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F Lefebvre

Publications and source records attributed to F Lefebvre.

At least 19 recordsLinked to original sources
PubMed2006

Design of low-frequency ultrasonic sensors for the analysis of the draining stage of cheese production.

This work presents a new ultrasonic system with a transmission mode in the 100-200 kHz frequency range. The system, composed of ultrasonic point sources, is used to monitor the mechanical properties of cheese during the early phases of production. First, our specialized sensor system is presented, and then the results obtained with the system model are compared to the results of actual experiments using the system to monitor the evolution of the medium during draining. The results of a rheological compliance test and shock pulse ultrasonic amplitude measurements agree and correspond quite accurately to the mechanical properties of the evolving physical state of the medium. This method could be used in the future to study the effect on the final cheese quality of the process parameters that interfere with cheese grain consolidation during the draining process.

Cheese↗
PubMed2006

Relative sensitivity factors of inorganic cations in frozen-hydrated standards in secondary ion MS analysis.

We describe the measurement, at 100 K, of the SIMS relative sensitivity factors (RSFs) of the main physiological cations Na+, K+, Mg2+, and Ca2+ in frozen-hydrated (F-H) ionic solutions. Freezing was performed by either plunge freezing or high-pressure freezing. We also report the measurement of the RSFs in flax fibers, which are a model for ions in the plant cell wall, and in F-H ionic samples, which are a model for ions in the vacuole. RSFs were determined under bombardment with neutral oxygen (FAB) for both the fibers and the F-H samples. We show that referencing to ice-characteristic secondary ions is of little value in determining RSFs and that referencing to K is preferable. The RSFs of Na relative to K and of Ca relative to Mg in F-H samples are similar to their respective values in fiber samples, whereas the RSFs of both Ca and Mg relative to K are lower in fibers than in F-H samples. Our data show that the physical factors important for the determination of the RSFs are not the same in F-H samples and in homogeneous matrixes. Our data show that it is possible to perform a SIMS relative quantification of the cations in frozen-hydrated samples with an accuracy on the order of 15%. Referencing to K permits the quantification of the ionic ratios, even when the absolute concentration of the referencing ion is unknown. This is essential for physiological studies of F-H biological samples.

Journal Article↗
PubMed2006

Alternative life-history and transmission strategies in a parasite: first come, first served?

Alternative transmission strategies are common in many parasitic organisms, often representing discrete phenotypes adopted in response to external cues. The facultative truncation of the normal 3-host life-cycle to a 2-host cycle in many trematodes provides an example: some individuals mature precociously, via progenesis, in their intermediate host and produce eggs without the need to reach a definitive host. The factors that determine how many and which individuals adopt the truncated life-cycle within a parasite population remain unknown. We investigated the occurrence of progenesis in the trematode Stegodexamene anguillae within its fish intermediate host. Location within the host was a key determinant of progenesis. Although the size and egg output of progenetic metacercariae encysted in host gonads did not differ from those of the few progenetic metacercariae in other host tissues, the likelihood of metacercariae becoming progenetic was much higher for those in the gonads than those elsewhere in the host. Progenetic parasites can only evacuate their eggs along with host eggs or sperm, providing a link between the parasite's transmission strategy and its location in the host. Host size and sex, and the presence of other parasite species in the host, did not affect the occurrence of progenesis in S. anguillae. However, the proportion of metacercariae in host gonads and the proportion of progenetic metacercariae both decreased with increasing numbers of S. anguillae per host. These results suggest that progenesis is adopted mostly by the parasites that successfully establish in host gonads. These are generally the first to infect a fish; subsequent arrivals settle in other tissues as the gonads quickly become saturated with parasites. In this system, the site of encystment within the fish host both promotes and constrains the adoption of a facultative, truncated life-cycle by the parasite.

Animals↗
PubMed2005

Progenesis in digenean trematodes: a taxonomic and synthetic overview of species reproducing in their second intermediate hosts.

