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Results for “Spatial pattern formation”

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PubMed1997

Self-organization of trajectory formation. II. Theoretical model.

Most studies of movement coordination deal with temporal patterns of synchronization between components, often without regard to the actual amplitudes the components make. When such a system is required to produce a composite action that is spatially constrained, coordination persists, but its stability is modulated by spatial requirements effected, we hypothesize, through the component amplitudes. As shown experimentally in part I, when a redundant three-joint system (wrist, elbow, and shoulder) is required to trace a specified arc in space, the joint angles may be frequency- and phased-locked even as the curvature of the trajectory is manipulated. Transitions between joint coordination patterns occur at a critical curvature, accompanied by a significant reduction in wrist amplitude. Such amplitude reduction is viewed as destabilizing the existing coordinative pattern under current task constraints, thereby forcing the joints into a more stable phase relationship. This paper presents a theoretical analysis of these multijoint patterns and proposes an amplitude mechanism for the transition process. Our model uses three linearly coupled, nonlinear oscillators for the joint angles and reproduces both the observed interjoint coordination and component amplitude effects as well as the resulting trajectories of the end effector.

Arm↗

PubMed2004

Formation mechanism and low-temperature instability of exciton rings.

The macroscopic rings observed in the photoluminescence patterns of excitons in coupled quantum wells are explained by a mechanism of carrier imbalance, transport, and recombination. The rings originate from the spatial separation of p and n carriers, and occur at the interface of the p and n domains, where excitons are generated. We explore the states of excitons in the ring over a range of temperatures down to 380 mK and report a transition of the ring into a periodic array of aggregates, a new low-temperature ordered exciton state.

Journal Article↗

PubMed1998

Developmental appearance and distribution of bone sialoprotein and osteopontin in human and rat cementum.

BACKGROUND: Bone sialoprotein (BSP) and osteopontin (OPN), two major noncollagenous proteins (NCPs) in collagen-based mineralized tissues, have been implicated in mineral deposition and cell- and matrix-matrix interactions during root development. However, their role in cementogenesis is still a subject of debate. Since distribution of proteins is indicative of function, we have analyzed their temporo-spatial appearance in relation to that of cementum collagen. METHODS: Human premolars and rat molars at various stages of root development characterized by differing rates of formation were fixed in aldehyde and embedded in epoxy and LR White resin. Sections were processed for ultrastructural analysis and postembedding colloidal gold (immuno)cytochemistry. RESULTS: Incubations with antibodies against BSP and OPN and with lectins recognizing prominent sugars in these proteins generally revealed similar labeling patterns in both human and rat teeth, with gold particles accumulating mainly in the interfibrillar spaces. The lectin Helix pomatia, specific for N-acetyl-D-galactosamine, was distinctive in that it consistently reacted with human cementum, but only sporadically labeled rat cementum. Regardless of both the species and the stage of root development, mineralization initiated in mantle predentin in association with distinct foci immunoreactive for BSP and OPN. In human teeth, the deposition of cementum collagen began before the start of dentin mineralization and thus prior to any detectable labeling for BSP and OPN. However, at early stages of root formation in the rat, cementum collagen appeared after BSP and OPN accumulated on the root surface, whereas at advanced stages the deposition of cementum collagen, BSP and OPN coincided. CONCLUSIONS: The temporo-spatial differences in the appearance of BSP and OPN relative to cementum collagen correlate well with known differences in the speed of root elongation and explain the variable appearance of the dentino-cemental junction. The data reveal no causal relationship between BSP and OPN and the differentiation of cementoprogenitor cells and indicate that the distribution of collagen fibrils ultimately determines the amount and pattern of accumulation of these NCPs. There also is no consistent planar accumulation of BSP and OPN between dentin and cementum such as the cement lines found between "old" and "new" bone. It is concluded that the interlacement of collagen fibrils at the dentino-cemental junction, across which mineralization spreads, represents the primary attachment mechanism between cementum and dentin.

Adolescent↗

PubMed2005

A segmental pattern of alkaline phosphatase activity within the notochord coincides with the initial formation of the vertebral bodies.

