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1.
The present paper addresses the following question can a simple regulatory bone remodeling model predict effects of viscosity on the trabecular morphology? For that, we propose an extension of a previous bone remodeling model by taking into account the viscosity properties of the tissue. Zener’s law is used to describe the mechanical behavior of the bone and a specific law of the apparent bone density rate is proposed. Based on stability analysis, numerical simulations are then performed to investigate the viscosity role on simulations of the bone remodeling process. We show that the viscous contribution affects the evolution of the apparent bone density, by slowing down the adaptation process, which seems to be confirmed by simulations with real data obtained from rat tibia. 相似文献
2.
Using string kernel to predict signal peptide cleavage site based on subsite coupling model 总被引:2,自引:0,他引:2
Summary. Owing to the importance of signal peptides for studying the molecular mechanisms of genetic diseases, reprogramming cells
for gene therapy, and finding new drugs for healing a specific defect, it is in great demand to develop a fast and accurate
method to identify the signal peptides. Introduction of the so-called {−3,−1, +1} coupling model (Chou, K. C.: Protein Engineering, 2001, 14–2, 75–79) has made it possible to take into account the coupling effect among some key subsites and hence can significantly
enhance the prediction quality of peptide cleavage site. Based on the subsite coupling model, a kind of string kernels for
protein sequence is introduced. Integrating the biologically relevant prior knowledge, the constructed string kernels can
thus be used by any kernel-based method. A Support vector machines (SVM) is thus built to predict the cleavage site of signal
peptides from the protein sequences. The current approach is compared with the classical weight matrix method. At small false
positive ratios, our method outperforms the classical weight matrix method, indicating the current approach may at least serve
as a powerful complemental tool to other existing methods for predicting the signal peptide cleavage site.
The software that generated the results reported in this paper is available upon requirement, and will appear at http://www.pami.sjtu.edu.cn/wm.
An erratum to this article is available at . 相似文献
3.
One of the most important problems in recovering DNA distribution from flow cytometric DNA measurements is the presence of
background noise. In this paper, we analyse a probabilistic model recently proposed for background debris distribution and
based on a specific probabilistic mechanism for the DNA fragmentation process of the cell nucleus. In particular, we carry
out some sufficient conditions to uniquely identify the original DNA distribution from the flow cytometric data.
Received: 15 June 1997 / Revised version: 18 November 1997 相似文献
4.
Structure and function of cells often depend critically on molecular signals arriving at their surface. There are universal
mechanisms of signal transduction and signal processing across cell membranes. In this paper the mechanisms involving guanine-nucleotide
regulatory proteins (“G-proteins”) and certain receptor-kinases are considered. On the basis of recent findings in molecular biology a mathematical
model is developed taking into account all essential components in the biochemical network between first and second messenger.
There are two coupled feedback loops inherent in this process.
The model finally consists of three nonlinear equations, which are obtained from a system of originally ten equations by using
conservation laws and quasi-steady state conditions.
The second part of the paper contains a mathematical analysis of the model. Solutions describing the temporal development
of the involved biochemical species are shown to be bounded, more specifically to remain, independent of the size of the input
signal, in a bounded domain of the state space.
For the situation of stationary input signals existence, uniqueness and asymptotic stability of steady states are derived.
We also demonstrate biologically relevant stimulus-response properties like monotonicity and saturation effects.
For temporally non-constant input signals we show numerically that the model is able to produce phenomena of hypersensitivity
and desensitization which are important characteristics of cellular responsiveness.
Received 18 March 1996; received in revised form 15 April 1996 相似文献
5.
Multiparametric bifurcations for a model in epidemiology 总被引:1,自引:0,他引:1
In the present paper we make a bifurcation analysis of an SIRS epidemiological model depending on all parameters. In particular
we are interested in codimension-2 bifurcations.
Received 8 April 1994; received in revised form 29 June 1995 相似文献
6.
