5,024 research outputs found
Implementing vertex dynamics models of cell populations in biology within a consistent computational framework
The dynamic behaviour of epithelial cell sheets plays a central role during development, growth, disease and wound healing. These processes occur as a result of cell adhesion, migration, division, differentiation and death, and involve multiple processes acting at the cellular and molecular level. Computational models offer a useful means by which to investigate and test hypotheses about these processes, and have played a key role in the study of cell–cell interactions. However, the necessarily complex nature of such models means that it is difficult to make accurate comparison between different models, since it is often impossible to distinguish between differences in behaviour that are due to the underlying model assumptions, and those due to differences in the in silico implementation of the model. In this work, an approach is described for the implementation of vertex dynamics models, a discrete approach that represents each cell by a polygon (or polyhedron) whose vertices may move in response to forces. The implementation is undertaken in a consistent manner within a single open source computational framework, Chaste, which comprises fully tested, industrial-grade software that has been developed using an agile approach. This framework allows one to easily change assumptions regarding force generation and cell rearrangement processes within these models. The versatility and generality of this framework is illustrated using a number of biological examples. In each case we provide full details of all technical aspects of our model implementations, and in some cases provide extensions to make the models more generally applicable
Non-Euclidean geometry in nature
I describe the manifestation of the non-Euclidean geometry in the behavior of
collective observables of some complex physical systems. Specifically, I
consider the formation of equilibrium shapes of plants and statistics of sparse
random graphs. For these systems I discuss the following interlinked questions:
(i) the optimal embedding of plants leaves in the three-dimensional space, (ii)
the spectral statistics of sparse random matrix ensembles.Comment: 52 pages, 21 figures, last section is rewritten, a reference to
chaotic Hamiltonian systems is adde
Dynamic Matrix Ansatz for Integrable Reaction-Diffusion Processes
We show that the stochastic dynamics of a large class of one-dimensional
interacting particle systems may be presented by integrable quantum spin
Hamiltonians. Generalizing earlier work \cite{Stin95a,Stin95b} we present an
alternative description of these processes in terms of a time-dependent
operator algebra with quadratic relations. These relations generate the Bethe
ansatz equations for the spectrum and turn the calculation of time-dependent
expectation values into the problem of either finding representations of this
algebra or of solving functional equations for the initial values of the
operators. We use both strategies for the study of two specific models: (i) We
construct a two-dimensional time-dependent representation of the algebra for
the symmetric exclusion process with open boundary conditions. In this way we
obtain new results on the dynamics of this system and on the eigenvectors and
eigenvalues of the corresponding quantum spin chain, which is the isotropic
Heisenberg ferromagnet with non-diagonal, symmetry-breaking boundary fields.
(ii) We consider the non-equilibrium spin relaxation of Ising spins with
zero-temperature Glauber dynamics and an additional coupling to an
infinite-temperature heat bath with Kawasaki dynamics. We solve the functional
equations arising from the algebraic description and show non-perturbatively on
the level of all finite-order correlation functions that the coupling to the
infinite-temperature heat bath does not change the late-time behaviour of the
zero-temperature process. The associated quantum chain is a non-hermitian
anisotropic Heisenberg chain related to the seven-vertex model.Comment: Latex, 23 pages, to appear in European Physical Journal
The Role of Intracellular Interactions in the Collective Polarization of Tissues and its Interplay with Cellular Geometry
Planar cell polarity (PCP), the coherent in-plane polarization of a tissue on
multicellular length scales, provides directional information that guides a
multitude of developmental processes at cellular and tissue levels. While it is
manifest that cells utilize both intracellular and intercellular mechanisms,
how the two produce the collective polarization remains an active area of
investigation. We study the role of intracellular interactions in the
large-scale spatial coherence of cell polarities, and scrutinize the role of
intracellular interactions in the emergence of tissue-wide polarization. We
demonstrate that nonlocal cytoplasmic interactions are necessary and sufficient
for the robust long-range polarization, and are essential to the faithful
detection of weak directional signals. In the presence of nonlocal
interactions, signatures of geometrical information in tissue polarity become
manifest. We investigate the deleterious effects of geometric disorder, and
determine conditions on the cytoplasmic interactions that guarantee the
stability of polarization. These conditions get progressively more stringent
upon increasing the geometric disorder. Another situation where the role of
geometrical information might be evident is elongated tissues. Strikingly, our
model recapitulates an observed influence of tissue elongation on the
orientation of polarity. Eventually, we introduce three classes of mutants:
lack of membrane proteins, cytoplasmic proteins, and local geometrical
irregularities. We adopt core-PCP as a model pathway, and interpret the model
parameters accordingly, through comparing the in silico and in vivo phenotypes.
