing apart FGF outputs. Targeted ectopic expression of p27Kip1, a cyclin-dependent kinase inhibitor, enhances myocardial proliferation defects observed in mice lacking Fgfr1/2. By contrast, it has no effect on observed coronary vascular defects, including vascular density and expression of vascular genes, Vegf-A and Vegfr1. Lavine also previously demonstrated that Hedgehog (HH) functions downstream of FGF signaling to regulate vascular development. Here they find that HH has no affect on myocardial proliferation both in vitro and in vivo. The work unequivocally shows that outputs of FGF signaling to the cardiomyoblast are the sum of two parts.
Stress management (Dev Dyn237:725–735) How do you react to stress? Some act out while others become quiet. As it turns out, chick heart endothelial cells (CHECs) do both, depending on what kind of stress they are under. Cardiac cells are typically exposed to shear stress, drag force imposed by blood flow, and to stretch forces, caused by hydrostatic and blood pressures. Using QRT-PCR, Hierck et. al show that expression profiles of a handful of previously tested stress-response genes differ under shear stress and stretch forces, with an overall more robust response to the former conditions. CHECs harbor primary cilia-membrane extensions that are connected to the cytoskeleton. To test the hypothesis that these structures function as stress mechanosensors, the authors compare effects of shear stress on ciliated and nonciliated cells. They find that both chicken arterial ECs (CAECs) that do not bear cilia, and CHECs where cilia are disrupted by chloral hydrate, show a twofold reduced expression of KLF2, a shear responsive gene. Moreover CHECs exposed to colchicine and taxol, drugs that disrupt or stabilize cytoskeletal microtubules, respectively, have opposing expression profiles. These findings show that primary cilia rely on their microtubule-based cytoskeletal connections to transmit mechanosensory information. Together their results suggest that ECs have a self-regulatory mechanism for coping with stress. And you?