Polymorphism, distribution, and segregation of AFLP markers in a doubled haploid rice population 总被引:18,自引:0,他引:18
M. Maheswaran P. K. Subudhi S. Nandi J. C. Xu A. Parco D. C. Yang N. Huang 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1997,94(1):39-45
We exploited the newly developed amplified fragment length polymorphism (AFLP) technique to study the polymorphism, distribution
and inheritance of AFLP markers with a doubled haploid rice population derived from ‘IR64’/‘Azucena’. Using only 20 pairs
of primer combinations, we detected 945 AFLP bands of which 208 were polymorphic. All 208 AFLP markers were mapped and distributed
over all 12 chromosomes. When these were compared with RFLP markers already mapped in the population, we found the AFLP markers
to be highly polymorphic in rice and to follow Mendelian segregation. As linkage map of rice can be generated rapidly with
AFLP markers they will be very useful for marker-assisted backcrossing.
Received: 11 April 1996 / Accepted: 14 June 1996 相似文献
7.
Glutaraldehyde-formaldehyde fixed undecalcified alveolar bone from 7-day-old rats was prepared for light and electron microscopy.
Colloidal lanthanum was used as an ultrastructural tracer, and both random and semi-serial sections were examined. Lanthanum
penetrated the infoldings of the ruffled border and some nearby vacuoles and vesicles. The majority of vacuoles and vesicles
were lanthanum-free. Some osteoclast profiles contained a large vacuole with a cell enclosed in its interior. The enclosed
cell exhibited an irregular nucleus containing condensed peripheral chromatin, intact cytoplasmic organelles, conspicuous
rough endoplasmic reticulum and large blebs on the cell surface. These features are characteristic of osteoblasts or bone-lining
cells or immature osteocytes which may be undergoing apoptosis or necrosis. The observation of remnants of cellular structures
within internalized osteoclast vacuoles, together with the above results, suggests that osteoclasts engulf and probably degrade
dying osteoblasts/bone-lining cells or immature osteocytes.
Received: 29 April 1997 / Accepted: 20 February 1998 相似文献
8.
Dickinson RB 《Journal of mathematical biology》2000,40(2):97-135
A generalized transport model is derived for cell migration in an anisotropic environment and is applied to the specific
cases of biased cell migration in a gradient of a stimulus (taxis; e.g., chemotaxis or haptotaxis) or along an axis of anisotropy (e.g., contact guidance). The model accounts for spatial or directional dependence of cell speed and cell turning behavior to predict a constitutive
cell flux equation with drift velocity and diffusivity tensor (termed random motility tensor) that are explicit functions of the parameters of the underlying random walk model. This model provides the connection between
cell locomotion and the resulting persistent random walk behavior to the observed cell migration on longer time scales, thus
it provides a framework for interpreting cell migration data in terms of underlying motility mechanisms.
Received: 8 April 1999 相似文献
9.
Bone marrow lesions (BMLs) are radiologic abnormalities in magnetic resonance images of subchondral bone that are correlated with osteoarthritis. Little is known about the physiologic processes within a BML, although BMLs are associated with mechanical stress, bone tissue microdamage and increased bone remodeling. Here we establish a rabbit model to study the pathophysiology of BMLs. We hypothesized that in vivo loads that generate microdamage in cancellous bone would also create BMLs and increase bone remodeling. In vivo cyclic loading (0.2–2.0 MPa in compression for 10,000 cycles at 2 Hz) was applied to epiphyseal cancellous bone in the distal femurs of New Zealand white rabbits (n = 3, right limb loaded, left limb controls experienced surgery but no loading). Magnetic resonance images were collected using short tau inversion recovery (STIR) and T1 weighted sequences at 1 and 2 weeks after surgery/loading and histological analysis of the BML was performed after euthanasia to examine tissue microdamage and remodeling. Loaded limbs displayed BMLs while control limbs showed only a small BML-like signal caused by surgery. Histological analysis of the BML at 2 weeks after loading showed increased tissue microdamage (p = 0.03) and bone resorption (p = 0.01) as compared to controls. The model described here displays the hallmarks of load-induced BMLs, supporting the use of the model to examine changes in bone during the development, progression and treatment of BMLs. 相似文献
10.
