1,720,960 research outputs found
Emergence of taxis and synergy in angiogenesis
No full textAngiogenesis, the expansion of the vascular system feeding a tumor, is crucial to both primary tumors long-time growth and for the successful implantation of metastases. We formulate a model that relates the energetic requirements of the cancer cells to the production and diffusion of an angiogenic factor and to the ensuing evolution of neighboring endothelial cells. The model yields predictions for the development of neovascularization and for the increase in the blood flow to the tumor. We show that the directed growth of the vascular net is an emergent property and that therapies targeting different stages of the angiogenic process might have a synergistic effect
Diffusion with evolving sources and competing sinks: Development of angiogenesis
No full textTumors ensure their long-time growth by emitting molecular messengers that induce cellular modifications in neighboring capillaries. These modifications are conducive to the enlargement of the vascular system feeding the tumor. This phenomenon, termed angiogenesis, is controlled by the diffusion and competitive trapping of nutrients and molecular messengers by several cell species. The number, location, and properties of these traps change continuously. The angiogenic process also implies that nutrient sources are time dependent. Starting from assumptions at the cellular level, we formulate a mathematical model that predicts the evolution of angiogenesis and the increase in the blood flow to the tumor. The model also predicts the emergence of directed growth and the possibility of therapeutical synergy. Simulations permit a careful analysis of the influence of the main parameters
Korteweg-de Vries solitons under additive stochastic perturbations
No full textThe evolution of solitonic solutions of the Korteweg-de Vries equation subject to additive noise is investigated using numerical techniques. Various types of additive white Gaussian noise are considered. The averaged solution amplitudes exhibit in all cases algebraic decay, verifying Wadati's universality conjecture. If the noise is time dependent, or position and time dependent, algebraic decay is obtained for intermediate times too. These intermediate-time results agree well with the outcome of an experiment on ion-acoustic soliton propagation in a noisy plasma. The distribution of soliton first passage times in a noisy medium is also discussed. [S1063-651X(98)10509-3]
Competition effects in the dynamics of tumor cords
No full textA general feature of cancer growth is the cellular competition for available nutrients. This is also the case for tumor cords, neoplasms forming cylindrical structures around blood vessels. Experimental data show that, in their avascular phase, cords grow up to a limit radius of about 100 mum, reaching a quasi - steady-state characterized by a necrotized area separating the tumor from the surrounding healthy tissue. Here we use a set of rules to formulate a model that describes how the dynamics of cord growth is controlled by the competition of tumor cells among themselves and with healthy cells for the acquisition of essential nutrients. The model takes into account the mechanical effects resulting from the interaction between the multiplying cancer cells and the surrounding tissue. We explore the influence of the relevant parameters on the tumor growth and on its final state. The model is also applied to investigate cord deformation in a region containing multiple nutrient sources and to predict the further complex growth of the tumor
A model for diffusion and competition in cancer growth and metastasis
No full textA master equation formalism is used to model the growth and metastasis of a tumor as a function of the diffusion and absorption of a nutrient. Healthy and cancerous (C-) cells compete to bind the nutrient, which is allowed to diffuse starting from a prescribed region. Two thresholds are defined for the quantity of nutrient bound by the C-cells. If this quantity falls below the lower threshold, the cell dies, while if it increases above the upper threshold, the cell divides according to a predefined stochastic mechanism. C-cells migrate when they record a low concentration of free nutrient in the local environment. The model is formulated in terms of a coupled system of equations for the cell populations and the free and bound nutrient. This system can be solved by using the Local Interaction Simulation Approach (LISA), a numerical procedure that permits an efficient and detailed solution and is easily adaptable to parallel processing. With suitable parameter variation, the model can describe multiple tumor configurations, ranging from the classical spheroid with a necrotic core favored by mathematicians to very anisotropic shapes with inhomogeneous concentrations of the various populations. This is important because the nature of the anisotropy may be crucial in determining whether and how the cancer metastasizes. The effects of stochasticity and the presence of additional nutrients or inhibitors can be easily incorporated
Nutrient competition as a determinant for cancer growth
No full textCompetition for available nutrients is known to be crucial for cancer development. Based on this fact, a model is proposed that can describe the manifold of morphologies and growth rates characteristic of tumoral growth. The formulation of a consistent set of rules governing the microscopic interactions leads to st system of coupled nonlinear iteration equations. These equations contain both deterministic and stochastic terms and are amenable to direct numerical simulation. They allow us to test the effects of such parameters as the availability, diffusivity, and binding rate of nutrients and the mobility, death, and multiplication rates of cancer cells on tumor morphology and development. Detailed numerical solutions are presented. [S1063-651X(99)03402-9]
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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