Showing papers by "National Autonomous University of Mexico published in 1990"

Site response, attenuation and source spectra of S waves along the Guerrero, Mexico, subduction zone

Abstract: Acceleration spectra from nine earthquakes and up to 11 stations per earthquake are inverted to find the source spectra, the site response, and the quality factor Q of S waves. This method is applied to moderate earthquakes (4.0 0.8 Hz does not follow a simple linear relation as observed in previous studies of attenuation in other regions. When the source spectra are interpreted in terms of Brune's model, the stress drops obtained are less than 20 bars for the four larger events analyzed. The site functions obtained indicate that although all the stations used are on rock, the effect of the near surface geology caused important amplifications at some stations.

Superior hypogastric plexus block for pelvic cancer pain.

Abstract: Blockade of the superior hypogastric nerve plexus was performed for relief of chronic cancer related pelvic pain. The targeted sympathetic nerves lie anterior to the sacral promontory. Twenty-eight patients with neoplastic involvement of pelvic viscera secondary to cervical, prostate, and testicular cancer or radiation injury were treated with neurolytic superior hypogastric plexus block. Sympathetically mediated pain was significantly reduced or eliminated in all cases and no serious complications occurred. Superior hypogastric plexus block is recommended for diagnostic/prognostic and therapeutic purposes in patients with chronic pelvic pain, particularly when pain is of neoplastic origin.

Stellar jets with intrinsically variable sources

Abstract: Models for high Mach number, isothermal, pressure-matched jets ejected from sources with variable velocities are presented. It is found that the relaxation of the assumption of a time-independent source allows the complex position-velocity diagrams and multiple bow shock structures observed in some stellar jets to be explained in a straightforward way. Analytic and numerical models are presented for jets with variable velocity sources. A comparison with previously published observations of the HH 46/47 jet is performed which shows how the velocity variations observed along a jet can be used to reconstruct the past time variability of the source. This technique is promising for future studies of the mechanism by which stellar jets are produced. 41 refs.

On the formation and expansion of H II regions

Abstract: The evolution of H II regions in spherical clouds with small, constant-density cores and power-law density distributions r exp -w outside the core is described analytically. It is found that there is a critical exponent above which the cloud becomes completely ionized. Its value in the formation phase depends on the initial conditions, but it has a well-defined value w(crit) = 3/2 during the expansion phase. For w less than w(crit), the radius of the H II region grows at a given rate, while neutral mass accumulates in the interphase between the ionization and shock fronts. For w = w(crit), the fronts move together without mass accumulation. Cases with w greater than w(crit) lead to the champagne phase: once the cloud is fully ionized, the expansion becomes supersonic. For self-gravitating disks without magnetic fields, the main features include a new 'variable-size' stage. The initial shape of the H II region has a critical point beyond which the disk becomes completely ionized.

Seed bank versus seed rain in the regeneration of a tropical pioneer tree.

Abstract: We used the tropical pioneer tree, Cecropia obtusifolia to evaluate the relative importance of different sources of seeds in the regeneration of species that depend on ephemeral sites. We studied seed production in a population established in a 5 ha plot, and dispersal, dormancy and seed predation in two recent treefall gaps ( 35 years since disturbed) for a one year period at Los Tuxtlas (Mexico). Flowers and fruits were counted at monthly intervals. Annual fecundity per tree ranged from 1.4×104 to 1.4×107 seeds. Seeds were continuously available on the trees and on the ground. Average annual seed rain per m2 (as measured by 0.5×0.5 m seed traps) varied from 184 to 1925 among the six sites. Distance to nearest seed source and patch type explained more than 60% of the seed rain variation among sites. Soil seed density, estimated by counting seeds from ten samples (78.5 cm2×10 cm deep) collected from each site in October and January, ranged among the six sites from 269 to 4485 seeds per m2 in January and from 204 to 5073 in October. Soil seed viabilities were much lower (17.1% in October and 5.1% in January) than those of rain seeds (48.26%). Annual survivorships of 2.2% were estimated for seeds artificially sown on the soil surface of a gap and a mature patch, and 3.75% in a building patch. In two other experiments seed removal rates ranged from 27% to 98% in 4 days. Removal rates were significantly higher in gap and mature patches than in building patches. Ants (Paratrechina vividula) and grasshopper nymphs (Hygronemobius. sp.) were the main predators. We draw three main conclusions from our data: (1) Pathogens and predators determine low survivorship of C. obtusifolia's seeds in the soil and a rapid turnover rate (1.07 to 1.02 years) of its seed bank; (2) a continuous and copious seed production and an abundant and extensive seed rain replenish the soil seed pool in patches with different disturbance ages at least up to 86 m from nearest source; (3) more than 90% of the seeds contributing to C. obtusifolia seedling recruitment in gaps are less than one year-old. We discuss our results in the context of previous similar studies for tropical forests.

