Showing papers by "University of Amsterdam published in 1990"

Flavonoid genes in petunia: addition of a limited number of gene copies may lead to a suppression of gene expression.

Abstract: To evaluate the effect of increased expression of genes involved in flower pigmentation, additional dihydroflavonol-4-reductase (DFR) or chalcone synthase (CHS) genes were transferred to petunia. In most transformants, the increased expression had no measurable effect on floral pigmentation. Surprisingly, however, in up to 25% of the transformants, a reduced floral pigmentation, accompanied by a dramatic reduction of DFR or CHS gene expression, respectively, was observed. This phenomenon was obtained with both chimeric gene constructs and intact CHS genomic clones. The reduction in gene expression was independent of the promoter driving transcription of the transgene and involved both the endogenous gene and the homologous transgene. The gene-specific collapse in expression was obtained even after introduction of only a single gene copy. The similarity between the sense transformants and regulatory CHS mutants suggests that this mechanism of gene silencing may operate in naturally occurring regulatory circuits.

Evidence for compartmentalization of functional subsets of CD2+ T lymphocytes in atopic patients

Abstract: Lymphokine secretion profiles were studied of human allergen-specific CD4+ T lymphocyte clones (TLC). To this aim, panels of house dust mite Dermatophagoides pteronyssinus (Dp)-specific TLC were generated from two atopic Dp-allergic patients, suffering from severe atopic dermatitis (AD1) and allergic asthma (AD2), respectively, and from a non-atopic individual (NAD). From AD1 additional TLC were cloned specific for tetanus toxoid or Candida albicans, both Ag that were not relevant for the atopic state of this patient. Secretion of IL-2, IL-4, and IFN-gamma was determined after specific stimulation of these TLC, using autologous monocytes as APC. With respect to the production of IL-4 and IFN-gamma, clearly distinct profiles were observed. All Dp-specific TLC from both atopic donors produced IL-4 but not IFN-gamma, whereas the Dp-specific TLC from NAD, as well as the tetanus toxoid- and C. albicans-specific TLC from AD1, all produced IFN-gamma but not or small quantities of IL-4. Most TLC from all panels produced IL-2. These lymphokine profiles were consistent for at least 3 days and were neither dependent on the dose of allergen nor on the atopic or nonatopic state of the donor of APC. The functional consequence of these restricted lymphokine profiles was stressed by the observation that, whereas Dp-specific TLC from AD1 and AD2 supported in vitro IgE production, this support could be abrogated by a Dp-specific TLC from NAD. The present results suggest that CD4+ T lymphocytes that produce IL-4, but not IFN-gamma, occur in high frequencies in the allergen-specific T cell repertoires of atopic donors, which may have important implications for the pathomechanism of atopic disease.

Activation of coagulation after administration of tumor necrosis factor to normal subjects.

Abstract: Tumor necrosis factor has been implicated in the activation of blood coagulation in septicemia, a condition commonly associated with intravascular coagulation and disturbances of hemostasis. To evaluate the early dynamics and the route of the in vivo coagulative response to tumor necrosis factor, we performed a controlled study in six healthy men, monitoring the activation of the common and intrinsic pathways of coagulation with highly sensitive and specific radioimmunoassays. Recombinant human tumor necrosis factor, administered as an intravenous bolus injection (50 micrograms per square meter of body-surface area), induced an early and short-lived rise in circulating levels of the activation peptide of factor X, reaching maximal values after 30 to 45 minutes (mean +/- SEM increase after 45 minutes, 34.2 +/- 18.2 percent; tumor necrosis factor vs. saline, P = 0.015). This was followed by a gradual and prolonged increase in the plasma concentration of the prothrombin fragment F1+2, peaking after four to five hours (mean increase after five hours, 348.0 +/- 144.8 percent; tumor necrosis factor vs. saline, P less than 0.0001). These findings signify the formation of factor Xa (activated factor X) and the activation of prothrombin. Activation of the intrinsic pathway could not be detected by a series of measurements of the plasma levels of factor XII, prekallikrein, factor XIIa-C1 inhibitor complexes, kallikrein-C1 inhibitor complexes, and the activation peptide of factor IX. The delay between the maximal activation of factor X and that of prothrombin amounted to several hours, indicating that neutralization of factor Xa activity was slow. We conclude that a single injection of tumor necrosis factor elicits a rapid and sustained activation of the common pathway of coagulation, probably induced through the extrinsic route. Our results suggest that tumor necrosis factor could play an important part in the early activation of the hemostatic mechanism in septicemia.

