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1.
A template matching model for pattern recognition is proposed. By following a previouslyproposed algorithm for synaptic modification (Hirai, 1980), the template of a stimulus pattern is selforganized as a spatial distribution pattern of matured synapses on the cells receiving modifiable synapses. Template matching is performed by the disinhibitory neural network cascaded beyond the neural layer composed of the cells receiving the modifiable synapses. The performance of the model has been simulated on a digital computer. After repetitive presentations of a stimulus pattern, a cell receiving the modifiable synapses comes to have the template of that pattern. And the cell in the latter layer of the disinhibitory bitory neural network that receives the disinhibitory input from that cell becomes electively sensitive to that pattern. Learning patterns are not restricted by previously learned ones. They can be subset or superset patterns of the ones previously learned. If an unknown pattern is presented to the model, no cell beyond the disinhibitory neural network will respond. However, if previously learned patterns are embedded in that pattern, the cells which have the templates of those patterns respond and are assumed to transmit the information to higher center. The computer simulation also shows that the model can organize a clean template under a noisy environment.  相似文献   

2.
3.
The formation of the nervous system in vertebrate embryos involves extensive morphogenetic movements that include the folding of the neural tube and the migration of neural crest cells. Changes in cell shape and cell movements underlie neural morphogenesis but the molecular mechanisms involved in these processes in vivo are not well understood. Here, we show that a new member of the hepatocyte growth factor family, which we name Livertine, is expressed in frog embryos in neural cells including neural crest and midline neural plate cells which are undergoing pronounced morphogenetic movements. The ectopic expression of Livertine perturbs gastrulation and leads to positional changes in injected cells without apparently changing cell type. These results suggest that one of the normal functions of Livertine is the control of neural morphogenesis in the vertebrate embryo.  相似文献   

4.
Cellular interactions during cartilage and bone development.   总被引:1,自引:0,他引:1  
Both interactions between like cells, as between chondrogenic cells in a developing cartilaginous rudiment, and between unlike cells, as in epithelial-mesenchymal interactions, are dealt with in this review. Such interactions may involve direct apposition of cell membranes or may be mediated via interaction with peri- or extracellular matrices. An ontogenetic approach is taken in which cellular interactions involved in five processes of the development of cartilage and bone are discussed, the five being (1) origin of the cells, (2) migration of the cells within the embryo, (3) localization of the cells at their final embryonic site, (4) differentiation, and (5) morphogenesis. Some emphasis is placed on interactions affecting neural crest-derived cells both before and during their migration and on interactions, especially epithelial-mesenchymal interactions, that precede cytodifferentiation of chondroblasts or osteoblasts. Whether epithelial or mesenchymal specificity is required for such interactions to occur is discussed with reference to the otic vesicle-otic mesenchyme interaction that leads to differentiation and morphogenesis of the cartilaginous otic capsule.  相似文献   

5.
The process that leads to embryo formation appears to follow a defined pattern, whose sequential developmental steps—under strict genetic control—can be analysed through the study of mutants affecting embryogenesis. We present the analysis of four embryo-specific (emb) mutants of maize, characterised by abnormal development not overcoming the proembryo or early transition stage, that define three separate genes on the basis of their chromosomal location and complementation pattern. A common feature emerging from histological analysis is that suppression of morphogenesis is accompanied by an uncontrolled pattern of cell division. The block in embryo development is associated with abnormal suspensor proliferation, possibly due to the absence of a signal elaborated by the embryo proper and required for suspensor cell identity maintenance. Mutant endosperm morphogenesis is not impaired, as shown by the formation of the expected domains, i.e. aleurone, starchy endosperm, embryo-surrounding region and basal endosperm transfer layer. The program of cell death appears impaired in the mutants, as expected if this process is essential in determining the shape and morphology of the developing organs. An unexpected result is obtained when mutant embryo rescue is attempted. Immature embryos transferred to a basal medium germinated, yielding small but otherwise normal seedlings, an observation not consistent with the histological evidence of a complete absence of morphogenetic potential. The analysis of emb mutants appears a promising tool to elucidate crucial points of embryo development such as the coupling of cell division with morphogenesis, cell-to-cell interactions, the relationship between embryo and endosperm development, and the interaction between embryo proper and suspensor.  相似文献   

