Embryonic axis elongation is a complex multi-tissue morphogenetic process responsible for the formation of the posterior part of the amniote body. How movements and growth are coordinated between the different posterior tissues (e.g. neural tube, axial and paraxial mesoderm, lateral plate, ectoderm, endoderm) to drive axis morphogenesis remain largely unknown. Here, we use quail embryos to quantify cell behavior and tissue movements during elongation. We quantify the tissue-specific contribution to axis elongation using 3D volumetric techniques, then quantify tissue-specific parameters such as cell density and proliferation. To study cell behavior at a multi-tissue scale, we used high-resolution 4D imaging of transgenic quail embryos expressing fluorescent proteins. We developed specific tracking and image analysis techniques to analyze cell motion and compute tissue deformations in 4D. This analysis reveals extensive sliding between tissues during axis extension. Further quantification of tissue tectonics showed patterns of rotations, contractions and expansions, which are consistent with the multi-tissue behavior observed previously. Our approach defines a quantitative and multi-scale method to analyze the coordination between tissue behaviors during early vertebrate embryo morphogenetic events.

BackgroundPlant biologists have long speculated about the mechanisms that guide pollen tubes to ovules. Although there is now evidence that ovules emit a diffusible attractant, little is known about how this attractant mediates interactions between the pollen tube and the ovules.ResultsWe employ a semi-in vitro assay, in which ovules dissected from Arabidopsis thaliana are arranged around a cut style on artificial medium, to elucidate how ovules release the attractant and how pollen tubes respond to it. Analysis of microscopy images of the semi-in vitro system shows that pollen tubes are more attracted to ovules that are incubated on the medium for longer times before pollen tubes emerge from the cut style. The responses of tubes are consistent with their sensing a gradient of an attractant at 100-150 μm, farther than previously reported. Our microscopy images also show that pollen tubes slow their growth near the micropyles of functional ovules with a spatial range that depends on ovule incubation time.ConclusionsWe propose a stochastic model that captures these dynamics. In the model, a pollen tube senses a difference in the fraction of receptors bound to an attractant and changes its direction of growth in response; the attractant is continuously released from ovules and spreads isotropically on the medium. The model suggests that the observed slowing greatly enhances the ability of pollen tubes to successfully target ovules. The relation of the results to guidance in vivo is discussed.

As the analog of the free energy for dynamical trajectories, the large deviation function plays a central role in the statistical mechanics of systems far from equilibrium. Here, we identify numerical issues that can arise when the model of interest evolves according to a continuous-time dynamics. This analysis motivates the introduction of an algorithm in which a list of previously visited states is used to resample the distribution of interest. We discuss the convergence properties of our algorithm in detail and demonstrate its application to the singlesite zero-range process and the many-site totally asymmetric exclusion process.

Cells often have tens of thousands of receptors, even though only a few activated receptors can trigger full cellular responses. Reasons for the overabundance of receptors remain unclear. We suggest that, in certain conditions, the large number of receptors results in a competition among receptors to be the first to activate the cell. The competition decreases the variability of the time to cellular activation, and hence results in a more synchronous activation of cells. We argue that, in simple models, this variability reduction does not necessarily interfere with the receptor specificity to ligands achieved by the kinetic proofreading mechanism. Thus cells can be activated accurately in time and specifically to certain signals. We predict the minimum number of receptors needed to reduce the coefficient of variation for the time to activation following binding of a specific ligand. Further, we predict the maximum number of receptors so that the kinetic proofreading mechanism still can improve the specificity of the activation. These predictions fall in line with experimentally reported receptor numbers for multiple systems.

Embryonic axis extension is a complex multi-tissue morphogenetic process responsible for the formation of the posterior part of the amniote body. Cells located in the caudal part of the embryo divide and rearrange to participate in the elongation of the different embryonic tissues (e.g. neural tube, axial and paraxial mesoderm, lateral plate, ectoderm, endoderm). We previously identified the paraxial mesoderm as a crucial player of axis elongation, but how movements and growth are coordinated between the different posterior tissues to drive morphogenesis remain largely unknown. Here we use the quail embryo as a model system to quantify cell behavior and movements in the various tissues of the elongating embryo. We first quantify the tissue-specific contribution to axis elongation by using 3D volumetric techniques, then quantify tissuespecific parameters such as cell density and proliferation at different embryonic stages. To be able to study cell behavior at a multi-tissue scale we used high-resolution 4D imaging of transgenic quail embryos expressing constitutively expressed fluorescent proteins. We developed specific tracking and image analysis techniques to analyze cell motion and compute tissue deformations in 4D. This analysis reveals extensive sliding between tissues during axis extension. Further quantification of "tissue tectonics" showed patterns of rotations, contractions and expansions, which are coherent with the multi-tissue behavior observed previously. Our results confirm the central role of the PSM in axis extension; we propose that the PSM specific cell proliferation and migration programs control the coordination of elongation between tissues during axis extension. Material and methodsQuail embryo and embryo culture Wild-type quail embryos (Japonica coturnix) were obtained from commercial sources and from the USC aviary. The PGK1:H2B-chFP quail line generation was described previously (Huss et al. 2015) and is maintained in the USC aviary. Embryos were staged according to (Ainsworth et al. 2010;Hamburger andHamilton 1992) Hamburger andHamilton, 1951). Embryos were cultured ex ovo with filter paper on albumen agar plates according to the EC (Early chick) technique (Chapman et al. 2001). Staining and immunodetectionsEmbryo were collected at the desired stages and fixed overnight in 4% formaldehyde [36% formaldehyde (47608, Sigma) diluted to 4% in PBS]. Blocking and tissues permeabilization were carried out for 2 hours in PBS/0.5% Triton/1% donkey serum. Primary antibodies against Sox2 (1/5000, millipore ab5603) and Bra (1/500, R&D AF2085) were incubated overnight at 4°C. After washing off primary antibody in PBT (PBS/O.1% Triton), embryos were incubated with secondary antibodies (donkey anti goat Alexa 594 and goat anti rabbit Alexa 488, 1/1000, Molecular probes) and DAPI (1/1000, D1306, Molecular probes) overnight at 4°C. The embryos were cleared in U2 scale (Hama et al. 2011) for at least 48h at 4°C and then mounted between slide and coverslip and imaged by confocal/2P microscopy. Proliferation ...