Plant strategies of manipulating predator- prey interactions through allelochemicals: prospects for application in pest control

Abstract: To understand the role of allelochemicals in predator-prey interactions it is not sufficient to study the behavioral responses of predator and prey. One should elucidate the origin of the allelochemicals and be aware that it may be located at another trophic level. These aspects are reviewed for predator-prey interactions in general and illustrated in detail for interactions between predatory mites and herbivorous mites. In the latter system there is behavioral and chemical evidence for the involvement of the host plant in production of volatile allelochemicals upon damage by the herbivores with the consequence of attracting predators. These volatiles not only influence predator behavior, but also prey behavior and even the attractiveness of nearby plants to predators. Herbivorous mites disperse away from places with high concentrations of the volatiles, and undamaged plants attract more predators when previously exposed to volatiles from infested conspecific plants rather than from uninfested plants. The latter phenomenon may well be an example of plant-to-plant communication. The involvement of the host plant is probably not unique to the predator-herbivore-plant system under study. It may well be widespread since it makes sense from an evolutionary point of view. If so, prospects for application in pest control are wide open. These are discussed, and it is concluded that crop protection in the future should include tactics whereby man becomes an ally to plants in their strategies to manipulate predator-prey interactions through allelochemicals.

Nitrogen metabolism and ornithine cycle function

Abstract: Article de synthese sur le metabolisme de l'azote et la synthese d'uree chez les mammiferes. Degradation des proteines. Transport des acides-amines entre les organes et a travers la membrane plasmique des hepatocytes. Cycle de l'ornithine et distribution des enzymes de ce cycle dans le foie. Ontogenese de la synthese d'uree. Pathophysiologie de l'hyperammonemie

Anatomic organization and physiology of the limbic cortex.

Abstract: Ce rapport analyse l'organisation anatomique du systeme limbique, ses principales connexions avec les autres compartiments du cerveau et sa physiologie en rapport avec le comportement de l'animal

Amoeba: a distributed operating system for the 1990s

Abstract: A description is given of the Amoeba distributed operating system, which appears to users as a centralized system but has the speed, fault tolerance, security safeguards, and flexibility required for the 1990s. The Amoeba software is based on objects. Objects are managed by server processes and named using capabilities chosen randomly from a sparse name space. Amoeba has a unique, fast file system split into two parts: the bullet service stores immutable files contiguously on the disk; the directory service gives capabilities symbolic names and handles replication and atomicity, eliminating the need for a separate transaction management system. To bridge the gap with existing systems, Amoeba has a Unix emulation facility consisting of a library of Unix system call routines that make calls to the various Amoeba server processes. >

Detection and Typing of Human Papillomavirus in Archival Cervical Cancer Specimens by DNA Amplification With Consensus Primers

Abstract: We developed a polymerase chain reaction DNA amplification system using two distinct consensus oligonucleotide primer sets for the improved detection and typing of a broad spectrum of human genital papillomavirus (HPV) sequences, including those of novel viruses. The system incorporates one primer set designed to amplify a highly conserved L1 domain and a second primer set designed to amplify a domain within the E6 gene. We used this system to analyze 48 fixed, paraffin-embedded tissue sections (41 specimens from 33 cervical carcinomas, four normal cervical tissues, and several control tissues) for the presence of HPV DNA. HPV sequences were detected in all carcinoma samples and none of the control samples. Hybridization analyses showed that the results obtained with the two amplification schemes concurred completely. This approach allowed rapid confirmation of typing results and may improve the likelihood of detecting a wide variety of HPV sequences, including those of novel HPVs.

The Use of General Primers in the Polymerase Chain Reaction Permits the Detection of a Broad Spectrum of Human Papillomavirus Genotypes

Abstract: A novel polymerase chain reaction (PCR) method was developed that permits the detection of 11 different human papillomavirus (HPV) genotypes using two general primer sets. By computer-assisted sequence analysis, two pairs of general primers were selected from the conserved L1 open reading frame and tested in the PCR on a set of cloned HPV genotypes. Experimental analysis showed that up to three mismatches between primers and target DNA did not influence the efficiency of the assay. The use of these primers in the PCR enabled the detection of HPV genotypes HPV-1a, -6, -8, -11, -13, -16, -18, -30, -31, -32 and -33, and was also successfully applied to well characterized cervical carcinoma cell lines and clinical samples. For the HPV types tested sub-picogram amounts of cloned DNA could be detected after general primer-mediated PCR and subsequent hybridization. The specificity of the amplification products was confirmed by blot hybridization procedures and RsaI restriction enzyme digestion. The results indicate that this PCR method can be a powerful tool for identifying novel HPV genotypes in dysplasias and squamous cell carcinomas suspected of having an HPV aetiology.

