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At least 541 records · Page 30Linked to original sources
PubMed1977

Breast secretory activity in nonlactating women, postpartum breast involution, and the epidemiology of breast cancer.

We found an association between the secretory activity of the nonlactating breast and genes related to apocrine function. Based on these findings, we developed a biologic model that relates epidemiologic evidence for an association of reproductive experience with risk of breast cancer to our findings of genetic variation of breast secretory activity and the fact that exogenously derived substances are secreted into breast fluid. The turnover rate of secreted substances was a primary determinant of exposure of the breast epithelium to environmental and endogenous carcinogens and promoters.

Adult↗
PubMed1984

Microdosimetry and thermoluminescence.

The mechanism for production of light from a thermoluminescent phosphor requires the combination of a trapped electron and hole at a luminescent center. The kinetics is that of a second-order process. Accordingly one may expect a similarity in the formalism for describing thermoluminescent emission (TL) and the theory of dual radiation action in radiobiology (TDRA). The TDRA was developed to provide a framework for describing the effects of ionizing radiation on biological systems. It requires the interaction of two primary units of radiation damage called "sublesions" to form a "lesion" which may then be expressed as a biological effect. The physical parameters used in the TDRA come from microdosimetry, which deals with the deposition of energy by ionizing radiation in domains whose dimensions are of the order of micrometers. This paper explores the relationships between microdosimetric concepts and a description of TL properties, such as TL dose-response curves and TL LET dependence. This is then compared with similar quantities in biology to determine the possible relevance of TL as a biological model.

Cell Survival↗
PubMed1994

[Measurement of nitric oxide and biological systems].

Interest in NO measurement strongly increased with the discovery that NO is endogenously produced by living tissues. This review describes the major techniques for quantification of NO and derivatives in biological models.

Electron Spin Resonance Spectroscopy↗
PubMed1999

Model of biological evolution with threshold dynamics and infinitely many absorbing states.

We study a model of biological evolution where the survival of a given species depends on its interactions with neighboring species. In the steady state the model has an active phase and an absorbing phase, which are separated by the critical point of the directed percolation universality class. The absorbing phase is infinitely degenerate and the dynamical behavior of our model is found to be nonuniversal.

Journal Article↗
PubMed1993

Advances in biologically based models for respiratory tract uptake of inhaled volatiles.

Physiologically based pharmacokinetic models for volatile organic chemicals typically describe the respiratory tract as a single compartment in which chemicals in the alveolar air space and the arterial blood are in instantaneous equilibrium. These models also assume that the distribution of chemical in the air-stream is uniform throughout the respiratory tract and that uptake is significant only in the alveolar region. A functional role for the upper respiratory tract in the uptake of volatile chemicals has been largely ignored. While these models have worked well for chemicals with low aqueous solubility in biological fluids, systemic uptake of highly soluble volatiles is overestimated. Thus, there is a significant effort to describe the critical determinants for uptake of soluble chemicals and to formulate models with more biologically relevant descriptions of respiratory tract structure and function. Investigators have addressed this problem from several viewpoints. Airflow patterns in the respiratory tract, regional metabolism, diffusion-dependent uptake, and the cyclic nature of respiration are now being incorporated into current models. Use of dosimetric models that incorporate relevant biology for inhaled chemicals will ultimately result in more meaningful human risk assessments.

Administration, Inhalation↗
PubMed1994

BUILD: a program generator for modelling experimental biological data.

BUILD is a program generator acting at source code level. The generated code corresponds to a whole application in order to model a biological process of interest using an iterative adjustment of experimental data. The program is designed to be executed in command line mode for processing of multiple data files with an individual execution control for each file. The results are completed by modular statistical and graphical functions. This approach has been shown to reduce the time and the amount of work needed for program development, debugging and maintenance. To date, BUILD has been successfully used in mathematical analysis of phenomenological approaches, but other fields of activity, such as educational software, are also conceivable.

Algorithms↗
PubMed1996

Models of biological pattern formation: common mechanism in plant and animal development.