This study shows that segmental expression of alkaline phosphatase (ALP) activity by the notochord of the Atlantic salmon (Salmo salar L.) coincides with the initial mineralization of the vertebral body (chordacentrum), and precedes ALP expression by presumed somite-derived cells external to the notochordal sheath. The early expression of ALP indicates that the notochord plays an instructive role in the segmental patterning of the vertebral column. The chordacentra form segmentally as mineralized rings within the notochordal sheath, and ALP activity spreads within the chordoblast layer from ventral to dorsal, displaying the same progression and spatial distribution as the mineralization process. No ALP activity was observed in sclerotomal mesenchyme surrounding the notochordal sheath during initial formation of the chordacentra. Our results support previous findings indicating that the chordoblasts initiate a segmental differentiation of the notochordal sheath into chordacentra and intervertebral regions.

Alkaline Phosphatase↗

PubMedDate not supplied

Growth tracks in dental enamel.

The present paper evaluates the enamel growth tracks as tools in the chronological mapping of dental development, with special reference to hominids. Dental enamel consists of tightly packed hydroxyapatite crystals organized by differential orientation into a pattern of prisms and interprisms. The crystal organization is probably under the influence of both cellular and physico-chemical factors. The structure of mature enamel testifies to events that took place during enamel formation. The prisms are the fossilized tracks traced out by ameloblasts. The tangential diameter of ameloblasts and the central distance of prisms increase from the enamel-dentine junction to the enamel surface. Available evidence suggests that prism cross-striations are light microscopic expressions of prism varicosities and/or compositional variations, that these are due to a rhythm in enamel formation, and that this rhythm is diurnal. In human enamel the mean daily rate of enamel production is about 3.5 micron, but increases from inner to outer enamel and decreases from incisal/cuspal to cervical enamel. Conclusive evidence has shown that Retzius lines are incremental lines. Evenly spaced Retzius lines probably represent a 6-11 day rhythm in enamel formation, while other Retzius lines may be due to various types of stress. The geometry of the enamel growth tracks and their chronological significance are valuable tools in chronological mapping of dental development and for understanding temporal and spatial patterns in tooth morphogenesis. The taxonomic significance of prism packing patterns, prism decussation and enamel thickness should be clarified through further systematic descriptive research.

Ameloblasts↗

PubMed1992

Molecular mechanisms of pattern formation in the vertebrate hindbrain.

During early stages of neural development a series of repeated bulges, termed rhombomeres, form in the vertebrate hindbrain. Studies in the chick have shown that rhombomeres are segments that underlie the patterning of nerves in the hindbrain, and this raises the question of the molecular basis of segment development. Several genes have been found with expression patterns consistent with roles in the formation or differentiation of rhombomeres. The zinc finger gene Krox-20 is expressed in two alternating rhombomeres, r3 and r5, in the mouse hindbrain; these stripes of gene expression are established prior to the morphological appearance of segments. Krox-20 is also expressed in this pattern in the chick and Xenopus, suggesting that it has a conserved role, possibly in the formation of rhombomeres. Four members of the Hox-2 homeobox gene cluster have limits of expression at rhombomere boundaries. Three genes, Hox-2.6, -2.7 and -2.8 have progressively more anterior limits of expression at two-segment intervals, whereas expression of Hox-2.9 is restricted to one rhombomere, r4. The Hox-2 genes are expressed in spatially restricted patterns in early neural crest cells. These findings suggest that the Hox genes have roles in specifying the identity of rhombomeres and of neural crest.

Animals↗

PubMed2004

Spatial patterns of transcriptional activity in the chromosome of Escherichia coli.

BACKGROUND: Although genes on the chromosome are organized in a fixed order, the spatial correlations in transcription have not been systematically evaluated. We used a combination of genomic and signal processing techniques to investigate the properties of transcription in the genome of Escherichia coli K12 as a function of the position of genes on the chromosome. RESULTS: Spectral analysis of transcriptional series revealed the existence of statistically significant patterns in the spatial series of transcriptional activity. These patterns could be classified into three categories: short-range, of up to 16 kilobases (kb); medium-range, over 100-125 kb; and long-range, over 600-800 kb. We show that the significant similarities in gene activities extend beyond the length of an operon and that local patterns of coexpression are dependent on DNA supercoiling. Unlike short-range patterns, the formation of medium and long-range transcriptional patterns does not strictly depend on the level of DNA supercoiling. The long-range patterns appear to correlate with the patterns of distribution of DNA gyrase on the bacterial chromosome. CONCLUSIONS: Localization of structural components in the transcriptional signal revealed an asymmetry in the distribution of transcriptional patterns along the bacterial chromosome. The demonstration that spatial patterns of transcription could be modulated pharmacologically and genetically, along with the identification of molecular correlates of transcriptional patterns, offer for the first time strong evidence of physiologically determined higher-order organization of transcription in the bacterial chromosome.