This comparison helps us shed light on the roles of the cytoplasmic proteins in
cell-cell communication, and make predictions regarding the cooperation of
cytoplasmic and membrane proteins in long-range polarization.Comment: 15 pages Main Text + 8 page Appendi
The role of topology and mechanics in uniaxially growing cell networks
In biological systems, the growth of cells, tissues, and organs is influenced
by mechanical cues. Locally, cell growth leads to a mechanically heterogeneous
environment as cells pull and push their neighbors in a cell network. Despite
this local heterogeneity, at the tissue level, the cell network is remarkably
robust, as it is not easily perturbed by changes in the mechanical environment
or the network connectivity. Through a network model, we relate global tissue
structure (i.e. the cell network topology) and local growth mechanisms (growth
laws) to the overall tissue response. Within this framework, we investigate the
two main mechanical growth laws that have been proposed: stress-driven or
strain-driven growth. We show that in order to create a robust and stable
tissue environment, networks with predominantly series connections are
naturally driven by stress-driven growth, whereas networks with predominantly
parallel connections are associated with strain-driven growth
Computerized Analysis of Magnetic Resonance Images to Study Cerebral Anatomy in Developing Neonates
The study of cerebral anatomy in developing neonates is of great importance for
the understanding of brain development during the early period of life. This
dissertation therefore focuses on three challenges in the modelling of cerebral
anatomy in neonates during brain development. The methods that have been
developed all use Magnetic Resonance Images (MRI) as source data.
To facilitate study of vascular development in the neonatal period, a set of image
analysis algorithms are developed to automatically extract and model cerebral
vessel trees. The whole process consists of cerebral vessel tracking from
automatically placed seed points, vessel tree generation, and vasculature
registration and matching. These algorithms have been tested on clinical Time-of-
Flight (TOF) MR angiographic datasets.
To facilitate study of the neonatal cortex a complete cerebral cortex segmentation
and reconstruction pipeline has been developed. Segmentation of the neonatal
cortex is not effectively done by existing algorithms designed for the adult brain
because the contrast between grey and white matter is reversed. This causes pixels
containing tissue mixtures to be incorrectly labelled by conventional methods. The
neonatal cortical segmentation method that has been developed is based on a novel
expectation-maximization (EM) method with explicit correction for mislabelled
partial volume voxels. Based on the resulting cortical segmentation, an implicit
surface evolution technique is adopted for the reconstruction of the cortex in
neonates. The performance of the method is investigated by performing a detailed
landmark study.
To facilitate study of cortical development, a cortical surface registration algorithm
for aligning the cortical surface is developed. The method first inflates extracted
cortical surfaces and then performs a non-rigid surface registration using free-form
deformations (FFDs) to remove residual alignment. Validation experiments using
data labelled by an expert observer demonstrate that the method can capture local
changes and follow the growth of specific sulcus
Graph-Theoretical Tools for the Analysis of Complex Networks
We are currently experiencing an explosive growth in data collection technology that threatens to dwarf the commensurate gains in computational power predicted by Moore’s Law. At the same time, researchers across numerous domain sciences are finding success using network models to represent their data. Graph algorithms are then applied to study the topological structure and tease out latent relationships between variables. Unfortunately, the problems of interest, such as finding dense subgraphs, are often the most difficult to solve from a computational point of view. Together, these issues motivate the need for novel algorithmic techniques in the study of graphs derived from large, complex, data sources. This dissertation describes the development and application of graph theoretic tools for the study of complex networks. Algorithms are presented that leverage efficient, exact solutions to difficult combinatorial problems for epigenetic biomarker detection and disease subtyping based on gene expression signatures. Extensive testing on publicly available data is presented supporting the efficacy of these approaches. To address efficient algorithm design, a study of the two core tenets of fixed parameter tractability (branching and kernelization) is considered in the context of a parallel implementation of vertex cover. Results of testing on a wide variety of graphs derived from both real and synthetic data are presented. It is shown that the relative success of kernelization versus branching is found to be largely dependent on the degree distribution of the graph. Throughout, an emphasis is placed upon the practicality of resulting implementations to advance the limits of effective computation
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