Precocious egg production, i.e. progenesis, has been documented for a number of species in scattered reports throughout the trematode literature. The last 2 extensive studies on the subject date from Buttner in the early 1950s (in French) and from Tang in the early 1980s (in Chinese). Overall, 43 species were then known for their ability to produce eggs at the metacercarial stage while still in the second intermediate host. Here, we update the list, and document the existence of progenesis in a total of 79 digenean trematode species, for which we provide information on the taxonomy of the hosts, the facultative or obligate character of progenesis, relevant references, as well as some other pertinent biological information. We then review the subject by asking 7 questions of fundamental evolutionary importance. These include: What favours progenetic development? What are the associated costs and benefits? How are progenetic eggs released from the host? While exposing the various opinions of previous authors, we attempt to give a synthetic overview and stress on the importance of the metacercarial cyst wall (whether it is present, and if so its thickness) in the evolution and the adoption of a progenetic life-cycle.

Animals↗
PubMed2005

Life history constraints on the evolution of abbreviated life cycles in parasitic trematodes.

Abbreviations of the complex life cycle of trematodes, from three to two hosts, have occurred repeatedly and independently among trematode lineages. This is usually facultative and achieved via progenesis: following encystment in the second intermediate host, the metacercaria develops precociously into an egg-producing adult, bypassing the need to reach a definitive host. Given that it provides relatively cheap insurance against a shortage of definitive hosts, it is not clear why facultative progenesis has only evolved in a few taxa. Here a comparative approach is used to test whether progenetic trematodes are characterized by larger body size and egg volumes, two traits that correlate with other key life history features, than other trematodes. These traits may constrain the evolution of progenesis, because precocious maturation might be impossible when the size difference between the metacercaria and a reproductive adult is too large. First, trematode species belonging to genera in which progenesis has been documented were found not to differ significantly from other trematode species. Second, using within-genus paired comparisons across 19 genera in which progenesis has been reported, progenetic species did not differ, with respect to body size or egg size, from their non-progenetic congeners. Third, using intraspecific paired comparisons in species where progenesis is facultative, no difference was observed in the sizes of eggs produced by worms in both the intermediate and definitive host, suggesting that opting for progenesis does not influence the size of a worm's eggs. Overall, the lack of obvious differences in body or egg size between trematodes with truncated life cycles and those with the normal three-host cycle indicates that basic life history characteristics are not acting as constraints on the evolution of progenesis; trematodes of all sizes can do it. Why facultative progenesis is not more widespread remains a mystery.

Animals↗
PubMed2002

Spatial and temporal variation in Anguillicola crassus counts: results of a 4 year survey of eels in Mediterranean lagoons.

We present the results of a survey of anguillicolosis in the Rh6ne River delta. From January 1997 to December 2000, a total of 13,319 eels (Anguilla anguilla from elver to silver phase) were examined, in which we found 22,227 swimbladder nematodes (Anguillicola crassus adults and pre-adults). A generalised linear model (GLM) framework was used to explore the relative contribution of various factors to the occurrence, intensity and abundance of the parasite. We reveal a major influence of the month of sampling, and we document the existence of a seasonal pattern with regular peaks in early summer and late winter. In contrast, the year of sampling is of secondary importance, and no particular trend in the development of the infection can be detected. More than a decade after the first record of A. crassus in the Rhĵne River delta, anguillicolosis has thus attained a constant infection rate of nearly 50%, with a mean number of 3 or 4 macroscopic lumen worms per infected eel. The eel length strongly influences the intensity and the abundance of the nematode, but has little if any effect on the probability of being infected. There exists a linear relationship between eel size and the number of parasites, but not between eel size and prevalence. We observe a decrease in the proportion of infected individuals among elver eels. We discuss this result in relation to the possible mortality of heavily infected individuals and/or a change in the eels' alimentary diet.

Age Factors↗
PubMed2002

Development of a new ultrasonic technique for bone and biomaterials in vitro characterization.

