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Biomedical subjects

B Chavez

Publications and source records attributed to B Chavez.

14 recordsLinked to original sources
PubMed2001

Plasmodium vivax: ookinete destruction and oocyst development arrest are responsible for Anopheles albimanus resistance to circumsporozoite phenotype VK247 parasites.

Anopheles albimanus and An. pseudopunctipennis differ in their susceptibilities to Plasmodium vivax circumsporozoite phenotypes. An. pseudopunctipennis is susceptible to phenotype VK247 but almost refractory to VK210. In contrast, An. albimanus is almost refractory to VK247 but susceptible to VK210. To investigate the site in the mosquito and the parasite stage at which resistance mechanisms affect VK247 development in An. albimanus, parasite development was followed in a series of experiments in which both mosquitoes species were simultaneously infected with blood from patients. Parasite phenotype was determined in mature oocysts and salivary gland sporozoites by use of immunofluorescence and Western blot assays and/or gene identification. Ookinete maturation and their densities within the bloodmeal bolus were similar in both mosquito species. Ookinete densities on the internal midgut surface of An. albimanus were 4.7 times higher than those in An. pseudopunctipennis; however, the densities of developing oocysts on the external midgut surface were 6.12 times higher in the latter species. Electron microscopy observation of ookinetes in An. albimanus midgut epithelium indicated severe parasite damage. These results indicate that P. vivax VK247 parasites are destroyed at different parasite stages during migration in An. albimanus midguts. A portion, accumulated on the internal midgut surface, is probably destroyed by the mosquito's digestive enzymes and another portion is most likely destroyed by mosquito defense molecules within the midgut epithelium. A third group, reaching the external midgut surface, initiates oocyst development, but over 90% of them interrupt their development and die. The identification of mechanisms that participate in parasite destruction could provide new elements to construct transgenic mosquitoes resistant to malaria parasites.

Animals↗
PubMed2000

Sex steroids modify working memory.

In the last ten years, numerous mechanisms by which sex steroids modify cortical function have been described. For example, estrogen replacement improves verbal memory in women, and animal studies have shown effects of estrogen on hippocampal synaptogenesis and function. Little is known about sex steroid effects on other aspects of memory, such as frontal lobe-mediated working memory. We examined the relationships between working memory and sex steroid concentrations and whether sex steroid supplementation would modify age-related loss of working memory in older men and women. Before hormone supplementation, working memory, tested with the Subject Ordered Pointing Test (SOP), was worse in older subjects than younger subjects, and there was no evidence of gender differences at either age. Testosterone supplementation improved working memory in older men, but a similar enhancement of working memory was not found in older women supplemented with estrogen. In men, testosterone and estrogen effects were reciprocal - with better working memory related to a higher testosterone to estrogen ratio. These results suggest that sex steroids can modulate working memory in men and can act as modulators of cognition throughout life.

Adult↗
PubMed1999

Description and morphometric analysis of the eggs of Anopheles (Anopheles) vestitipennis (Diptera: Culcidae) from southern Mexico.

Light and scanning electron microscopy were used to compare the eggs of Anopheles vestitipennis Dyar & Knab females collected from human and animal baits in 9 villages of southern Mexico. An. vestitipennis eggs are boat-shaped, with lateral floats extending the length of the egg. Both the deck and dorsal surface are covered with hexagonal and pentagonal chorionic cells that contain round tubercles in the cell field. Crowns that enclose 3-5 lobed tubercles are present at both egg poles. By light microscopy, the mean length/width ratio of eggs of females caught at human bait were statistically different from those of females caught in horse-baited animal traps. In a regression tree model that included 19 egg attributes, the same egg groups could be separated by their posterior crown length/width ratio and mean anterior cell deck form factor (an index of their roundness). These findings support of the possible existence of 2 An. vestitipennis subpopulations with different host preferences in southern Mexico.

Animals↗
PubMed1998

Effects of toxin A from Clostridium difficile on mast cell activation and survival.