Contemporary Neotropical Defaunation and Forest Structure, Function, and Diversity—A Sequel to John Terborgh*

Abstract: A series of long-term studies on the demography of tree seedlings and patterns of herbivory in the forest understory have been carried out at the Los Tuxtlas Tropical Research Station (southern Veracruz, Mexico) (see de la Cruz & Dirzo 1987; Dirzo 1984, 1987; Dirzo & Miranda 1990; Niiuiez-Farfan & Dirzo 1988). As a result of these studies, a number of features of this forest's understory became evident which, we believe, support Terborgh's contention (1988) that the "big [animal] things" of the tropical forest play an important though largely unappreciated role in the diversity and structure of these natural systems. Such features of the forest understory at Los Tuxtlas include the following (1) A consistently low or total absence of damage by folivorous vertebrates. Practically none of the seedlings, which were individually marked and monitored for several years, show evidence of damage, other than by invertebrates (insects in several orders). Moreover, none of our permanent plots has shown damage by trampling, which is characteristic of other areas of tropical Mexico in which tapirs, white-

Volume-sensitive release of taurine from cultured astrocytes: properties and mechanism.

Abstract: Release of taurine in response to cell swelling induced by hyposmolarity was observed in cultured astrocytes. Efflux of 3H-taurine increased by 30% and 70% upon reductions in osmolarity of only 5% and 10%. Reductions in osmolarity of 20%, 30%, and 50% stimulated basal taurine release by 300%, 500%, and 1,500%, respectively. The properties of this volume-sensitive release of taurine were examined to investigate: 1) its association with K+ and Cl- fluxes, currently activated during volume regulation: 2) its relationship with Ca2(+)-dependent reactions; and 3) the mechanism of the taurine efflux process. Taurine release was unaffected by removal of Na+, Ca2+, or Cl-, by pimozide and trifluoperazine, or by agents disrupting the cytoskeleton. The K+ channel inhibitors barium, quinidine, tetraethylammonium, and gadolinium had no effect. Taurine release was reduced by furosemide, a blocker of K+/Cl- cotransport, but not by the more specific inhibitor, bumetanide. It was markedly reduced by the inhibitors of Cl- channels DIDS, SITS, and anthracene-9-carboxylate. Taurine efflux was pH-dependent, being reduced at low pH values. It was decreased at 4 degrees C but not at 14 degrees C or 20 degrees C. These results suggest that the volume-sensitive release of taurine is independent of K+ fluxes but may be associated with Cl- conductances. It also seems unrelated to Ca2(+)-dependent transduction mechanisms. The Na(+)-dependent taurine carrier apparently is not involved in the swelling-induced release process.

Geometry of subduction and depth of the seismogenic zone in the Guerrero gap, Mexico

Abstract: THE western coast of Guerrero in southern Mexico has been identified as a seismic gap on the Middle American Trench in which no large earthquakes have occurred at least since 1908. It has been suggested that the seismic energy accumulated since 1908 will eventually be released by a large earthquake. A permanent seismic network was installed to monitor the seismicity of this mature seismic gap and to understand the geometry of the subducted slab beneath this region. The seismicity defines an unusual distribution along two bands of seismic activity parallel to the coast. The resulting geometry of the subduction zone shows that the Cocos plate dips at a shallow angle beneath the North American plate to a depth of ∼40 km; from there the subducted slab is bent upward, following a subhorizontal trajectory extending inland at a depth of ∼50 km. This plate geometry is reminiscent of that found in Peru and central Argentina, two other regions where a young oceanic plate is being subducted. In Mexico, however, the slab underplates an overriding plate which is only half as thick as that observed in South America. A possible oceanic origin of the allochtonous terranes comprising southern Mexico may explain the presence of the anomalously thin lithosphere in this region.