Decomposition and nitrogen mineralization in natural and agro- ecosystems: the contribution of soil animals

Abstract: The present article centres on the contribution of soil animals to organic. matter decomposition and nitrogen mineralization in natural and agro-ecosystems. Criteria are presented for the categorisation of the soil fauna in functional groups in order to be able to quantify the contribution of the soil fauna. Three types of classifications: size, habitat and food, are discussed. For various natural ecosystems, such as prairies and forests, and for agro-ecosystems a rather similar outcome of the faunal contribution to nitrogen mobilization of approximately 30% appears to exist. This value is dependent on various types of interactions among functional groups, changes in population density of microorganisms and soil fauna, seasonally changing abiotic factors and management, such as fertilization, harvesting and addition of harvest residues to the soil. Finally, to improve management of ecosystems as related to soil faunal activity in decomposition, lines are set out for further research such as the development of dynamic models, studies concerning the effects of perturbation in relation to microbial dominance and the integration of the study of below-ground food webs with ecological theories.

High-quality recording of bioelectric events. Part 1. Interference reduction, theory and practice.

Abstract: In the first part of the review the various mechanisms that could be the cause of interference in bioelectric recordings are considered. It is demonstrated that the performance of a good amplifier can be seriously degraded in its functioning if the whole measurement situation is not taken into account. Several techniques used to reduce interference, of which guarding and driven right leg circuits are the most important, are analysed. In the second part of the review some examples of the application of the theory in practical situations are presented. The instrumentation amplifier circuit normally used in bioelectric recordings is improved for measurements under difficult circumstances. Another application is a low-cost 64-channel amplifier for multichannel ECG recordings. The third application is a device that can be added to bioelectric measurement systems and will provide a major reduction in interference.

Complete Marginal Seal of Class V Resin Composite Restorations Effected by Increased Flexibility

Abstract: The use of adhesive resin composites for restoration of Class V lesions often results in cervical marginal gap formation due to polymerization contraction. In this laboratory study, flexible linings applied between the bonding agent and the bulk restorative appeared to preserve the marginal integrity of Class V restorations during curing. Measurements of the polymerization contraction stress showed 20 to 50% relief due to various flexible intermediate materials. Apparently, reduction of the total stiffness by application of an intermediate layer can render the total restoration sufficiently flexible to compensate for that part of the stress that would otherwise exceed the bond strength.

De‐individuation and group polarization in computer‐mediated communication

Abstract: A computer-mediated communication system (CMCS) was used to explore the effects of de-individuation on group polarization. Reicher (1984) argued that de-individuating members of a group should increase the salience of group identity and hence normative behaviour, while de-individuating subjects treated as individuals should have the reverse effect. We extended this idea to the group polarization paradigm and in addition independently manipulated group salience and de-individuation, which were confounded factors in Reicher's study. It was reasoned that the visual anonymity created by isolating discussants in separate rooms would be de-individuating compared to seating them together in the same room. At the same time either the subject's group or individual identity was made salient. A computer-mediated communication system provided text-based communication for discussants in all four conditions. Assuming that group polarization reflects conformity to a group norm (Turner, Hogg, Oakes, Reicher & Wetherell, 1987), we predicted an interaction between the de-individuation and group salience factors, such that greatest polarization in the direction of a pre-established group norm would be obtained in the de-individuated—group condition and least in the de-individuated—individual condition. This prediction was confirmed. Explanations of the findings in terms of Reicher's earlier study and in terms of self-attention processes are considered within the general framework of social identity theory. Finally, the relevance of this research to the realm of human communication via computer networks is evaluated.

