Gene regulatory networks that control the specification of neural-crest cells in the lamprey

The lamprey is the only basal vertebrate in which large-scale gene perturbation analyses are feasible at present. Studies on this unique animal model promise to contribute both to the understanding of the basic neural-crest gene regulatory network architecture, and evolution of the neural crest. In this review, we summarize the currently known regulatory relationships underlying formation of the vertebrate neural crest, and discuss new ways of addressing the many remaining questions using lamprey as an experimental model

Fast algorithms for matching CCD images to a stellar catalogue

Two new algorithms are described for matching two dimensional coordinate lists of point sources that are signifcantly faster than previous methods. By matching rarely occurring triangles (or more complex shapes) in the two lists, and by ordering searches by decreasing probability of success, it is demonstrated that very few candidates need be considered to find a successful match. Moreover, by immediately testing the suitability of a potential match using an efficient mechanism, the need to process the entire candidate set is avoided, yielding considerable performance improvements. Triangles are described by a cosine metric that reduces the density of triangle space, permitting efficient searches. An alternative shape characterization method that reduces computational overhead in the construction phase is discussed. The algorithms are tested on a set of 10 063 wide-field survey images, with fields-of-view up to 4.8 x 3.6 deg, successfully matching 100% of the images in a mean elapsed time of 6 ms (2.4 GHz Athlon CPU). The elapsed time of the searching phase is shown to vary by less than 1 ms for list sizes between 10 and 200 points, demonstrating that fast, robust searches may be completed in nearly constant time, independent of list size.Comment: Accepted for publication in Publications of the Astronomical Society of Australi

Establishing neuronal identity in vertebrate neurogenic placodes

The trigeminal and epibranchial placodes of vertebrate embryos form different types of sensory neurons. The trigeminal placodes form cutaneous sensory neurons that innervate the face and jaws, while the epibranchial placodes (geniculate, petrosal and nodose) form visceral sensory neurons that innervate taste buds and visceral organs. In the chick embryo, the ophthalmic trigeminal (opV) placode expresses the paired homeodomain transcription factor Pax3 from very early stages, while the epibranchial placodes express Pax2. Here, we show that Pax3 expression in explanted opV placode ectoderm correlates at the single cell level with neuronal specification and with commitment to an opV fate. When opV (trigeminal) ectoderm is grafted in place of the nodose (epibranchial) placode, Pax3-expressing cells form Pax3-positive neurons on the same schedule as in the opV placode. In contrast, Pax3-negative cells in the grafted ectoderm are induced to express the epibranchial placode marker Pax2 and form neurons in the nodose ganglion that express the epibranchial neuron marker Phox2a on the same schedule as host nodose neurons. They also project neurites along central and peripheral nodose neurite pathways and survive until well after the main period of cell death in the nodose ganglion. The older the opV ectoderm is at the time of grafting, the more Pax3-positive cells it contains and the more committed it is to an opV fate. Our results suggest that, within the neurogenic placodes, there does not appear to be a two-step induction of 'generic' neurons followed by specification of the neuron to a particular fate. Instead, there seems to be a one-step induction in which neuronal subtype identity is coupled to neuronal differentiation

Competence, specification and induction of Pax-3 in the trigeminal placode

Placodes are discrete regions of thickened ectoderm that contribute extensively to the peripheral nervous system in the vertebrate head. The paired-domain transcription factor Pax-3 is an early molecular marker for the avian ophthalmic trigeminal (opV) placode, which forms sensory neurons in the ophthalmic lobe of the trigeminal ganglion. Here, we use collagen gel cultures and heterotopic quail-chick grafts to examine the competence, specification and induction of Pax-3 in the opV placode. At the 3-somite stage, the whole head ectoderm rostral to the first somite is competent to express Pax-3 when grafted to the opV placode region, though competence is rapidly lost thereafter in otic-level ectoderm. Pax-3 specification in presumptive opV placode ectoderm occurs by the 8-somite stage, concomitant with robust Pax-3 expression. From the 8-somite stage onwards, significant numbers of cells are committed to express Pax-3. The entire length of the neural tube has the ability to induce Pax-3 expression in competent head ectoderm and the inductive interaction is direct. We propose a detailed model for Pax-3 induction in the opV placode

Early- and late-migrating cranial neural crest cell populations have equivalent developmental potential in vivo