The origin and early evolution of life on earth

Abstract: Results of the studies that have provided insights into the cosmic and primitive earth environments are reviewed with emphasis on those environments in which life is thought to have originated. The evidence bearing on the antiquity of life on the earth and the prebiotic significance of organic compounds found in interstellar clouds and in primitive solar-system bodies such as comets, dark asteroids, and carbonaceous chondrites are assessed. The environmental models of the Hadean and early Archean earth are discussed, as well as the prebiotic formation of organic monomers and polymers essential to life. The processes that may have led to the appearance in the Archean of the first cells are considered, and possible effects of these processes on the early steps of biological evolution are analyzed. The significance of these results to the study of the distribution of life in the universe is evaluated.

Role of oxygen in PrBa2Cu3O7-y: Effect on structural and physical properties.

Abstract: ${\mathrm{PrBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathit{y}}$ is the only homomorphic member of the entire rare-earth 1:2:3 family which is insulating and not metallic or superconducting. With this unusual behavior in mind, and with an eye to resolving certain questions regarding the widely held notion of otherwise mobile holes held captive in a supratrivalent Pr ionic state as its cause, we examined the effect of varying oxygen concentration on the structural, electrical, and magnetic properties of this compound. Powder x-ray diffraction studies revealed an orthorhombic-to-tetragonal order-disorder transition at oxygen levels close to those found for the superconducting lanthanide compounds. From Rietveld-refined powder-neutron-diffraction data taken on a fully oxygenated sample, key insights into the ambivalent character of Pr valency were obtained. Other aspects of Pr valency and carrier dynamics were uncovered by electrical and magnetic measurements as a function of y. We found that the room-temperature resistivity increases by about three orders of magnitude from y\ensuremath{\simeq}0 to y\ensuremath{\simeq}0.5, while the temperature dependence of the paramagnetic susceptibility remains quantitatively the same. We conclude from these results that removal of oxygen from ${\mathrm{PrBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathit{y}}$ does not substantially alter the valence state of Pr, which we assert to be nominally 3+, but with strong overlap of its outer-lying 4f orbital with neighboring oxygen 2p levels results in a characteristically mixed or fluctuating valence situation reminiscent of heavy-fermion systems, and the principal effect on transport properties is on carrier concentration in the chains and steric hindrance of their motion by the resulting oxygen vacancies.

The Study of Activities in Classic Households: Two Case Studies from Coba and Teotihuacan

Abstract: Mesoamerican household sites were often deserted gradually, leaving very little de facto refuse for analysis. Starting with archaeological and paleobiological distribution patterns, this study adds a new dimension to the spatial study of two Classic households by plotting chemical concentrations in probable activity areas. The study of activity areas and domestic structures has become a fertile field of archaeological research. Yet, in some cases, this boom has also led to careless analyses of data. Though drawn from examinations of household structure, conclusions about surface distributions of artifacts and domestic mounds do not often rest on firm functional, contextual, social, or chronological evidence. Suppositions about elite versus common items, residential versus storage or cult architecture, have yet to be tested rigorously. Worse still, the archaeological literature consistently confuses refuse areas with workshops, common domestic structures with high status residences, and administrative sectors with residential areas. The result: small-scale excavations of floors have provided a voluminous number of untested and debatable hypotheses concerning household size, the degree of cooperation between families, and the developmental cycle of the unit, without, however, benefiting from a full understanding of the activity repertoire, its spatial distribution, the functions of particular structures, and the spatial limits of domestic

Mangrove ecology, aquatic primary productivity, and fish community dynamics in the Teacapán-Agua Brava Lagoon-Estuarine system (Mexican Pacific)