Chromosome Mapping↗

PubMed2001

Cytochrome P450s: coupling development and environment.

There is growing evidence that some members of the cytochrome P450 superfamily could be involved in the regulation of basic developmental processes such as pattern formation, morphogenesis, cell differentiation and growth. This development calls attention to the myriad small molecules metabolized by cytochrome P450s, some of which might function as the morphogens proposed by the Local Source-Dispersed Sink hypothesis. This new information also suggests a mechanism for the developmental toxicity of drugs and environmental pollutants: such compounds could interfere with normal development by altering the spatial and temporal expression patterns of cytochrome P450s required for normal development.

Animals↗

PubMed1993

The structure and expression of the Xenopus Krox-20 gene: conserved and divergent patterns of expression in rhombomeres and neural crest.

Recent studies in the chick have indicated that rhombomeres (r) are segments that underlie the patterning of hindbrain nerves. These segments may also be important for the specification of branchial arch structures since alternating rhombomeres, r2, r4 and r6, each contribute crest to a specific arch. Krox-20 has been implicated in the segmental patterning of the hindbrain in the mouse by its expression prior to segment formation in alternating domains, which later correspond to r3 and r5. Here, we describe the sequence and developmental expression of the Xenopus Krox-20 gene, XKrox-20. Alternating domains of XKrox-20 expression appear in the early neurula, later correspond to r3 and r5, and persist until late tadpole stages. In contrast to this conserved spatial expression in rhombomeres, we find a pattern in the neural crest of Xenopus that appears different from that found in the mouse: expression occurs in crest that migrates from r5 into the third visceral arch. We speculate that this may reflect a distinct route of neural crest migration due to anatomical differences between these systems, rather than a difference in the site of origin of Krox-20-expressing crest.

Amino Acid Sequence↗

PubMed2001

Spatio-temporal pattern formation on spherical surfaces: numerical simulation and application to solid tumour growth.

In this paper we examine spatio-temporal pattern formation in reaction-diffusion systems on the surface of the unit sphere in 3D. We first generalise the usual linear stability analysis for a two-chemical system to this geometrical context. Noting the limitations of this approach (in terms of rigorous prediction of spatially heterogeneous steady-states) leads us to develop, as an alternative, a novel numerical method which can be applied to systems of any dimension with any reaction kinetics. This numerical method is based on the method of lines with spherical harmonics and uses fast Fourier transforms to expedite the computation of the reaction kinetics. Numerical experiments show that this method efficiently computes the evolution of spatial patterns and yields numerical results which coincide with those predicted by linear stability analysis when the latter is known. Using these tools, we then investigate the rĵle that pre-pattern (Turing) theory may play in the growth and development of solid tumours. The theoretical steady-state distributions of two chemicals (one a growth activating factor, the other a growth inhibitory factor) are compared with the experimentally and clinically observed spatial heterogeneity of cancer cells in small, solid spherical tumours such as multicell spheroids and carcinomas. Moreover, we suggest a number of chemicals which are known to be produced by tumour cells (autocrine growth factors), and are also known to interact with one another, as possible growth promoting and growth inhibiting factors respectively. In order to connect more concretely the numerical method to this application, we compute spatially heterogeneous patterns on the surface of a growing spherical tumour, modelled as a moving-boundary problem. The numerical results strongly support the theoretical expectations in this case. Finally in an appendix we give a brief analysis of the numerical method.

Animals↗

PubMed1984

Models for positional signalling, the threefold subdivision of segments and the pigmentation pattern of molluscs.