De Gioia Elena M. Ghibaudi Enzo Laurenti Mario Salmona R. P. Ferrari 《Journal of biological inorganic chemistry》1996,1(5):476-485
Lactoperoxidase (LPO), eosinophil peroxidase (EPO) and myeloperoxidase (MPO) belong to the class of haloperoxidases, a group
of mammalian enzymes able to catalyze the peroxidative oxidation of halides and pseudohalides, such as thiocyanate. They all
play a key role in the development of antibacterial activity. The homology in their functional role is emphasized by the striking
similarity of their primary structures. A theoretical model for the three-dimensional structure of LPO and EPO has been developed
on the basis of the X-ray structure of MPO, a high degree of similarity having been found in their sequences. Evidence supporting
the hypothesis of an ester linkage between heme and apoprotein in LPO and EPO, originally proposed by Hultquist and Morrison
is discussed.
Received: 2 May 1996 / Accepted: 25 July 1996 相似文献
11.
Edoardo Beretta Fortunata Solimano Yasuhiro Takeuchi 《Journal of mathematical biology》1996,35(1):1-19
A mathematical model for the delivery of drug directly to the macrophages by using the phagocytosis of senescent red blood
cells is proposed. The model is based on the following assumption: At time t=0 a preassigned red blood cell population n(0, a)=φ(a), a>0, loaded by the drug, is injected in the blood circulation. Among the cells of that population only those with an age a≧ā (ā=120 days) will be phagocytosed by macrophages. Of course, the lifetime of the drug must be higher than ā. Within the
red blood cells it cannot be metabolized, neither can it diffuse through their membranes. The emphasis of the paper is on
the mathematical properties and on the formulation of the control problem.
Received 15 December 1994; received in revised form 20 July 1995 相似文献
12.
Extinction of top-predator in a three-level food-chain model 总被引:5,自引:0,他引:5
In this paper we extend the Lyapunov functions, constructed by A. Ardito and P. Ricciardi for predator–prey system [1], to
the three level food chain models. We first consider a general three-level food-chain model. A criterion for the extinction
of top predator will be given. Then we restrict our attentions to the case in which the prey is of logistic growth and predators
have Holling’s type II functional responses.
Received: 10 October 1997 相似文献
13.
In an unpredictable environment, the distributions of alleles from which polymorphism can be maintained forever belong to
a certain set, the C-viability kernel. Such a set is calculated in the two-locus haploid model, as well as the corresponding fitnesses at any time which make this
maintenance possible. The dependence of the C-viability kernel on the set U of admissible fitnesses and on the recombination rate r is studied. Notably, the C-viability kernel varies rapidly in the neighborhood of equal fitness of AB and ab; it becomes empty when ab has a fitness below a certain function, which is delineated, of the recombination rate. The properties of the two-locus model
under constraints, out of equilibrium and with unpredictable selection are thus presented.
Received: 20 May 1999 相似文献
14.
A mathematical, computer-based, dynamic sway model of a Sitka spruce (Picea sitchensis) tree was developed and tested against measurements of the movement of a tree within a forest. The model tree was divided
into segments each with a stiffness, mass and damping parameter. Equations were formulated to describe the response of every
segment which together form a system of coupled differential equations. These were solved with the aid of matrices and from
the resulting modes, the transfer function of the tree was found and used to calculate the movement of the tree in the wind.
Comparison of the modelled movement of a tree in response to the measured wind speed above a forest canopy gave good agreement
with the measured movement of the top of the tree but less satisfactory agreement close to the base. The comparison also pointed
to the complexity of tree response to the wind and inadequacies in the model. In particular, the branches need to be treated
as coupled cantilevers attached to the stem rather than simply as masses lumped together.
Received: 18 February 1997 / Accepted: 16 December 1997 相似文献
15.