Abstract: Aquatic primary productivity, mangrove ecology, and fish community dynamics were investigated in the Teacapan-Agua Brava lagoon-estuarine system, the most extensive mangrove ecosystem on the Pacific coast of Mexico with three species of mangroves distributed heterogeneously (Laguncularia racemosa, Rhizophora mangle, andAvicennia germinans). Tree density was 3,203 trees ha−1 and basal area was 14.0 m2 ha−1. Litterfall was 1,417 g m−2 yr−1, characteristic of a productive riverine forest. The degradation constant forLaguncularia racemosa leaves varied from 1.71 to 4.7 yr−1 and mean annual net aquatic productivity was 0.41 g C m−3 d−1. There were high concentrations of humic substances (up to 150 mg l−1) early in the wet season. Seasonal variations of the above parameters seemed closely related to the ecology of fish populations. There were 75 fish species distributed in two principal assemblages associated with wet and dry seasons. Diversity and biomass analysis indicated 18 dominant species. Total biomass of the community in this coastal system was estimated at 10 g wet wt m−2. The highest biomass occurred in the wet season. The most common fish species wereMugil curema, Achirus mazatlanus, Galeichthys caerulescens, Arius liropus, Diapterus peruvianus, Lile stolifera, Centropomus robalito, andEucinostomus sp., all of which have fishery importance. Primary productivity and fish community ecology are controlled by habitat characteristics, river discharge, and climatic seasonality.

Immunodiagnosis of human cysticercosis in cerebrospinal fluid. Antigens from murine Taenia crassiceps cysticerci effectively substitute those from porcine Taenia solium.

Abstract: Tapeworm antigens from Taenia crassiceps performed as well as those antigens from Taenia solium in an enzyme-linked immunosorbent assay for the detection of Cysticercus antibodies in 96 cerebrospinal fluid samples from patients with neurocysticercosis and in 96 CSF samples from patients with other varied neurological ailments. Thus, this manageable murine model of experimental cysticercosis solved the problem of antigen supply for clinical and epidemiological applications, and it provided an immediate means of abundant production of antigens for the wide distribution and standardization of immunodiagnostic tests for cysticercosis.

Determination of the sensitive stages for gonadal sex-reversal in Xenopus laevis tadpoles.

Abstract: The response of developing gonads of the clawed toad Xenopus laevis tadpoles to estradiol benzoate (EB) was studied between stages 44 and 67 using high resolution techniques. In presumptive genetic males the following results were obtained: 1) 100% sex reversal was induced when EB was administered before translocation of primordial germ cells (PGCs) from the gonadal epithelium into the medullary region (stages 44-50). 2) Ambiguous gonads were formed when EB treatment was initiated at stages 51-54, when PGCs were migrating into the medullary region. 3) Finally, normal testes differentiated when EB treatment began after the primordial germ cells had completed their translocation into the medulla (stages 55-56). These results suggest that EB might induce sex-reversal in genetic males by disruption of early somatic-germ cell interactions in the medullary region of the gonad. Consequently, later morphogenetic events might be deranged, preventing differentiation of testis. We propose a hypothesis in which precocious production of estradiol (E2) by genotypic females is the mechanism for primary sex differentiation.

Correlation Between Eeg Spectral Parameters and an Educational Evaluation

Abstract: EEG spectral parameters were computed in a group of children with different degrees of difficulty in learning to read and write. For statistical analyses, Z-transformed values according to normative age-regression equations were used to control the age effects. Canonical Correlation Analysis between absolute power in different bands and the categories of the educational evaluation (good, regular, poor and very poor) showed that more delta was probably related to a poor evaluation and more alpha in occipital areas to a good one. MONOVA also showed highly significant differences in the absolute power in many leads between children with different evaluations. As children with a poor evaluation very frequently had antecedents of risk factors related to brain damage and were from a low socioeconomic status, and both factors have been shown to affect absolute power, it may be that the differences observed were due to these causes. However, relative power correlated more with the learning problems. Children with...