Models of biological pattern formation are discussed. The regulatory features expected from the models are compared to those observed experimentally. It will be shown that: (i) Stable gradients appropriate to supply positional information can be produced by local autocatalysis and long-range inhibition. (ii) Spatially ordered sequences of differentiated cell states can emerge if these cell states mutually activate each other on long range but exclude each other locally. Segmentation results from the repetition of three such cell states, S, A and P (and not of only two, as is usually assumed). With a repetition of three states, each segment has a defined polarity. The confrontation of P cells and S cells lead to the formation of a segment border (...P/SAP/SAP/S...) while the A-P confrontation is a prerequisite for appendage formation. Mutations of Drosophila affecting larval segmentation are discussed in terms of this model. (iii) The two models for the generation of sequences of structures in space (positional information including interpretation versus mutual activation) lead to different predictions with respect to intercalary regeneration. This allows a distinction between the two models on the basis of experiments. (iv) The pigmentation patterns of certain molluscs emerge from a coupled oscillation of cells (that is, a lateral inhibition in time, instead of space). The oblique lines result from a chain of triggering events.

Animals↗

PubMed1999

Spatially regulated translation in embryos: asymmetric expression of maternal Wnt-11 along the dorsal-ventral axis in Xenopus.

Transition from symmetry to asymmetry is a central theme in cell and developmental biology. In Xenopus embryos, dorsal-ventral asymmetry is initiated by a microtubule-dependent cytoplasmic rotation during the first cell cycle after fertilization. Here we show that the cytoplasmic rotation initiates differential cytoplasmic polyadenylation of maternal Xwnt-11 RNA, encoding a member of the Wnt family of cell-cell signaling factors. Translational regulation of Xwnt-11 mRNA along the dorsal-ventral axis results in asymmetric accumulation of Xwnt-11 protein. These results demonstrate spatially regulated translation of a maternal cell-signaling factor along the vertebrate dorsal-ventral axis and represent a novel mechanism for Wnt gene regulation. Spatial regulation of maternal RNA translation, which has been established in invertebrates, appears to be an evolutionarily conserved mechanism in the generation of intracellular asymmetry and the consequential formation of the multicellular body pattern.

Animals↗

PubMed2001

A large-scale in situ screen provides molecular evidence for the induction of eye anterior segment structures by the developing lens.

The anterior segment of the vertebrate eye includes the cornea, iris, ciliary body, trabecular meshwork, and lens. Although malformations of these structures have been implicated in many human eye diseases, little is known about the molecular mechanisms that control their development. To identify genes involved in anterior segment formation, we developed a large-scale in situ hybridization screen and examined the spatial and temporal expression of over 1000 genes during eye development. This screen identified 62 genes with distinct expression patterns in specific eye structures, including several expressed in novel patterns in the anterior segment. Using these genes as developmental markers, we tested for the presence of inductive signals that control the differentiation of anterior segment tissues. Organ culture recombination experiments showed that a chick lens is capable of inducing the expression of markers of the presumptive iris and ciliary body in the developing mouse neural retina. The inducing activity from the lens acts only over short ranges and is present at multiple stages of eye development. These studies provide molecular evidence that an evolutionarily conserved signal from the lens controls tissue specification in the developing optic cup.

Animals↗

PubMed1979

Cataractogenesis: developmental inputs and constraints.

The developmental period during which the lens is susceptible to one or another cataractogenic influence, the types of cataracts that result, and the spatial and temporal patterns in which these cataracts become manifest are better understood in the context of the orderly sequence of steps by which the vertebrate lens develops. Thus, events attending lens induction, formation of the lens vesicle, deposition of the lens capsule, maturation of lens fibers, elaboration of lens sutures, and developmental changes in the relationship of the lens to the optic cup have implications for cataractogenesis. We have also seen that events in juxtalenticular tissues (eg, the zonule, the temporary hyaloid-lenticular vascular, the neural retina) can contribute to the development of lenticular opacities. Hereditary cataracts, as well as congenital cataracts produced in response to untoward environmental input, can be properly understood only within a dynamic developmental context.

Animals↗

PubMed1989

[Digital image intensifier radiography--one year's experience with a Polytron system].

Since January 1988, digital image intensifier radiography has been used in the Clinic in Mannheim for DSA examinations and also in place of conventional screen/film examinations. Measurements have shown that compared with 100 mm and film/screen formats, digital radiography has poorer spatial resolution, but improved contrast resolution. The most common use of digital radiography was for examinations of the gastrointestinal tract. Using the demonstration of the mucosal fine relief pattern as a criterion of image quality, digital image intensifier radiography was able to achieve this satisfactorily. Comparison with film/screen examinations showed no loss of diagnostic information. Advantages of image intensifier radiography are reduced radiation dose, the possibility of postprocessing and economy. On the basis of 399 examinations, digital image intensifier radiography is now firmly established as part of the daily routine of the Mannheim Clinic.