6.
Molecular and cellular analysis of early mammalian development is compromised by the experimental inaccessibility of the embryo. Pluripotent embryonic stem (ES) cells are derived from and retain many properties of the pluripotent founder population of the embryo, the inner cell mass. Experimental manipulation of these cells and their environment in vitro provides an opportunity for the development of differentiation systems which can be used for analysis of the molecular and cellular basis of embryogenesis. In this review we discuss strengths and weaknesses of the available ES cell differentiation methodologies and their relationship to events in vivo. Exploitation of these systems is providing novel insight into embryonic processes as diverse as cell lineage establishment, cell progression during differentiation, patterning, morphogenesis and the molecular basis for cell properties in the early mammalian embryo.  相似文献   

7.
The neural crest provides a useful paradigm for cell migration and modulations in cell adhesion during morphogenesis. In the present review, we describe the major findings on the role of the extracellular matrix glycoprotein fibronectin and its corresponding integrin receptor in the locomotory behavior of neural crest cells. In vivo, fibronectin is associated with the migratory routes of neural crest cells and, in some cases, it disappears from the environment of the cells as they stop migrating. In vitro, neural crest cells show a great preference for fibronectin substrates as compared to other matrix molecules. Both in vivo and in vitro, neural crest cell migration can be specifically inhibited by antibodies or peptides that interfere with the binding of fibronectin to its integrin receptor. However, the migratory behavior of neural crest cells cannot result solely from the interaction with fibronectin. Thus, neural crest cells exhibit a particular organization of integrin receptors on their surface and develop a cytoskeletal network which differs from that of non-motile cells. These properties are supposed to permit rapid changes in the shape of cells and to favor a transient adhesion to the substratum. Recent findings have established that different forms of fibronectin may occur, which differ by short sequences along the molecule. The functions of most of these sequences are not known, except for 1 of them which carries a binding site for integrin receptors. We have demonstrated that this site is recognized by neural crest cells and plays a crucial role in their displacement. It is therefore possible that the forms of fibronectin carrying this sequence are not evenly distributed in the embryo, thus allowing migrating neural crest cells to orientate in the embryo. Fibronectin would then not only play a permissive role in embryonic cell motility, but have an instructive function in cell behavior.  相似文献   

8.
Chordates originated from a common ancestor(s) shared with two other deuterostome groups, echinoderms and hemichordates, by creating a novel type of tadpole-like larva, which was characterized by a dorsal hollow neural tube and notochord. Recent molecular phylogeny supports the notion that echinoderms and hemichordates form a clade named the Ambulacraria and that, among the chordates, cephalochordates are more basal than urochordates and vertebrates. An aboral-dorsalization hypothesis is proposed to explain how the tadpole-type larva evolved. Embryological comparison of cephalochordates with nonchordate deuterostomes suggests that, because of limited space on the oral side of the ancestral embryo, morphogenesis to form the neural tube and notochord occurred on the aboral side of the embryo. Namely, the dorsalization of the aboral side of the ancestral embryo may have been a key developmental event that led to the formation of the basic chordate body plan.  相似文献   

9.
The application of computer simulation to molecular systems of biochemical interest is reviewed. It is shown that computer simulation is a tool complementary to experimental methods, which can be used to access atomic details inaccessible to experimental probes. Examples are given in which computer simulation augments the experimental information by providing an atomic picture of high resolution with respect to space, energy or time. The usefulness of a computer simulation largely depends on its quality. The most important factors that limit the accuracy of simulated results are discussed. The accuracy of different simulation studies can differ by orders of magnitude. The accuracy will depend on the type of biomolecular system and process studied. It will also depend on the choice of force field, the simulation set-up and the protocol that is used. A list of quality-determining factors is given, which may be useful when interpreting simulation studies appearing in the literature.  相似文献   