Projection from the nucleus reuniens thalami to the hippocampal region: Light and electron microscopic tracing study in the rat with the anterograde tracer Phaseolus vulgaris‐leucoagglutinin

Abstract: In order to study the morphological substrate of possible thalamic influence on the cells of origin and area of termination of the projection from the entorhinal cortex to the hippocampal formation, we examined the pathways, terminal distribution, and ultrastructure of the innervation of the hippocampal formation and parahippocampal region by the nucleus reuniens of the thalamus (NRT). We employed anterograde tracing with Phaseolus vulgaris-leucoagglutinin (PHA-L). Injections of PHA-L in the NRT produce fiber and terminal labeling in the stratum lacunosum-moleculare of field CAI of the hippocampus, the molecular layer of the subiculum, layers I and III/IV of the dorsal subdivision of the lateral entorhinal area (DLEA), and layers I and 111-VI of the ventral lateral (VLEA) and medial (MEA) divisions of the entorhinal cortex. Terminal labeling is most dense in the stratum lacunosum-moleculare of field CA1, the molecular layer of the ventral part of the subiculum, MEA, and layer I of the perirhinal cortex. In layer I of the caudal part of DLEA and in MEA, terminal labeling is present in clusters. Injections in the rostral half of the NRT produce the same distribution in the hippocampal region as those in the caudal half of the NRT, although the projections from the rostral half of the NRT are much stronger. A topographical organization is present in the projections from the head of the NRT, so that the dorsal part projects predominantly to dorsal parts of field CA1 and the subiculum and to lateral parts of the entorhinal cortex, whereas the ventral part projects in greatest volume to ventral parts of field CA1 and the subiculum and to medial parts of the entorhinal cortex. The distribution of the reuniens fibers coursing in the cingulate bundle was determined by comparing cases with and without transections of this bundle. The fibers carried by the cingulate bundle exclusively innervate field CA1 of the hippocampus, the dorsal part of the subiculum, and the presubiculum and parasubiculum. They participate in the innervation of the ventral part of the subiculum and MEA. Electron microscopy was used to visualize the axon terminals of PHA-L-labeled reuniens fibers. These terminals possess spherical synaptic vesicles and form asymmetric synaptic contacts with dendritic spines or with thin shafts of spinous dendrites. Following lesions of the cingulate bundle, large numbers of degenerating axon terminals are present in MEA. Scattered degenerating axon terminals are visible in the areas that in the light microscopic tracinghesion experiments are depleted of PHA-L-labeled fibers, i.e., field CA1 of the hippocampus, the dorsal part of the subiculum, and MEA. Also, the molecular layer of the ventral part of the subiculum contains degenerating terminals. The results of the present study suggest that the NRT may simultaneously influence the parent cell bodies of the perforant pathway in the entorhinal cortex and the target neurons of this pathway in field CA1 and the subiculum of the hippocampal formation. The perforant

Experiences with the Amoeba distributed operating system

Abstract: The Amoeba project is a research effort aimed at understanding how to connect multiple computers in a seamless way [16, 17, 26, 27, 31]. The basic idea is to provide the users with the illusion of a single powerful timesharing system, when, in fact, the system is implemented on a collection of machines, potentially distributed among several countries. This research has led to the design and implementation of the Amoeba distributed operating system, which is being used as a prototype and vehicle for further research. In this article we will describe the current state of the system (Amoeba 4.0), and show some of the lessons we have learned designing and using it over the past eight years. We will also discuss how this experience has influenced our plans for the next version, Amoeba 5.0.Amoeba was originally designed and implemented at the Vrije Universiteit in Amsterdam, and is now being jointly developed there and at the Centrum voor Wiskunde en Informatica, also in Amsterdam. The chief goal of this work is to build a distributed system that is transparent to the users. This concept can best be illustrated by contrasting it with a network operating system, in which each machine retains its own identity. With a network operating system, each user logs into one specific machine—his home machine. When a program is started, it executes on the home machine, unless the user gives an explicit command to run it elsewhere. Similarly, files are local unless a remote file system is explicitly mounted or files are explicitly copied. In short, the user is clearly aware that multiple independent computers exist, and must deal with them explicitly.In contrast, users effectively log into a transparent distributed system as a whole, rather than to any specific machine. When a program is run, the system—not the user—decides upon the best place to run it. The user is not even aware of this choice. Finally, there is a single, system-wide file system. The files in a single directory may be located on different machines, possibly in different countries. There is no concept of file transfer, uploading or downloading from servers, or mounting remote file systems. A file's position in the directory hierarchy has no relation to its location.The remainder of this article will describe Amoeba and the lessons we have learned from building it. In the next section, we will give a technical overview of Amoeba as it currently stands. Since Amoeba uses the client-server model, we will then describe some of the more important servers that have been implemented so far. This is followed by a description of how wide-area networks are handled. Then we will discuss a number of applications that run on Amoeba. Measurements have shown Amoeba to be fast, so we will present some of our data. After that, we will discuss the successes and failures we have encountered, so that others may profit from those ideas that have worked out well and avoid those that have not. Finally we conclude with a very brief comparison between Amoeba and other systems.Before describing the software, however, it is worth saying something about the system architecture on which Amoeba runs.