Classical techniques based on bone imaging allow visual examination or provide quantitative parameters like bone mineral density, which is a mass per unit of surface. Unfortunately, these techniques are generally expensive. A new method of bone characterization that uses ultrasound techniques is presented in this article. This method is able to evaluate mechanical properties like Young's modulus E and cortical bone thickness with low-frequency transducers and does not require the use of a coupling medium. Some results of Young's modulus measurements are presented and compared to the literature values and the pulse echo method.

Animals↗
PubMed2002

The health state of the eel swimbladder as a measure of parasite pressure by Anguillicola crassus.

We have developed an index based on macroscopic criteria to easily assess the health state of the eel swimbladder as a result of infection by the nematode Anguillicola crassus. In the sampling area (brackish lagoons of the French Mediterranean coast), 92% of the host sample (1,251 eels covering all size classes) showed pathological signs of infection. The general trend was for increasing damage as eel size increased, thus suggesting accumulation of pathological effects. We also revealed a non-linear relationship between the swimbladder index and the abundance of living worms. In particular, we showed that severely damaged swimbladders harboured very few living nematodes. We argue that the swimbladder degenerative index more closely reflects the parasite pressure than does classic parasite count. We found seasonal variation in the swimbladder index, with maximum damage occurring in July and thereafter a trend for healthy individuals. We discuss the possibility that the seasonal decrease in the swimbladder index could reflect the death of the more severely affected individuals during the warmest months.

Air Sacs↗
PubMed2001

Activity of the forensic entomology department of the French Gendarmerie.

The criminal research Institute of the French Gendarmerie deals with several disciplines which are part of forensic science. Forensic entomology is one of the last laboratories created in Rosny-Sous-Bois. As with all other departments of the Institute, its main mission is carrying out its expertise on behalf of justice. Another important mission is to educate on one hand the "scene of crime" officers in order to increase the quality of the specimens collected and on the other hand to magistrates to make forensic entomology better known. At last, a research program is available to follow scientific development and to improve services to justice.

Animals↗
PubMed2001

Supported metallocene catalysts by surface organometallic chemistry. Synthesis, characterization, and reactivity in ethylene polymerization of oxide-supported mono- and biscyclopentadienyl zirconium alkyl complexes: establishment of structure/reactivity relationships.

The reactions of CpZr(CH(3))(3), 1, and Cp(2)Zr(CH(3))(2), 2, with partially dehydroxylated silica, silica-alumina, and alumina surfaces have been carried out with careful identification of the resulting surface organometallic complexes in order to probe the relationship between catalyst structure and polymerization activity. The characterization of the supported complexes has been achieved in most cases by in situ infrared spectroscopy, surface microanalysis, qualitative and quantitative analysis of evolved gases during surface reactions with labeled surface, solid state (1)H and (13)C NMR using (13)C-enriched compounds, and EXAFS. 1 and 2 react with silica(500) and silica-alumina(500) by simple protonolysis of one Zr-Me bond by surface silanols with formation of a single well-defined neutral compound. In the case of silica-alumina, a fraction of the supported complexes exhibits some interactions with electronically unsaturated surface aluminum sites. 1 and 2 also react with the hydroxyl groups of gamma-alumina(500), leading to several surface structures. Correlation between EXAFS and (13)C NMR data suggests, in short, two main surface structures having different environments for the methyl group: [Al](3)-OZrCp(CH(3))(2) and [Al](2)-OZrCp(CH(3))(mu-CH(3))-[Al] for the monoCp series and [Al](2)-OZrCp(2)(CH(3)) and [Al]-OZrCp(2)(mu-CH(3))-[Al] for the bisCp series. Ethylene polymerization has been carried out with all the supported complexes under various reaction conditions. Silica-supported catalysts in the absence of any cocatalyst exhibited no activity whatsoever for ethylene polymerization. When the oxide contained Lewis acidic sites, the resulting surface species were active. The activity, although improved by the presence of additional cocatalysts, remained very low by comparison with that of the homogeneous metallocene systems. This trend has been interpreted on the basis of various possible parameters, including the (p-pi)-(d-pi) back-donation of surface oxygen atoms to the zirconium center.