Toxins A and B from Clostridium difficile are the main cause of antibiotic-associated diarrhea and pseudomembranous colitis. They cause fluid accumulation, necrosis, and a strong inflammatory response when inoculated in intestinal loops. Since mast cells are a rich source of inflammatory mediators, abundant in the gut, and known to be involved in C. difficile-induced enteritis, we studied the in vitro effect of toxin A on isolated mast cells. Normal rats sensitized by infection with Nippostrongilus brasiliensis were used to isolate peritoneal mast cells (PMC). PMC from naive rats were stimulated with calcium ionophore A23187 as a model of antigen-independent activation, and PMC from sensitized rats were stimulated with N. brasiliensis antigens to study immunoglobulin E-dependent mast cell activation. After 4 h, toxin A did not induce release of nitric oxide or histamine in naive PMC. However, 10 ng of toxin per ml caused a significant release of tumor necrosis factor alpha (TNF-alpha). In contrast, 1 microg of toxin per ml inhibited antigen or A23187-induced histamine release by PMC. Toxin A at 1 microg/ml for 4 h caused disruption of actin which aggregated in the cytoplasm and around the nucleus. After 24 h, chromatin condensation, cytoplasmic blebbing, and apoptotic-like vesicles were observed; DNA fragmentation was documented also. These results suggest that mast cells may participate in the initial inflammatory response to C. difficile infection by releasing TNF-alpha upon interaction with toxin A. However, longer exposure to toxin A affects the release of inflammatory mediators, perhaps because of the alteration of the cytoskeleton and induction of apoptosis. The impaired functions and survival of mast cells by C. difficile toxin A could hamper the capacity of these cells to counteract the infection, thus prolonging the pathogenic effects of C. difficile toxins.

Animals↗
PubMed1992

Scanning electron microscopic observations of Anopheles albimanus (Diptera: Culicidae) eggs.

To investigate the existence of subspecies of Anopheles albimanus Wiedeman in southern Mexico, the egg morphology of specimens obtained from several field populations and from insectary-adapted colonies of uniform pupal phenotype was examined. Scanning electron microscopic observations have shown that the eggs of An. albimanus are polymorphic in respect to the size and shape of their floats, but not in their ornamentation. Four types of eggs were found. Differences in the proportion of the various morphological types were statistically significant, although proportions of egg types were variable among individuals within the same population. These observations are suggestive of distinctive populations and warrant further studies using more sensitive methods to investigate sibling species in An. albimanus sensu lato.

Animals↗
PubMed1992

Scanning electron microscopy of egg hatching of Anopheles albimanus (Diptera: Culicidae).

Scanning electron and light microscopic observations showed that egg hatching in Anopheles albimanus Wiedemann is aided by a chisel-shaped spine. This hatching tooth is surrounded by a thin flexible membrane fixed to a groove in the head of the larvae. Increased intracranial pressure may force the spine against the egg shell until a fissure is produced. Further opening of the egg is achieved by movements of the head and the entire body of the larva.

Animals↗
PubMed1990

Incomplete regression of müllerian ducts in the androgen insensitivity syndrome.

The biochemical and histological features of two related patients with the complete form of the androgen insensitivity syndrome (AIS) coexisting with incomplete regression of the Müllerian ducts are described. Both patients presented unilateral Müllerian derivatives (fallopian tube) identified by microscopic examination of surgically excised internal genital tissue. Biochemical studies performed in genital skin-derived fibroblasts from one of the affected subjects showed the existence of a specific and saturable 8.2 to 8.4 S cytosolic and 3.4 S nuclear androgen receptor exhibiting a Kd of 1.32 nmol/L. These mutant cells, however, clearly presented a significantly low maximal nuclear [3H]-5 alpha-dihydrotestosterone uptake (71.0 fmol/mg of deoxyribonucleic acid [DNA]; control strain, 284 fmol/mg DNA). Thus, an impaired uptake of the androgen receptor complex at the nuclear level was probably the cause of the complete absence of phenotypic expression of androgen action in this family. The overall findings are on line with the well-demonstrated genetic and molecular heterogeneity of the AIS.