The timing of cell differentiation can be controlled both by cell-intrinsic mechanisms and by cell-extrinsic signals. Oligodendrocyte
type-2 astrocyte progenitor cells are known to be the precursor cells that give rise to oligodendrocytes. When stimulated
to divide by purifed cortical astrocytes or by platelet-derived growth factor, these progenitor cells generate oligodendrocytes
in vitro with a timing like that observed in vivo. The most widely accepted model of this process assumes a cell-intrinsic biological clock that resides in the progenitor
cell. The intrinsic clock model originally proposed in 1986 remains as the dominant theoretical concept for the analysis of
timed differentiation in this cell lineage. However, the results of a recent experimental study (Ibarrola et al., Developmental
Biology, vol. 180, 1–21, 1996) are most consistent with the hypothesis that the propensity of a clone of dividing O-2A progenitor
cells initially to generate at least one oligodendrocyte may be regulated by cell-intrinsic mechanisms, but that environmental
signals regulate the extent of further oligodendrocyte generation. We propose a stochastic model of cell differentiation in
culture to accommodate the most recent experimental findings. Our model is an age-dependent branching stochastic process with
two types of cells. The model makes it possible to derive analytical expressions for the expected number of progenitor cells
and of oligodendrocytes as functions of time. The model parameters were estimated by fitting these functions through data
on the average (sample mean) number of both types of cells per colony at different time intervals from start of experiment.
Using this method we provide a biologically meaningful interpretation of the observed pattern of oligodendrocyte generation
in vitro and its modification in the presence of thyroid hormone.
Received: 18 April 1997 / Revised version: 30 November 1997 相似文献
16.
Aggregation, the formation of large particles through multiple collision of smaller ones is a highly visible phenomena in
oceanic waters which can control material flux to the deep sea. Oceanic aggregates more than 1 cm in diameter have been observed
and are frequently described to consist of phytoplankton cells as well as other organic matter such as fecel pellets and mucus
nets from pteropods. Division of live phytoplankton cells within an aggregate can also increase the size of aggregate (assuming
some daughter cells stay in the aggregate) and hence could be a significant factor in speeding up the formation process of
larger aggregate. Due to the difficulty of modeling cell division within aggregates, few efforts have been made in this direction.
In this paper, we propose a size structured approach that includes growth of aggregate size due to both cell division and
aggregation. We first examine some basic mathematical issues associated with the development of a numerical simulation of
the resulting algal aggregation model. The numerical algorithm is then used to examine the basic model behavior and present
a comparison between aggregate distribution with and without division in aggregates. Results indicate that the inclusion of
a growth term in aggregates, due to cell division, results in higher densities of larger aggregates; hence it has the impact
to speed clearance of organic matter from the surface layer of the ocean.
Received 1 July 1994; received in revised form 23 February 1996 相似文献
17.
H. L. Smith 《Journal of mathematical biology》1997,35(5):545-556
It is proved that the periodically forced Droop model for phytoplankton growth in a chemostat has precisely two dynamic regimes
depending on a threshold condition involving the dilution rate. If the dilution rate is such that the sub-threshold condition
holds, the phytoplankton population is washed out of the chemostat. If the super-threshold condition holds, then there is
a unique periodic solution, having the same period as the forcing, characterized by the presence of the phytoplankton population,
to which all solutions approach asymptotically. Furthermore, this result holds for a general class of models with monotone
growth rate and monotone uptake rate, the latter possibly depending on the cell quota.
Received 10 October 1995; received in revised form 26 March 1996 相似文献
18.
19.
The relationship between developmental stress caused by intraspecific larval density and fluctuating asymmetry (FA) was examined
in the tachinid parasitoid Homotrixa alleni. Individuals reared in larger clutches were significantly smaller in body size when they emerged as adults. As predicted,
smaller individuals were shown to have significantly higher levels of FA in their tibiae. This negative relationship was consistent
across each pair of tibia and the magnitude of FA was also significantly correlated between tibiae. The exact nature of the
observed relationship between larval density and FA in H. alleni remains speculative.
Received: 20 December 1999 / Received in revised form: 17 February 2000 / Accepted: 17 February 2000 相似文献
20.
We modify a simple mathematical model for natural selection originally formulated by Robert M. May in 1983 by permitting
one homozygote to have a larger selective advantage when rare than the other, and show that the new model exhibits dynamical
chaos. We determine an open region of parameter space associated with homoclinic points, and prove that there are infinite
sequences of period-doubling bifurcations along selected paths through parameter space. We also discuss the possibility of
chaos arising from imbalance in the homozygote fitnesses in more realistic biological situations, beyond the constraints of
the model.
Received 3 February 1995; received in revised form 1 November 1995 相似文献