Nerves and nerve plexuses of the human vertebral column

Abstract: The origin, distribution, and termination pattern of nerves supplying the vertebral column and its associated structures have been studied in the human fetus by means of an acetylcholinesterase whole-mount method. The vertebral column is surrounded by ventral and dorsal nerve plexuses which are interconnected. The ventral nerve plexus consists of the nerve plexus associated with the anterior longitudinal ligament. This longitudinally oriented nerve plexus has a bilateral supply from many small branches of the sympathetic trunk, rami communicantes, and perivascular nerve plexuses of segmental arteries. In the thoracic region, the ventral nerve plexus also is connected to the nerve plexuses of costovertebral joints. The dorsal nerve plexus is made up of the nerve plexus associated with the posterior longitudinal ligament. This nerve plexus is more irregular and receives contributions only from the sinu-vertebral nerves. The sinu-vertebral nerves originate from the rami communicantes and, in the cervical region, also from the nerve plexus of the vertebral artery. Thick and thin sinu-vertebral nerves are found. Most frequently three types of thick sinu-vertebral nerves are observed, i.e., ascending, descending, or dichotomizing ones. Finally, the distribution of the branches of the ventral and dorsal nerve plexuses and of the sinu-vertebral nerves is described.

Peroxisomal protein import is conserved between yeast, plants, insects and mammals.

Abstract: We have previously demonstrated that firefly luciferase can be imported into peroxisomes of both insect and mammalian cells. To determine whether the process of protein transport into the peroxisome is functionally similar in more widely divergent eukaryotes, the cDNA encoding firefly luciferase was expressed in both yeast and plant cells. Luciferase was translocated into peroxisomes in each type of organism. Experiments were also performed to determine whether a yeast peroxisomal protein could be transported to peroxisomes in mammalian cells. We observed that a C-terminal segment of the yeast (Candida boidinii) peroxisomal protein PMP20 could act as a peroxisomal targeting signal in mammalian cells. These results suggest that at least one mechanism of protein translocation into peroxisomes has been conserved throughout eukaryotic evolution.

Tumor Necrosis Factor: A Putative Mediator of the Sick Euthyroid Syndrome in Man

Abstract: Tumor necrosis factor-alpha (TNF) is believed to be an important mediator in many diseases that are associated with the sick euthyroid syndrome. To investigate the effect of TNF on thyroid hormone metabolism, we performed a controlled study in six healthy postabsorptive males, in whom plasma thyroid hormones and TSH were sequentially measured after iv bolus injections of recombinant human TNF (50 micrograms/m2) and isotonic saline. During the 10.5-h study TNF produced the characteristic changes in circulating thyroid hormones and TSH observed in the sick euthyroid syndrome. Compared with the control experiment, TNF induced significant decreases in T3 (-36 +/- 2%; saline, -20 +/- 3%; P less than 0.05) and TSH levels (-68 +/- 3%; saline, -44 +/- 8%; P less than 0.05) and a significant increase in rT3 values (+48 +/- 11%; saline, -12 +/- 7%; P less than 0.05). T4 and free T4 index were not affected by TNF. Free T4 showed a mean transient increase of 18% in five subjects (nonsignificant), which occurred synchronically with a transient 3.5-fold rise in circulating FFA levels. Our results suggest that TNF is involved, either directly or indirectly, in the pathogenesis of the sick euthyroid syndrome.

Cloning and sequence analysis of beta‐4 cDNA: an integrin subunit that contains a unique 118 kd cytoplasmic domain.

Abstract: The alpha 6 beta 4 complex is a member of the integrin superfamily of adhesion receptors. A human keratinocyte lambda gt11 cDNA library was screened using a monoclonal antibody directed against the beta 4 subunit. Two cDNAs were selected and subsequently used to isolate a complete set of overlapping cDNA clones. The beta 4 subunit consists of 1778 amino acids with a 683 amino acid extracellular domain, a 23 amino acid transmembrane domain and an exceptionally long cytoplasmic domain of 1072 residues. The deduced amino-terminal sequence is in good agreement with the published amino-terminal sequence of purified beta 4. The extracellular domain contains five potential N-linked glycosylation sites and four cysteine-rich homologous repeat sequences. The extracellular part of the beta 4 subunit sequence shows 35% identify with other integrin beta subunits, but is the most different among this class of molecules. The transmembrane region is poorly conserved, whereas the cytoplasmic domain shows no substantial identity in any region to the cytoplasmic tails of the known beta sequences or to other protein sequences. The exceptionally long cytoplasmic domain suggests distinct interactions of the beta 4 subunit with cytoplasmic proteins.