Journal Article↗
PubMed2001

Mediators of behavioral problems in 7-year-old children born after 24 to 28 weeks of gestation.

We tested the hypothesis that prematurity acts through its association with neuromotor and intellectual functioning to explain behavior problems at school age. Sixty-one extremely preterm (EP) very low birth weight (VLBW) children (< 29 wk and < 1,500 g) born in 1987-1990 and 44 normal birth weight children (NBW) (> 37 wk and > 2,500 g) were matched for age, sex, and socioeconomic status (SES). Mediator variables were evaluated at a hospital at 5 years and 9 months. Behaviors were evaluated at school at 7 years by peers, teachers, and parents. When compared with NBW children, EP/VLBW children had poorer IQ and neuromotor development. At school, EP/VLBW children were evaluated by peers as more sensitive/ isolated, and by teachers and parents as more inattentive and hyperactive than NBW. When mediators were introduced, the previously significant relation between prematurity and behavior problems disappeared. Hyperactive and inattentive behaviors were explained by a specific working memory factor for the latter, and by a general intellectual delay for the former, whereas sensitive/isolated behaviors were best explained by neuromotor delays. Inattentive behaviors were also related to family adversity. At school age, extreme prematurity had thus an indirect effect on behaviors via specific and nonspecific intellectual and neuromotor delays.

Age Factors↗
PubMed2000

Computerized ultrasound B-scan characterization of breast nodules.

The ability of computerized parameters to discriminate benign from malignant breast nodules from digitized ultrasonic acquisitions has been assessed. The images of 75 lesions, including 19 lesions proved to be malignant at histology and 56 found to be benign, were digitized and characterized by morphometric and texture parameters. The texture parameters, derived from first-order statistics, run-length matrices and co-occurrence matrices, were computed in the entire lesion and in a ring-shape region surrounding the contour of the lesion. The strongest features were found to be issued from the second region. Further investigations confirmed that the discriminant information was contained in the external part of the lesion and, to a lesser extent, in the neighboring tissue. A linear discriminant analysis using three features yielded a sensitivity of 94.7% for a specificity of 80.4% and the "leave-one-out" technique confirmed the results. Comparison with the classifications given by radiologists let us assume that information revealed by texture features is able to help the physician in reducing the number of unnecessary biopsies.

Breast Diseases↗
PubMed2000

Secondary ion mass spectrometry imaging of the fixation of 15N-labelled NO in pollen grains.

We used secondary ion mass spectrometry to image cellular targets of nitrogen oxides (widespread air pollutants) in pollen grains of birch (Betula verrucosa Ehrh.) and cockfoot (Dactylis glomerata L.). The pollen samples were exposed to air supplemented with high doses of 15NO. The pollen grains were then fixed, dehydrated using a newly developed 'vapour phase' preparation method and embedded in LRW resin. Semithin sections were then analysed. Imaging was performed in scanning mode. As usual, the two isotopes 14N and 15N were imaged as 12C14N- and 12C15N-, respectively. The isotopic percentages of 15N were quantitatively determined either by image processing or by direct analysis. We show that the preferential areas of NO fixation in the pollen cell are the sporoderm and discrete intracytoplasmic structures that we tentatively describe as globoid-like structures similar to those encountered in seeds.

Air Pollutants↗
PubMed2000

Evaluation of error bounds on calcaneal speed of sound caused by surrounding soft tissue.