Ecological significance of light controlled seed germination in two contrasting tropical habitats

Abstract: The effects of temperature, photoperiod, phytochrome photoreversion and the response to a R/FR ratio gradient were investigated in seeds of four species from two contrasting tropical habitats; two species from a rain forest (Cecropia obtusifolia and Piper umbellatum) and two from a high altitude lava field covered by low vegetation (Buddleja cordata and Chenopodium ambrosioides). In the rain forest seed species the photoblastic response seems to be adapted to light quality changes due to canopy destruction, on the other hand, the lava field seed species seem to be adapted to instantaneous light stimulus such as would be produced by the sudden exposure of a buried seed to the soil surface light environment.

Production of fine roots and the seasonality of their growth in a Mexican deciduous dry forest

Abstract: Very limited information regarding fine-root growth and production of tropical dry forests is available. Fine roots and small roots are defined as rootlets with diameters < 1 mm and 1.1 to 5 mm, respectively. Live and dead fine-and small-root mass fluctuations were studied over one year by means of soil core analyses in the deciduous dry forest of Chamela, Mexico, at 19° 30′, 2 km inland from the Pacific Ocean. By means of systematically varying the distance of soil core extraction points from tree stems, it was shown that random core collection is justified. A difference between fine-root biomass on south and north facing slopes was documented, although this difference was significant only during the rainy season. The live/dead ratio of fine roots was highest during the rainy period. The annual fine-root production for 1989 was estimated at 4.23 Mg ha-1 by summing significant fine-root biomass changes between sampling dates. This value is higher than most of the comparable data from other ecosystems.

Salmonella typhimurium Penetration through the Eggshell of Hatching Eggs

Abstract: This study was undertaken to demonstrate penetration of Salmonella typhimurium through the eggshell of newly laid broiler hatching eggs. Eggs were challenged either by lightly spraying the bacteria over the blunt end of the egg or by contact with contaminated dry nest litter. Exposure time for both groups was 10 minutes; afterward, all eggs were disinfected and incubated 19 days under normal conditions. Chorioallantoic membranes and yolk sacs were cultured in brain-heart infusion broth on day 19 to demonstrate penetration. Isolation of the bacteria from chorioallantoic membranes and yolk sacs, respectively, were as follows: sprayed group: 100% and 83%; contact group: 59% and 29%. These results showed that although water enhanced S. typhimurium penetration, its presence on the eggshell is not essential for penetration to occur.

Ordering of latex particles during film formation

Abstract: Small angle neutron scattering and electron microscopy are used to investigate the ordering of soft latex particles which have been dispersed in water and then brought into contact through evaporation of the solvent. We observe a succession of metastable states where the particles are kept separated by their hydrophilic surface layers: colloidal liquid (short range order only) to colloidal crystal (long range order) to foam (dense packing of polyhedral cells with latex interior and hydrophilic walls). At each stage the structure may collapse into a homogeneous latex phase through coalescence of the particles and expulsion of the hydrophilic material.

Constitutive exo-pectinase produced by Aspergillus sp. CH-Y-1043 on different carbon source.

Abstract: The production of a constitutive exo-pectinase byAspergillus sp. CH-Y-1043 grown on glucose, sucrose, fructose, glycerol and galacturonic acid is reported. The specific activity was found to be in the range of 26% to 75% of that produced with pectin or poly-galacturonic acid. The production of this exo-pectinase is strictly correlated to the exponential growth phase and it is highly sensitive to the pH of the culture medium

Characterization by XPS and XAS of supported Pt/TiO2CeO2 catalysts

Abstract: TiO2 and CeO2 are will known for inducing strong metal-support interactions J(SMSI) of a different nature when used as supports for Pt catalysts. X-ray photoelectron spectroscopy analysis of mixtures of TiO2CeO2 oxide supports (80:20 and 20:80) reveals at the surface a remarkable stabilization of cerium in the +3 oxidation state; the stabilization is more pronounced with lower cerium oxide concentration. X-ray absorption spectroscopy measurements done on the LIII edge show the same variation as a function of cerium oxide concentration, but in the bulk the reduction is much lower. Cerium in Pt/TiO2CeO2 catalysts exhibits the same behaviour.