10.
Mechanisms for shaping and folding sheets of cells during development are poorly understood. An example is the complex reorganisation of the forebrain neural plate during neurulation, which must fold a sheet into a tube while evaginating two eyes from a single contiguous domain within the neural plate. We, for the first time, track these cell rearrangements to show that forebrain morphogenesis differs significantly from prior hypotheses. We postulate a new model for forebrain neurulation and demonstrate how mutations affecting two signalling pathways can generate cyclopic phenotypes by disrupting normal cell movements or introducing new erroneous behaviours.  相似文献   

11.
Parallels between tissue repair and embryo morphogenesis   总被引:18,自引:0,他引:18  
Wound healing involves a coordinated series of tissue movements that bears a striking resemblance to various embryonic morphogenetic episodes. There are several ways in which repair recapitulates morphogenesis. We describe how almost identical cytoskeletal machinery is used to repair an embryonic epithelial wound as is involved during the morphogenetic episodes of dorsal closure in Drosophila and eyelid fusion in the mouse foetus. For both naturally occurring and wound-activated tissue movements, JNK signalling appears to be crucial, as does the tight regulation of associated cell divisions and adhesions. In the embryo, both morphogenesis and repair are achieved with a perfect end result, whereas repair of adult tissues leads to scarring. We discuss whether this may be due to the adult inflammatory response, which is absent in the embryo.  相似文献   

12.
Through the direct analysis of cell behaviors, we address the mechanisms underlying anterior neural tube morphogenesis in the zebrafish and the role of the cell adhesion molecule N-cadherin (N-cad) in this process. We demonstrate that although the mode of neurulation differs at the morphological level between amphibians and teleosts, the underlying cellular mechanisms are conserved. Contrary to previous reports, the zebrafish neural plate is a multi-layered structure, composed of deep and superficial cells that converge medially while undergoing radial intercalation, to form a single cell-layered neural tube. Time-lapse recording of individual cell behaviors reveals that cells are polarized along the mediolateral axis and exhibit protrusive activity. In N-cad mutants, both convergence and intercalation are blocked. Moreover, although N-cad-depleted cells are not defective in their ability to form protrusions, they are unable to maintain them stably. Taken together, these studies uncover key cellular mechanisms underlying neural tube morphogenesis in teleosts, and reveal a role for cadherins in promoting the polarized cell behaviors that underlie cellular rearrangements and shape the vertebrate embryo.  相似文献   

13.
The use of computer simulations as a neurophysiological tool creates new possibilities to understand complex systems and to test whether a given model can explain experimental findings. Simulations, however, require a detailed specification of the model, including the nerve cell action potential and synaptic transmission. We describe a neuron model of intermediate complexity, with a small number of compartments representing the soma and the dendritic tree, and equipped with Na+, K+, Ca2+, and Ca2+ dependent K+ channels. Conductance changes in the different compartments are used to model conventional excitatory and inhibitory synaptic interactions. Voltage dependent NMDA-receptor channels are also included, and influence both the electrical conductance and the inflow of Ca2+ ions. This neuron model has been designed for the analysis of neural networks and specifically for the simulation of the network generating locomotion in a simple vertebrate, the lamprey. By assigning experimentally established properties to the simulated cells and their synapses, it has been possible to verify the sufficiency of these properties to account for a number of experimental findings of the network in operation. The model is, however, sufficiently general to be useful for realistic simulation also of other neural systems.  相似文献   

