Mise en evidence de Campanien continental (facies provencal) a Salavas (Ardeche,S-E de la France): Description,sedimentologie, paleogeographie

The continental sediments in the Northern Alès Basin are dated as Campanian age based on gastropod Melania praelonga MATHERON. The lithology is made up by red shales, conglomerates and pedologic limestones (crusts and nodules) with microcodiums. The conglomerates are heterometric and made up by limestone (Urgonian facies) and standstone boulders of Cenomanian and Turonian age. Also sandy limestones of coniacian age are found, all belonging to the local substratum. This discovery helps to understand the evolution of the Upper Cretaceous paleogeography.     

PROLIFERATION KINETICS OF AN EXPERIMENTAL ASCITES TUMOUR OF THE MOUSE

The cell cycles of an experimental ascitic tumour of the C3H mouse (NCTC 2472) were determined at various times after the intraperitoneal injection of 10⁶ cells. It was found that, contrary to results in solid NCTC 2472 tumours, obtained with the same NCTC cells, the duration of the cell cycle and its phases lengthened with the age of the tumour while the growth fraction remained relatively constant. G₁ was the first phase to lengthen, while later T_s and T_G2 increased also. The amount of DNA per cell was determined by cytospectrophotometry. This method provides data on the evolution during growth of the relative number of cells in each phase of the cell cycle.     

Modelers' perception of mathematical modeling in epidemiology: a web-based survey

Background

Mathematical modeling in epidemiology (MME) is being used increasingly. However, there are many uncertainties in terms of definitions, uses and quality features of MME.

Methodology/Principal Findings

To delineate the current status of these models, a 10-item questionnaire on MME was devised. Proposed via an anonymous internet-based survey, the questionnaire was completed by 189 scientists who had published in the domain of MME. A small minority (18%) of respondents claimed to have in mind a concise definition of MME. Some techniques were identified by the researchers as characterizing MME (e.g. Markov models), while others–at the same level of sophistication in terms of mathematics–were not (e.g. Cox regression). The researchers'' opinions were also contrasted about the potential applications of MME, perceived as higly relevant for providing insight into complex mechanisms and less relevant for identifying causal factors. The quality criteria were those of good science and were not related to the size and the nature of the public health problems addressed.

Conclusions/Significance

This study shows that perceptions on the nature, uses and quality criteria of MME are contrasted, even among the very community of published authors in this domain. Nevertheless, MME is an emerging discipline in epidemiology and this study underlines that it is associated with specific areas of application and methods. The development of this discipline is likely to deserve a framework providing recommendations and guidance at various steps of the studies, from design to report.     

Cell cycle models and mother-daughter correlation

Among the different experimental features that are usually used to measure the variability of cell cycle duration (Tc), the mother-daughter correlation is a special case since apparently contradictory results have been reported from different experimental systems. Indeed, positive, null and negative experimental values have been found, raising problems in cell cycle modeling in as much as one would like a general, unified view of the control of the cell cycle. Such views are provided by mathematical models. We present a method to derive the value(s) of the correlation between mother and daughter Tcs predicted from any such model, provided Tc can be analytically expressed. An exact calculation is proposed for the case when Tc can be expressed as a linear combination of the parameters involved. Otherwise, using Taylor series expansion, an approximate expression for the correlation can be derived. In both cases, the predicted values can be compared with experimental ones. The proposed method is illustrated using the Growth Controlled model of Koch (1980, Nature 286: 80) and the Continuum model of Cooper (1982, J. theor. Biol. 94: 783). Whereas Koch's model leads to inflexible negative values of the correlation between mother and daughter Tcs, our results demonstrate that differences in the mean duration of the G1 phase may account for differences in the sign of this correlation in Cooper's model. Furthermore, the proposed method can also be applied to other correlations such as the correlation between sibling cell Tcs.(ABSTRACT TRUNCATED AT 250 WORDS)     

A COMPARISON OF COMPUTER METHODS FOR THE ANALYSIS OF FRACTION LABELLED MITOSES CURVES

Computer methods developed by the authors for analysis of fraction labelled mitoses curves (FLM curves) have been compared. Four test examples were used in the study; the first example was the synthesis of a FLM curve with fixed parameters and the others involved fitting actual data. Experimental FLM curves showing various degrees of damping were used in the curve fitting tests. In each test example the comparison was based on the assumptions of exponential growth, a growth fraction of unity and no cell loss. In three of the test examples good agreement between the methods was observed but in one example some important discrepancies arose in the analysis of a heavily damped FLM curve.     

Cell proliferation in EMT6 tumors treated with single doses of x-rays or hydroxyurea. I. Experimental results

EMT6 mouse mammary tumors were treated in vivo with 5 mg/mouse of hydroxyurea (HU) or 300 rads of X-rays. The proliferation of the tumor cells was followed for 28 hr after treatment. Changes in the 3H-TdR labeling index, the mitotic index, the specific activity of the 3H-TdR-labeled DNA, and the proportion of suspended, clonogenic cells in the S phase of the cell cycle were examined and compared. Evidence was found for reassortment of the surviving cells in treated tumors into partially synchronous cohorts. The partial synchrony in the proliferation of the surviving cells was not accurately predicted by the changes in the labeling index and the mitotic index. The changes in DNA specific activity proved unacceptable as an indicator of cell proliferation in solid EMT6 tumors treated with low doses of radiation or HU.     

COMPUTER SIMULATION OF GROWING CELL POPULATIONS

A new technique of computer simulation of growing population of cells is presented. This method can be used to study various biological problems the examples of which are discussed in this paper.     

A compartmental model for the study of diurnal rhythms in cell proliferation

A mathematical model taking into account the observed diurnal variations in cell kinetics is presented. Its principle is to divide each phase of the cell cycle into a definite number of compartments and to assume time-dependent probabilities of transition from one compartment to the following; general properties of the model are derived.The particular case where the only time-dependent transition probabilities are those corresponding to the G₁ phase is studied. A characterization of the joint percentages of S and M cells variations is given. The application of the model to interpretation of published experimental data obtained in hamster cheek pouch epithelium is given.