For absorptiometry measurements, soft tissue may have an impact on quantitative ultrasound (QUS) measurements. In the present study, we focused primarily on the quantification of measurement error on speed of sound (SOS) caused by surrounding soft tissue. The relevant soft tissue parameters affecting the inherent SOS inaccuracies are thickness and sound velocity. To meet our goal, SOS measurements were taken at the right heel using a QUS imaging device in 21 healthy subjects. Site-matched measurements of soft tissue thickness (STT) and bone width were performed using magnetic resonance imaging of the heel. Several bone velocities were calculated either by accounting for bone width (SOSBW) only or by taking into account the exact path lengths of all major components traversed by ultrasound &lapr;V(b)). Given that soft tissue composition is difficult to determine in vivo, we chose to estimate lower and upper error bounds on bone velocity (V(b lower) and V(b upper)) by spanning the full range of available values in the literature. The mean BW was 30.7 +/- 2.7 mm and the mean medial and external STTs were 8.8 +/- 1.7 and 8.5 +/- 1.5 mm, respectively. Accounting for true BW only resulted in no significant difference between SOS (1533 +/- 37) and SOSBW (1531 +/- 33). By contrast, accounting for both true BW and surrounding soft tissue resulted in an increase in the calculated bone velocity and statistically significant differences between SOS and V(b upper) (1568 +/- 36) and V(b lower) (1542 +/- 34). Root mean square errors between SOS and the calculated velocities were 0.34, 2. 32, and 0.70% for SOSBW, V(b upper), and V(b lower), respectively. We report here measurement errors caused by soft tissue to be 3 to 20 times higher than the SOS short-term precision (SOS coefficient of variation of 0.1%). Our results suggest that inaccuracies in SOS measurement caused by overlying soft tissue cannot be neglected. Overlying soft tissues may influence outcomes of longitudinal studies, especially if variations in tissue thickness and composition occur during the longitudinal follow-up. A practical way of minimizing the measurement error could be to perform an adequate correction for the overlying soft tissue. However, ideally, this should require knowing both the thickness and sound velocity in soft tissue. One might preferably conduct experimental investigations that directly control soft tissue thickness and composition to resolve this problem.

Adult↗
PubMed1999

Development of RGD peptides grafted onto silica surfaces: XPS characterization and human endothelial cell interactions.

The attachment of human umbilical vein endothelial cells (HUVECs) on substrates that had been covalently grafted with the cell adhesion peptides Arg-Gly-Asp (RGD) was investigated. This approach was used to provide substrates that are adhesive to cells even in the absence of serum proteins and to cells that have had no prior treatment of the surface with proteins that promote cell adhesion. We wanted to improve control of cellular interactions with cell-adhesive materials by providing fixedly bound adhesion ligands. Silica was examined as a model surface. The peptides were grafted using three different steps: grafting of aminosilane molecules; reaction with a maleimide molecule; and immobilization of cell-binding peptides containing the RGD sequence. The RGD-grafted surface was characterized by X-ray photoelectron spectroscopy (XPS) and contact-angle measurements.

Amino Acid Sequence↗
PubMed1999

Potentialities of ultrasounds for the nondestructive evaluation of cell adhesion.

The aim of this paper is to present the potentialities of ultrasounds to investigate the mechanical properties of a cell/substrate interface. The adhesion process plays a major role in the development of osteoblastic cells on various substrates used in orthopedic applications such as metals, bioceramics, etc. Particularly, cell adherence appears to be a critical factor in the colonization process. High-frequency and low-power ultrasounds seem to be an appropriate tool for a nondestructive evaluation of interface properties. First, we present the results obtained with bulk longitudinal and shear waves under an arbitrary incidence over an aluminum-adhesive interface. This study was performed for an industrial application of bonding. The results clearly show the sensitivity of shear waves for the evaluation of the adhesion quality owing to the shear solicitations at the interface they induce. A model of ultrasound interactions with a boundary subject to varying degrees of adhesion has been developed and compared to the experiments. Second, we investigated osteoblastic cell cultures with a high-frequency acoustic microscope working at 50 MHz. The images obtained in the shear mode reveal a better contrast than those obtained in the longitudinal mode. For the time being, these results are qualitative, and theoretical models have to be developed according to the point of view of biologists.

Aluminum↗