We present the first in vivo study of the long-term fate and potential of early-migrating and late-migrating mesencephalic neural crest cell populations, by performing isochronic and heterochronic quail-to-chick grafts. Both early- and late-migrating populations form melanocytes, neurons, glia, cartilage and bone in isochronic, isotopic chimeras, showing that neither population is lineage-restricted. The early-migrating population distributes both dorsally and ventrally during normal development, while the late-migrating population is confined dorsally and forms much less cartilage and bone. When the late-migrating population is substituted heterochronically for the early-migrating population, it contributes extensively to ventral derivatives such as jaw cartilage and bone. Conversely, when the early-migrating population is substituted heterochronically for the late-migrating population, it no longer contributes to the jaw skeleton and only forms dorsal derivatives. When the late-migrating population is grafted into a late-stage host whose neural crest had previously been ablated, it migrates ventrally into the jaws. Thus, the dorsal fate restriction of the late-migrating mesencephalic neural crest cell population in normal development is due to the presence of earlier-migrating neural crest cells, rather than to any change in the environment or to any intrinsic difference in migratory ability or potential between early- and late-migrating cell populations. These results highlight the plasticity of the neural crest and show that its fate is determined primarily by the environment

Cryptic MHC Polymorphism Revealed but Not Explained by Selection on the Class IIB Peptide-Binding Region

The immune genes of the major histocompatibility complex (MHC) are characterized by extraordinarily high levels of nucleotide and haplotype diversity. This variation is maintained by pathogen-mediated balancing selection that is operating on the peptide-binding region (PBR). Several recent studies have found, however, that some populations possess large clusters of alleles that are translated into virtually identical proteins. Here, we address the question of how this nucleotide polymorphism is maintained with little or no functional variation for selection to operate on. We investigate circa 750–850 bp of MHC class II DAB genes in four wild populations of the guppy Poecilia reticulata. By sequencing an extended region, we uncovered 40.9% more sequences (alleles), which would have been missed if we had amplified the exon 2 alone. We found evidence of several gene conversion events that may have homogenized sequence variation. This reduces the visible copy number variation (CNV) and can result in a systematic underestimation of the CNV in studies of the MHC and perhaps other multigene families. We then focus on a single cluster, which comprises 27 (of a total of 66) sequences. These sequences are virtually identical and show no signal of selection. We use microsatellites to reconstruct the populations' demography and employ simulations to examine whether so many similar nucleotide sequences can be maintained in the populations. Simulations show that this variation does not behave neutrally. We propose that selection operates outside the PBR, for example, on linked immune genes or on the “sheltered load” that is thought to be associated to the MHC. Future studies on the MHC would benefit from extending the amplicon size to include polymorphisms outside the exon with the PBR. This may capture otherwise cryptic haplotype variation and CNV, and it may help detect other regions in the MHC that are under selection

Vascular remodeling of the mouse yolk sac requires hemodynamic force

The embryonic heart and vessels are dynamic and form and remodel while functional. Much has been learned about the genetic mechanisms underlying the development of the cardiovascular system, but we are just beginning to understand how changes in heart and vessel structure are influenced by hemodynamic forces such as shear stress. Recent work has shown that vessel remodeling in the mouse yolk sac is secondarily effected when cardiac function is reduced or absent. These findings indicate that proper circulation is required for vessel remodeling, but have not defined whether the role of circulation is to provide mechanical cues, to deliver oxygen or to circulate signaling molecules. Here, we used time-lapse confocal microscopy to determine the role of fluid-derived forces in vessel remodeling in the developing murine yolk sac. Novel methods were used to characterize flows in normal embryos and in embryos with impaired contractility (Mlc2a^(–/–)). We found abnormal plasma and erythroblast circulation in these embryos, which led us to hypothesize that the entry of erythroblasts into circulation is a key event in triggering vessel remodeling. We tested this by sequestering erythroblasts in the blood islands, thereby lowering the hematocrit and reducing shear stress, and found that vessel remodeling and the expression of eNOS (Nos3) depends on erythroblast flow. Further, we rescued remodeling defects and eNOS expression in low-hematocrit embryos by restoring the viscosity of the blood. These data show that hemodynamic force is necessary and sufficient to induce vessel remodeling in the mammalian yolk sa

University networks in the context of their academic excellence and openness: A comparative study of leading Czech and German universities

A simple methodology of multi-dimensional vector analysis for the comparison of the academic performance and the openness of university networks of the identical dimension was developed, which is illustrated by the example of the leading universities in the Czech Republic and GermanyyesBelgorod State Universit