Consecutive inactivation of both alleles of the pim-1 proto-oncogene by homologous recombination in embryonic stem cells

Abstract: Specific genes can be inactivated or mutated in the mouse germ line. The phenotypic consequences of the mutation can provide pivotal information on the function of the gene in development and maintenance of the mammalian organism. The procedure entails homologous recombination in embryonic stem cells, which, on fusion to recipient blastocysts, give rise to chimaeric mice that can transmit the mutant gene to their offspring. Inbreeding can then yield mice carrying the mutation in both alleles allowing the phenotypic analysis of recessive mutations. In addition to mice lacking a particular gene function, cell lines carrying null alleles of normally expressed genes can be instrumental in assessing the function of the gene. These cell lines can either be obtained from homozygous animals or, should the mutation be lethal early in embryonic development, be generated by consecutive inactivation of both alleles by homologous recombination in cultured cells. Here we illustrate the feasibility of this latter approach by the efficient consecutive inactivation of both alleles of the pim-1 proto-oncogene in embryonic stem cells.

Open Problems in Topology

Abstract: This is a cumulative status report on the 1100 problems listed in the volume Open Problems in Topology (North-Holland, 1990), edited by J. van Mill and G.M. Reed.

Rapid detection of human papillomavirus in cervical scrapes by combined general primer-mediated and type-specific polymerase chain reaction.

Abstract: A two-step polymerase chain reaction (PCR) procedure was used as a new screening strategy for the detection of human papillomavirus (HPV) genotypes in cervical scrapes omitting prior DNA extraction. Sample preparation consisted of a freeze-thaw step followed by boiling the cells before the PCR mixture was added. This pretreatment was as efficient and reproducible for HPV DNA amplification as DNA purification. By using crude cell suspensions, a prescreening of the samples with the general primer-mediated PCR method (GP-PCR) was performed to detect a broad spectrum of sequenced and still unsequenced HPV types at the subpicogram level. HPV-containing scrapes by GP-PCR were subjected to HPV 6, 11, 16, 18, 31, and 33 type-specific PCR (TS-PCR) to identify the sequenced HPV types. This direct GP/TS-PCR method was tested on a large group of cervical scrapes (n = 459) from women visiting a gynecologic outpatient clinic. The results were compared with HPV data obtained by a method using modified filter in situ hybridization and TS-PCR in which the PCR was mainly used to confirm HPV positivity. A substantially higher HPV prevalence rate was found by direct GP/TS-PCR strategy. The results indicate that GP/TS-PCR is a rapid, sensitive, and reliable detection method for HPV in cervical scrapes. The easy performance on crude cell suspensions makes this strategy applicable for large HPV-screening programs.

The hydrological response of soil surfaces to rainfall as affected by cover and position of rock fragments in the top layer

Abstract: Rainfall experiments have been conducted in the laboratory in order to assess the hydrological response of top soils very susceptible to surface sealing and containing rock fragments in different positions with respect to the soil surface. For a given cover level, rock fragment position in the top soil has an ambivalent effect on water intake and runoff generation. Compared to a bare soil surface rock fragments increase water intake rates as well as time of runoff concentration and decrease runoff volume if they rest on the soil surface. For the same cover level, rock fragments reduce infiltration rate and enhance runoff generation if they are well embedded in the top layer. The effects of rock fragment position on infiltration rate and runoff generation are proportional to cover percentage. Micromorphological analysis and measurements of the saturated hydraulic conductivity of bare top soils and of the top layer underneath rock fragments resting on the soil surface reveal significant differences supporting the mechanism proposed by Poesen (1986): i.e. runoff generated as rock flow or as Horton overland flow can (partly) infiltrate into the unsealed soil surface under the rock fragments, provided that they are not completely embedded in the top layer. Hence, rock fragment position, beside other rock fragment properties, should be taken into account when assessing the hydrological response of soils susceptible to surface sealing and containing rock fragments in their surface layers. A simple model, based on the proportions of bare soil surface, soil surface occupied by embedded rock fragments, and soil surface covered with rock fragments resting on the soil surface, describes the runoff coefficient data relatively well.