Earlier proposed models for primary pattern formation, for gene activation and for segmentation are summarized and compared with recent molecular-genetic observations. A model for head, foot, tentacle and bud formation in Hydra illustrates that complex patterns can be reliably generated. Stable cell determination requires autocatalytic (autoregulatory) genes. Segmentation in insects has been proposed to result from a reiteration of (at least three) cell states. Their patterning is achieved by a mutual activation of cell states that locally exclude each other. A model for accretion of new segments by proliferation at the posterior pole is proposed that accounts for the generation of a periodic and a sequential pattern in register with each other. The assumption of a process analogous to segmentation in plants can account for the initiation of leaves with an intrinsic polarity that eventually leads to the upper and lower leaf surfaces. The model accounts also for the formation of axillary buds in correct relation to a leaf and for the much smaller spacing of leaves within a whorl when compared with the spacing between two successive whorls along the shoot. It is concluded that the generation of complex structures in distantly related organisms may be based on similar mechanisms.

Animals↗
PubMed1993

Dioxin hepatic carcinogenesis: biologically motivated modeling and risk assessment.

There are several key portions of the exposure-dose-response continuum with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD, dioxin) that have to be described quantitatively in developing a comprehensive mechanistically based dose-response model. These include: (i) the accumulation of TCDD in the target tissue, (ii) formation of a complex between dioxin and the Ah receptor, (iii) activation of transcription of growth regulatory genes by the TCDD-Ah receptor complex, (iv) cellular events on tumor initiation, promotion, and progression. Physiologically based pharmacokinetic (PBPK) models have been used as tools to integrate knowledge of the determinants of dioxin disposition, including specific binding to dioxin-inducible hepatic cytochromes, and to link TCDD tissue dosimetry with gene activation by pharmacodynamic (PD) models crafted to examine dioxin-regulated gene expression. Biological studies on growth factor regulation suggest hypotheses for the role of these gene products in transient cell proliferation, prolonged growth suppression, and hepatic tumor promotion. We have used these hypotheses as the basis for stochastic cell growth models of the promotional events with TCDD and to suggest experimental strategies for future research. The combination of PBPK, PBPD and stochastic cell growth models provides a seamless exposure-dose-response model for TCDD induction of liver tumors in rodents. This comprehensive exposure-dose-response model should prove useful for risk assessment, experimental design, and analysis of noncancer endpoints with this potent, ubiquitous environmental contaminant. This paper outlines progress in formulating and evaluating these models for TCDD.

Animals↗
PubMed1980

Application of 31P-NMR saturation transfer techniques to investigate phospholipid motion and organization in model and biological membranes.

The potential of 31P-NMR saturation transfer experiments for determining motional characteristics (in the millisecond to second time scale) of phospholipids in model and biological membranes is demonstrated. A technique to separate membrane phospholipid 31P-NMR signals from those of small water-soluble phosphates in intact cells in liver tissue is also illustrated.

Animals↗
PubMed2004

Biological nutrient removal model No.1 (BNRM1).

This paper presents the results of the work carried out by the CALAGUA Group on Mathematical Modelling of Biological Treatment Processes: the Biological Nutrient Removal Model No.1. This model is based on a new concept for dynamic simulation of wastewater treatment plants: a unique model can be used to design, simulate and optimize the whole plant, as it includes most of the biological and physico-chemical processes taking place in all treatment operations. The physical processes included are: settling and clarification processes (flocculated settling, hindered settling and thickening), volatile fatty acids elutriation and gas-liquid transfer. The chemical interactions included comprise acid-base processes, where equilibrium conditions are assumed. The biological processes included are: organic matter, nitrogen and phosphorus removal; acidogenesis, acetogenesis and methanogenesis. Environmental conditions in each operation unit (aerobic, anoxic or anaerobic) will determine which bacterial groups can grow. Thus, only the model parameters related to bacterial groups able to grow in any of the operation units of a specific WWTP will require calibration. One of the most important advantages of this model is that no additional analysis with respect to ASM2d is required for wastewater characterization. Some applications of this model have also been briefly explained in this paper.