Evaluation Studies as Topic↗

PubMed2005

Emergence of large-scale vorticity during diffusion in a random potential under an alternating bias.

Conventional wisdom indicates that the presence of an alternating driving force will not change the long-term behavior of a Brownian particle moving in a random potential. Although this is true in one dimension, here we offer direct evidence that the inevitable local symmetry breaking present in a two-dimensional random potential leads to the emergence of a local ratchet effect that generates large-scale vorticity patterns consisting of steady-state net diffusive currents. For small fields the spatial correlation function of the current follows a logarithmic distance dependence, while for large external fields both the vorticity and the correlations gradually disappear. We uncover the scaling laws characterizing this unique pattern formation process, and discuss their potential relevance to real systems.

Journal Article↗

PubMed1994

Spatial and temporal distribution of insulin-like growth factors I and II during development of rat lung.

Insulin-like growth factors (IGF) I and II genes are expressed in developing lung tissue, but the details of distribution during different stages of lung development have not been delineated. To define the role of IGFs in lung development, the expression of IGF-I and IGF-II mRNAs and peptides was examined in rat lungs from 16-day gestation through adulthood. The expression of stable IGF mRNAs was investigated by Northern analysis, IGF mRNAs were localized by in situ hybridization histochemistry, and IGF peptides were localized by immunohistochemistry. Four IGF-II transcripts (3.5, 2.2, 1.8, and 1.0 kb) were identified; IGF-II mRNA levels were highest early in fetal life (day 16-18) and decreased gradually until 28 days postnatal. Four IGF-I transcripts (7.8, 4.4, 2.1, and 0.6 kb) were detected but were of low abundance. IGF-II mRNA was localized mainly in mesenchymal cells but was also identified in the airway epithelium, with peak expression in the perinatal lung. IGF-II peptide was prominent in late fetal life and for 5 days after birth and was identified in both mesenchymal and epithelial cells. In contrast, there was little evidence of IGF-I mRNA and peptide until after birth: IGF-I was most easily identified between 5 and 14 days postnatal (the height of alveolar multiplication and formation), when both mRNA and peptide were localized to mesenchymal and epithelial cells. Our studies demonstrate that IGF-I and IGF-II are synthesized and distributed in spatially and temporally different patterns in the developing lung. IGF-I and IGF-II peptides are more widely identified than their respective mRNAs, suggesting a paracrine distribution in the developing lung.

Aging↗

PubMed1993

[Determination of the dorso-ventral polarity of the Drosophila embryo].

Embryonic pattern formation has been studied extensively in many organisms. In Drosophila, the powerful combination of genetics cytoplasm transplantation experiments, as well as recent molecular data, have helped to elucidate the mechanisms responsible for the establishment of embryonic polarity. A small number of genes, most of them maternally expressed, are involved in this process and participate in four independent systems--three for the antero-posterior axis (A/P) and one for dorsoventral axis (D/V)--which define various embryonic territories by specifically localized cues. This review concerns the definition of the dorsoventral polarity responsible for the establishment of the germ layers of the embryo. Dorsoventral development is regulated by a single group of maternally expressed genes: the "dorsal group" of genes. It includes 11 genes, the loss of function of any of which results in a dorsalized development, whereas mutation of the 12th gene, cactus, results in a ventralized development. These genes are arranged according to a functional hierarchy, and have been shown to cooperate in the formation of a graded nuclear concentration of the dorsal gene product. The dorsal product corresponds to the dorsoventral morphogen and is homologous to the transcription factor NF-kappa B. Among the 11 genes of the dorsal group, 3 are required in the somatic line. This suggests the existence of inductive signals originating during oogenesis from the follicle cells that surround the developing oocyte. This somatically expressed spatial information probably controls dorsoventral development by defining the polarity of a signal transducing pathway that specifically activates the nuclear uptake of the dorsal product. This model, highlights the importance of the polarity of the egg chamber, and suggests that it is the oocyte nucleus due to its asymmetrical localization, that determines the dorsoventral pattern formation of the embryo.

Animals↗