14.
Marwan W 《Genetics》2003,164(1):105-115
Mutants of Physarum polycephalum can be complemented by fusion of plasmodial cells followed by cytoplasmic mixing. Complementation between strains carrying different mutational defects in the sporulation control network may depend on the signaling state of the network components. We have previously suggested that time-resolved somatic complementation (TRSC) analysis with such mutants may be used to probe network architecture and dynamics. By computer simulation it is now shown how and under which conditions the regulatory hierarchy of genes can be determined experimentally. A kinetic model of the sporulation control network is developed, which is then used to demonstrate how the mechanisms of TRSC can be understood and simulated at the kinetic level. On the basis of theoretical considerations, experimental parameters that determine whether functional complementation of two mutations will occur are identified. It is also shown how gene dosage-effect relationships can be employed for network analysis. The theoretical framework provided may be used to systematically analyze network structure and dynamics through time-resolved somatic complementation studies. The conclusions drawn are of general relevance in that they do not depend on the validity of the model from which they were derived.  相似文献   

15.
Marr's theory of the cerebellar cortex as an associative learning device is one of the best examples of a theory that directly relates the function of a neural system to its neural structure. However, although he assigned a precise function to each of the identified cell types of the cerebellar cortex, many of the crucial aspects of the implementation of his theory remained unspecified. We attempted to resolve these difficulties by constructing a computer simulation which contained a direct representation of the 13,000 mossy fibres and the 200,000 granule cells associated with a single Purkinje cell of the cerebellar cortex, together with the supporting Golgi, basket and stellate cells. In this paper we present a detailed explanation of Marr's theory based upon an analogy between Marr's cerebellar model and an abstract model called the associative net. Although some of Marr's assumptions contravene neuroanatomical findings, we found that in general terms his conclusion that each Purkinje cell can learn to respond to a large number of different patterns of activity in the mossy fibres is substantially correct. However, we found that this system has a lower capacity and acts more stochastically than he envisaged. The biologically realistic simulated structure that we designed can be used to assess the computational capabilities of other network theories of the cerebellum.  相似文献   

16.
The factors governing the pattern formation process in the early morphogenesis of a marine colonial hydroid, Dynamena pumila, have been studied. Two different types of morphogenesis have been distinguished. Morphogenesis of the first type goes on via changes in cell shape and cell axis orientation, while morphogenesis of the second type is based upon the active coordinated cell movements associated with cell rearrangements. It was shown that morphogenesis of both types can be considered as cascades in which any event is a consequence of the previous one. The spatial structure of each developmental stage contains information about the direction and the initial conditions of further morphogenesis. So, an "epigenetic program" of morphogenesis gradually originates in the course of development and provides the stable reproduction of spatial structures. It is reasonable to consider the activity of epigenetic factors guiding Dynamena morphogenesis (geometry/topology of an embryo, heterogeneity of an embryo spatial structure, configuration of the field of mechanical stresses of the embryo surface) as "morphomechanical programming" of morphogenesis.  相似文献   

17.
Cells in the Drosophila retina have well-defined morphologies that are attained during tissue morphogenesis. We present a computer simulation of the epithelial tissue in which the global interfacial energy between cells is minimized. Experimental data for both normal cells and mutant cells either lacking or misexpressing the adhesion protein N-cadherin can be explained by a simple model incorporating salient features of morphogenesis that include the timing of N-cadherin expression in cells and its temporal relationship to the remodeling of cell-cell contacts. The simulations reproduce the geometries of wild-type and mutant cells, distinguish features of cadherin dynamics, and emphasize the importance of adhesion protein biogenesis and its timing with respect to cell remodeling. The simulations also indicate that N-cadherin protein is recycled from inactive interfaces to active interfaces, thereby modulating adhesion strengths between cells.  相似文献   

