Retinoic acid induces changes in the localization of homeobox proteins in the antero-posterior axis of Xenopus laevis embryos.

We have studied the localization of the proteins of Xeb1 and Xeb2, two homeobox (hbx)-containing genes that are expressed during the early development of Xenopus laevis. Both proteins are expressed in juxtaposed and partially overlapping domains along the antero-posterior axis of Xenopus laevis embryos, with clearly defined anterior boundaries. Xeb2 is predominantly expressed in the caudal region of the hindbrain, whereas the Xeb1 protein is located in the most rostral region of the spinal cord. Furthermore, both proteins are expressed in single cells dispersed in the lateral flanks of the embryo in positions that correlate with the expression domains in the neural tube. We suggest that these cells are migratory neural crest cells that have acquired positional information in the neural tube prior to migration. The Xeb2 protein was also detected in the most posterior branchial arches and the pronephros. In stage 45 embryos, nuclei of the IX-X cranial ganglia, the lung buds and cells spreading into the forelimb rudiment express the Xeb2 antigen. The Xeb1 protein was also detected in the lung buds and the forelimb rudiment. To examine the effect of retinoic acid on expression, gastrula embryos were treated with all-trans retinoic acid (RA). Increasing concentrations of RA caused progressive truncation of anterior structures. The most severely affected embryos lacked eyes, nasal pits, forebrain, midbrain and otic vesicles, and the anterior boundary of the hindbrain seemed to be displaced rostrally. This alteration correlates with a progressive displacement of the anterior boundary of the expression domain of Xeb2. On the other hand, 10(-6) M RA induces an ectopic site of Xeb1 expression at the anterior end of the central nervous system, located just anterior to the extended domain of Xeb2 whereas expression in the spinal cord remains unaffected.     

Novel patterns of the gene expression regulation in the prestalk region along the antero-posterior axis during multicellular development of Dictyostelium

Simultaneous hybridization with differentially labeled fluorescent probes for in situ hybridization analysis revealed several novel expression patterns of prestalk genes during multicellular development of Dictyostelium. Seven prestalk genes and one prespore gene (pspA) were analyzed in this study. The patterns identified here indicate that prestalk cells are more heterogeneous than previously thought. Heterogeneity was observed in peripheral prestalk tissues such as the pstAO domain of a slug and the prestalk region surrounding a stalk tube of a culminant. Heterogeneity was also observed in the core pstAB cells of the slug and immature stalk cells within the stalk tube. The upper- and lower-cups of a late culminant were also composed of several subdomains.     

Cohesin Smc1beta determines meiotic chromatin axis loop organization

Meiotic chromosomes consist of proteinaceous axial structures from which chromatin loops emerge. Although we know that loop density along the meiotic chromosome axis is conserved in organisms with different genome sizes, the basis for the regular spacing of chromatin loops and their organization is largely unknown. We use two mouse model systems in which the postreplicative meiotic chromosome axes in the mutant oocytes are either longer or shorter than in wild-type oocytes. We observe a strict correlation between chromosome axis extension and a general and reciprocal shortening of chromatin loop size. However, in oocytes with a shorter chromosome axis, only a subset of the chromatin loops is extended. We find that the changes in chromatin loop size observed in oocytes with shorter or longer chromosome axes depend on the structural maintenance of chromosomes 1β (Smc1β), a mammalian chromosome–associated meiosis-specific cohesin. Our results suggest that in addition to its role in sister chromatid cohesion, Smc1β determines meiotic chromatin loop organization.     

Wnt signaling is required for antero-posterior patterning of the planarian brain

Wnt signaling functions in axis formation and morphogenesis in various animals and organs. Here we report that Wnt signaling is required for proper brain patterning during planarian brain regeneration. We showed here that one of the Wnt homologues in the planarian Dugesia japonica, DjwntA, was expressed in the posterior region of the brain. When DjwntA-knockdown planarians were produced by RNAi, they could regenerate their heads at the anterior ends of the fragments, but formed ectopic eyes with irregular posterior lateral branches and brain expansion. This suggests that the Wnt signal may be involved in antero-posterior (A-P) patterning of the planarian brain, as in vertebrates. We also investigated the relationship between the DjwntA and nou-darake/FGFR signal systems, as knockdown planarians of these genes showed similar phenotypes. Double-knockdown planarians of these genes did not show any synergistic effects, suggesting that the two signal systems function independently in the process of brain regeneration, which accords with the fact that nou-darake was expressed earlier than DjwntA during brain regeneration. These observations suggest that the nou-darake/FGFR signal may be involved in brain rudiment formation during the early stage of head regeneration, and subsequently the DjwntA signal may function in A-P patterning of the brain rudiment.     

