Biomedical subjects

C Zonneveld

Publications and source records attributed to C Zonneveld.

6 recordsLinked to original sources

PubMed2003

The embedded tumour: host physiology is important for the evaluation of tumour growth.

The growth potential of a tumour can significantly depend on host features such as age, cell proliferation rates and caloric intake. Although this is widely known, existing mathematical models for tumour growth do not account for it. We therefore developed a new model for tumour growth, starting from a mathematical framework that describes the host's physiology. The resulting tumour-in-host model allowed us to study the implications of various specific interactions between the energetics of tumour and host. The model accounts for the influence of both age and feeding regimen of the host organism on the behaviour of a tumour. Concerning the effects of a tumour on its host, it explains why tumour-mediated body-weight loss is often more dramatic than expected from the energy demands of the tumour. We also show how the model can be applied to study enhanced body-weight loss in presence of cachectic factors. Our tumour-in-host model thus appears a proper tool to unite a wide range of phenomena in tumour-host interactions.

Age Factors↗

PubMed2001

Analysis of bioassays with time-varying concentrations.

In the analysis of ecotoxicological bioassays the concentration of the test compounds is assumed to be constant. In many situations this assumption is questionable, as various processes may cause a substantial decline in the concentration during exposure. This leads to difficulties in the estimation of parameters that characterise the toxicity of the test compound. As a solution to this problem, time-varying concentrations are often replaced by their mean values for the estimation of toxicity parameters. However, Monte-Carlo simulations show that this approach results in biased estimates of the toxicity parameters. As an alternative approach, we propose models to estimate one important toxicity parameter, the no effect concentration, in situations where the concentration of the compound varies in time. These models are extensions of the DEBtox model (Kooijman and Bedaux, 1996) which is based on biological assumptions about toxicokinetics and toxic effects. We also propose a new approach for the estimation of toxicity parameters for strongly accumulating non-metabolisable compounds. This approach does not require any kinetics assumption. Computer simulation and experimental data confirm the relevance of the different proposals.

Biological Assay↗

PubMed2001

From exposure to effect: a comparison of modeling approaches to chemical carcinogenesis.

Standardized long-term carcinogenicity tests aim to reveal the relationship between exposure to a chemical and occurrence of a carcinogenic response. The analysis of such tests may be facilitated by the use of mathematical models. To what extent current models actually achieve this purpose is difficult to evaluate. Various aspects of chemically induced carcinogenesis are treated by different modeling approaches, which proceed very much in isolation of each other. With this paper we aim to provide for the non-mathematician a comprehensive and critical overview of models dealing with processes involved in chemical carcinogenesis. We cover the entire process of carcinogenesis, from exposure to effect. We succinctly summarize the biology underlying the models and emphasize the relationship between model assumptions and model formulations. The use of mathematics is restricted as far as possible with some additional information relegated to boxes.

Animals↗

PubMed1997

Modeling effects of Photoadaption on the Photosynthesis-Irradiance Curve

In this article, I analyse how photoadaption affects the photosynthesis-irradiance curve in phytoplankton. Four parameters are presumably affected by photoadaption: (1) the size of a photosynthetic unit; (2) the number of photosynthetic units per cell; (3) the average turnover time of a photosynthetic unit; and (4) the chlorophyll-specific absorption cross-section. Prezelin's well-known conceptual model of photoadaption deals with variation in size and number of photosynthetic units, but not with the other two parameters. Prezelin's model predicts that the photosynthesis-irradiance curve is differentially affected by variations in size or number of photosynthetic units. By contrast, my analysis shows that if photoadaptional variation in the turnover time is also accounted for, only the cellular chlorophyll concentration is relevant; variation in the size vs. number of photosynthetic units is immaterial. The photoadaptional response of the chlorophyll-specific absorption cross-section allows a simple description consistent with experimental data. I use this description to derive a single expression for the rate of photosynthesis as dependent on irradiance and the cellular chlorophyll concentration. This model for the photosynthesis-irradiance-curve accounts for the photoadaptional alterations of all four parameters. Copyright 1997 Academic Press Limited

Journal Article↗

PubMed1993

Comparative kinetics of embryo development.

A model of embryo energetics was fitted to data from the literature for species as different as snails and mammals. The model is based on assumptions about energy uptake, storage and utilization. It describes the animal by two state variables: volume and energy storage. Embryo weight is taken to be proportional to volume, yolk weight to energy storage, and respiration rate to storage utilization rate. The fits were good, with minor deviations occurring only in the early phases of development. For altricial birds, good model fits were obtained, but the parameter values markedly differed from those of other species. We hypothesized that, due to an increase in energy utilization towards hatching, the temperature of the embryo increases. As a result, metabolic processes are accelerated. When this was taken into account, parameter values were obtained that correspond better with those of other animals.

Animals↗

PubMed1991

Models on butterfly protandry: virgin females are at risk to die.

Current models on protandry in butterflies assume that females are mated instantaneously upon eclosion. However, for most butterfly species this assumption is not realistic. In this paper a model is formulated in which the mating rate depends on both male and female density. Given the female presence curve, protandry is an evolutionarily stable strategy (ESS) for males. The evolutionarily stable amount of protandry decreases with increasing death rate and decreasing encounter rate. Given the male presence curve, protandry also is an ESS for females. However, male and female ESS are not identical; moreover, in the present model a simultaneous ESS does not exist. Protandry critically depends on the assumption that females mate only once, whereas males are capable of multiple mating. If females too are capable of multiple mating, absence of protandry is the ESS for males as well as females. The model predicts that protandry depends on population density: protandry should be more pronounced in populations with high density than in populations with low density. Protandry also depends on sex ratio. It becomes more pronounced when the proportion of males among emerging adults increases.

Animals↗