18.
Avian embryos provide a unique platform for studying many vertebrate developmental processes, due to the easy access of the embryos within the egg. Chimeric avian embryos, in which quail donor tissue is transplanted into a chick embryo in ovo, combine the power of indelible genetic labeling of cell populations with the ease of manipulation presented by the avian embryo.Quail-chick chimeras are a classical tool for tracing migratory neural crest cells (NCCs)1-3. NCCs are a transient migratory population of cells in the embryo, which originate in the dorsal region of the developing neural tube4. They undergo an epithelial to mesenchymal transition and subsequently migrate to other regions of the embryo, where they differentiate into various cell types including cartilage5-13, melanocytes11,14-20, neurons and glia21-32. NCCs are multipotent, and their ultimate fate is influenced by 1) the region of the neural tube in which they originate along the rostro-caudal axis of the embryo11,33-37, 2) signals from neighboring cells as they migrate38-44, and 3) the microenvironment of their ultimate destination within the embryo45,46. Tracing these cells from their point of origin at the neural tube, to their final position and fate within the embryo, provides important insight into the developmental processes that regulate patterning and organogenesis.Transplantation of complementary regions of donor neural tube (homotopic grafting) or different regions of donor neural tube (heterotopic grafting) can reveal differences in pre-specification of NCCs along the rostro-caudal axis2,47. This technique can be further adapted to transplant a unilateral compartment of the neural tube, such that one side is derived from donor tissue, and the contralateral side remains unperturbed in the host embryo, yielding an internal control within the same sample2,47. It can also be adapted for transplantation of brain segments in later embryos, after HH10, when the anterior neural tube has closed47.Here we report techniques for generating quail-chick chimeras via neural tube transplantation, which allow for tracing of migratory NCCs derived from a discrete segment of the neural tube. Species-specific labeling of the donor-derived cells with the quail-specific QCPN antibody48-56 allows the researcher to distinguish donor and host cells at the experimental end point. This technique is straightforward, inexpensive, and has many applications, including fate-mapping, cell lineage tracing, and identifying pre-patterning events along the rostro-caudal axis45. Because of the ease of access to the avian embryo, the quail-chick graft technique may be combined with other manipulations, including but not limited to lens ablation40, injection of inhibitory molecules57,58, or genetic manipulation via electroporation of expression plasmids59-61, to identify the response of particular migratory streams of NCCs to perturbations in the embryo''s developmental program. Furthermore, this grafting technique may also be used to generate other interspecific chimeric embryos such as quail-duck chimeras to study NCC contribution to craniofacial morphogenesis, or mouse-chick chimeras to combine the power of mouse genetics with the ease of manipulation of the avian embryo.62  相似文献   

19.
Angiosperm embryogenesis generates the basic body organization of flowering plants. The underlying processes of pattern formation, which establishes the diversity of position-dependent cell fates, and morphogenesis, which brings about the shape of the embryo, may not only involve intercellular communication and controlled cell expansion but also non-random cell divisions. Genetic analysis ofArabidopsisembryogenesis which displays a large invariant pattern of cell divisions suggests that unequal cell divisions segregate cell fates and are thus involved in pattern formation whereas other oriented cell divisions and differential mitotic rates reflect patterning and rather play a role in morphogenesis.  相似文献   

20.
In the zebrafish embryo, cells fated to give rise to the rostral brain move in a concerted fashion and retain tissue coherence during morphogenesis. We demonstrate here that Otx proteins have a dramatic effect on cell-cell interactions when expressed ectopically in the zebrafish embryo. Injection of zebrafish Otx1 or Drosophila otd RNAs into a single cell at the 16-cell stage results in aggregation of descendants of the injected cell. The Otx/Otd homeodomain is necessary for aggregation and appears to be sufficient for the effect when substituted for the homeodomain of an unrelated homeodomain protein. When cells containing injected zOtx1 RNA are limited to the area that is normally fated to become the anterior brain and neural retina, the induced aggregates contribute to anterior brain and retina tissues. In many other embryonic regions, which do not express endogenous zOtx1, the aggregates appear to be incompatible with normal development and do not integrate into developing tissues. By using an activatable Otx1-glutocorticoid receptor fusion protein that results in the stimulation of cell association, we demonstrate that cell aggregates can form as a result of Otx1 activity even after gastrulation is completed. Time-lapse analysis of cell movements show that cell aggregation occurs with only a slight inhibition of the rate of convergence. These results suggest that promotion of cell adhesion or mediation of cell repulsion may be one of the normal functions of the Otx proteins in the establishment of the anterior brain.  相似文献   

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