Mapping the organization of axis of motion selective features in human area MT using high-field fMRI

Functional magnetic resonance imaging (fMRI) at high magnetic fields has made it possible to investigate the columnar organization of the human brain in vivo with high degrees of accuracy and sensitivity. Until now, these results have been limited to the organization principles of early visual cortex (V1). While the middle temporal area (MT) has been the first identified extra-striate visual area shown to exhibit a columnar organization in monkeys, evidence of MT's columnar response properties and topographic layout in humans has remained elusive. Research using various approaches suggests similar response properties as in monkeys but failed to provide direct evidence for direction or axis of motion selectivity in human area MT. By combining state of the art pulse sequence design, high spatial resolution in all three dimensions (0.8 mm isotropic), optimized coil design, ultrahigh field magnets (7 Tesla) and novel high resolution cortical grid sampling analysis tools, we provide the first direct evidence for large-scale axis of motion selective feature organization in human area MT closely matching predictions from topographic columnar-level simulations.     

Heads and tails: evolution of antero-posterior patterning in insects

In spite of their varied appearances, insects share a common body plan whose layout is established by patterning genes during embryogenesis. We understand in great molecular detail how the Drosophila embryo patterns its segments. However, Drosophila has a type of embryogenesis that is highly derived and varies extensively as compared to most insects. Therefore, the study of other insects is invaluable for piecing together how the ancestor of all insects established its segmented body plan, and how this process can be plastic during evolution. In this review, we discuss the evolution of Antero-Posterior (A-P) patterning mechanisms in insects. We first describe two distinct modes of insect development - long and short germ development - and how these two modes of patterning are achieved. We then summarize how A-P patterning occurs in the long-germ Drosophila, where most of our knowledge comes from, and in the well-studied short-germ insect, Tribolium. Finally, using examples from other insects, we highlight differences in patterns of expression, which suggest foci of evolutionary change.     

The basic organization of the Plathelminthes

The basic organization of the Plathelminthes is summarized. Special attention is given to epidermal structures, musculature, extracellular matrices, nervous system and sensory structures, digestive system, totipotent stem cells, protonephridia, reproductive system and life cycle. The latest common ancestor of the Plathelminthes lacked any parenchymal cells and tissues; Plathelminthes do not represent Parenchymia. Discussions concerning the relationships of the Plathelminthes with other Metazoa must be based on the characteristics of the plathelminth stem species.     

Body-plan evolution in the Bilateria: early antero-posterior patterning and the deuterostome-protostome dichotomy

Recent molecular analyses reveal common themes in early antero-posterior patterning in the four major groups of invertebrate deuterostomes and vertebrates in spite of large differences in the mode of gastrulation. Comparisons with Drosophila and Cnidarians suggest a scheme for evolution of the Bilaterian body plan and emphasize the pressing need for similar studies in a wider variety of organisms, especially more basal protostomes.     

The organization of the prosystemin gene

The organization of the gene encoding tomato prosystemin, a 200 amino acid protein precursor of the 18 amino acid polypeptide inducer of proteinase inhibitor synthesis in tomato and potato plants, is reported. The prosystemin sequence reveals that the gene, which is composed of five homologous pairs of exons plus a non-homologous exon at the C-terminus containing the systemin sequence, has evolved by several gene duplication-elongation events from a much smaller ancestral gene. The nucleotide and amino acid sequence homologies among the exons suggest that a small ancestral gene was duplicated to form at least two tandem repeats, followed by subsequent duplication-elongation events that resulted in five tandemly repeated nucleotide sequences and three duplicated amino acid sequence elements. Since the systemin nucleotide or amino acid sequence was not duplicated, it was either not part of the gene duplication-elongation events or its coding region evolved separately and may even have been added to the tandemly repeated part of the gene at a later time.     

The polarisation of the anteroposterior axis in Drosophila

The polarisation of the embryonic anteroposterior (AP) axis requires the establishment of positional cues with spatial information, and often involves complex intercellular communications, cell adhesion and cell movement. Recent work on several fronts has begun to shed light on how the initial asymmetries are established and maintained. In this review, I discuss the polarisation of the AP axis during Drosophila oogenesis, focusing on the function of the Notch signalling pathway and its relationship to the activation of the epidermal growth factor receptor. I make special reference to some aspects of Notch activity regulation during oogenesis that appear to depart from the canonical pathway. Finally, I hypothesise on possible similarities between these activities of Notch signalling during Drosophila oogenesis and vertebrate somitogenesis.     

The functional organization of the barrel cortex

The tactile somatosensory pathway from whisker to cortex in rodents provides a well-defined system for exploring the link between molecular mechanisms, synaptic circuits, and behavior. The primary somatosensory cortex has an exquisite somatotopic map where each individual whisker is represented in a discrete anatomical unit, the "barrel," allowing precise delineation of functional organization, development, and plasticity. Sensory information is actively acquired in awake behaving rodents and processed differently within the barrel map depending upon whisker-related behavior. The prominence of state-dependent cortical sensory processing is likely to be crucial in our understanding of active sensory perception, experience-dependent plasticity and learning.