Acid-Base Equilibrium↗
PubMedDate not supplied

Synthesis of biomass and utilization of plants wastes in a physical model of biological life-support system.

The paper considers problems of biosynthesis of higher plants' biomass and "biological incineration" of plant wastes in a working physical model of biological LSS. The plant wastes are "biologically incinerated" in a special heterotrophic block involving Californian worms, mushrooms and straw. The block processes plant wastes (straw, haulms) to produce soil-like substrate (SLS) on which plants (wheat, radish) are grown. Gas exchange in such a system consists of respiratory gas exchange of SLS and photosynthesis and respiration of plants. Specifics of gas exchange dynamics of high plants--SLS complex has been considered. Relationship between such a gas exchange and PAR irradiance and age of plants has been established. Nitrogen and iron were found to the first to limit plants' growth on SLS when process conditions are deranged. The SLS microflora has been found to have different kinds of ammonifying and denitrifying bacteria which is indicative of intensive transformation of nitrogen-containing compounds. The number of physiological groups of microorganisms in SLS was, on the whole, steady. As a result, organic substances--products of exchange of plants and microorganisms were not accumulated in the medium, but mineralized and assimilated by the biocenosis. Experiments showed that the developed model of a man-made ecosystem realized complete utilization of plant wastes and involved them into the intrasystem turnover.

Agaricales↗
PubMed1993

Construction of biological surface models from cross-sections.

An approach improving on existing techniques is presented for blending cross-sections of biological objects to produce a polynomial surface model. As intermediate steps to the final surface skinning, representative data points on the cross-sections are selected for defining piecewise cubic B-splines providing an immediate reduction in storage and computational requirements for the contour representation of the objects. A mesh of quadrilateral patches is subsequently formed over adjacent cross-sections using bicubic B-spline surfaces which exhibit second parametric derivative (C2) continuity. The surface model provides a complete and robust representation with significant data reduction. The resulting algorithm is demonstrated using bone data of a human hand.

Biomechanical Phenomena↗
PubMed2006

Lipid microdomains in model and biological membranes: how strong are the connections?

The concept of 'lipid rafts' and related liquid-ordered membrane microdomains has attracted great attention in the field of membrane biology, both as a novel paradigm in models of membrane organization and for the potential importance of such domains in phenomena such as membrane signaling and the differential trafficking of various membrane components. Studies of biological and of model membranes have gone hand in hand in shaping our current picture of the possible organization and functions of liquid-ordered lipid microdomains in membranes. This essay discusses some important current questions concerning the existence and functional importance of lipid microdomains in mammalian cell membranes, and the potential as well as the limitations of using model systems to help to address such questions.

Animals↗
PubMed2001

The role of physiologically based toxicokinetic models in biologically based risk assessment.

Biologically-based cancer risk assessment relies on mathematical models that represent the toxicokinetics and toxicodynamics of the xenobiotic in the body. Physiologically-based toxicokinetic (PBTK) models are used as a tool for predicting the target tissue dose of a xenobiotic from different routes of exposure, extrapolating from high doses to low doses and extrapolating between species. This paper reviews the role of PBTK models and their limitations in biologically-based risk assessment.

Animals↗
PubMed2004

Solvable biological evolution models with general fitness functions and multiple mutations in parallel mutation-selection scheme.

In a recent paper [Phys. Rev. E 69, 046121 (2004)]], we used the Suzuki-Trottere formalism to study a quasispecies biological evolution model in a parallel mutation-selection scheme with a single-peak fitness function and a point mutation. In the present paper, we extend such a study to evolution models with more general fitness functions or multiple mutations in the parallel mutation-selection scheme. We give some analytical equations to define the error thresholds for some general cases of mean-field-like or symmetric mutation schemes and fitness functions. We derive some equations for the dynamics in the case of a point mutation and polynomial fitness functions. We derive exact dynamics for two-point mutations, asymmetric mutations, and the four-value spin model with a single-peak fitness function. The same method is applied for the model with a royal road fitness function. We derive the steady-state distribution for the single-peak fitness function.

Biological Evolution↗