Adolescent↗
PubMed1988

Inherited impairment of nuclear androgen uptake as a cause of familial androgen insensitivity.

The endocrine and biochemical characteristics of four related 46,XY pseudohermaphrodite patients with the Reifenstein Syndrome are presented. All of them (6 and 9 years old, first generation, and 9 and 12 months old, second generation) exhibited ambiguity of external genitalia and a family pedigree characteristic of an X-linked pattern of inheritance. Serum basal levels of LH, FSH, testosterone (T), androstenedione and 5 alpha-dihydrotestosterone (DHT) were within normal limits. Administration of hCG induced a normal response in terms of serum T in three of the patients, with a concomitant increase in serum DHT. However, an abnormally elevated T: DHT ratio was found in two of these subjects on the day of maximal T response (T: DHT ratio, 24 and 27; normal range, 4-21). Genital skin-derived fibroblasts from all patients were studied for [3H]DHT uptake in a whole-cell monolayer assay. Three of the mutant strains exhibited values of [3H]DHT uptake at 37 degrees C within the lower limits of normality (39.4-47.05 fmol/mg protein/h; normal strains, 36-101 fmol/mg protein/h), whereas fibroblasts from the remaining patient presented a slightly decreased uptake (31.66 fmol/mg protein); when studied at 42 degrees C, all mutant strains behaved as the normal controls. The existence of a specific 4.6 S cytosol androgen receptor was clearly seen in the two mutant strains when analysed by sucrose gradient centrifugation. Nevertheless, in one of the mutant strains, a significantly low maximal nuclear [3H]DHT uptake was detected (173.6 fmol/mg DNA; control strain, 301.6 fmol/mg DNA). The overall data were interpreted as demonstrating the existence of an impaired uptake of the androgen-receptor complex at the nuclear levels as the cause of the incomplete phenotypic expression of androgen action in this family. In this setting, the presence of low peripheral 5 alpha-reductase activity may be considered as a secondary manifestation of the androgen insensitivity.

Androgens↗
PubMed1978

[Interaction between Entamoeba histolytica and the cecal epithelium in the guinea pig].

The early interaction between E. histolytica trophozoites and the intestinal mucosa was studied by light and electron microscopy using sheets of isolated epithelium of the guinea pig colonic mucosa. The integrity of the isolated epithelium was confirmed by scanning electron microscopy. After 30 minutes of interaction, the amebas adhered first, and penetrated subsecuently the epithelium of the colon at the interglandular regions. The results suggest that the trophozoites of E. histolytica invade the epithelial barrier exclusively at the regions of lowest resistence, represented by the interglandular zones of epithelial shedding.

Animals↗
PubMedDate not supplied

Giardia lamblia: ultrastructural study of the in vitro effect of benzimidazoles.

Axenically grown Giardia lamblia trophozoites treated with low concentrations of the benzimidazole carbamates albendazole and mebendazole detach from glass culture tubes and lose viability. Scanning electron microscopic observations revealed that these drugs produce grotesque modifications of the cell shape of the parasite and disassembly of the adhesive disc. Transmission electron microscopy showed several stages of the fragmentation of adhesive discs with dispersion of microtubules and microribbons in the cytoplasm. Flagella appeared undamaged. In drug-treated trophozoites electron-dense precipitates were selectively deposited on microtubules and microribbons. The results indicate that the antigiardial effect of benzimidazoles is the result of binding to microtubules and subsequent alterations of the cytoskeleton. The electron microscopic observations also suggest that the drugs may bind to microribbon components of the adhesive disc, possibly giardin proteins.

Albendazole↗
PubMedDate not supplied

Giardia lamblia: freeze-fracture ultrastructure of the ventral disc plasma membrane.

The close contact of Giardia lamblia trophozoites with mucosal surfaces produces surface indentations of epithelial cells, that progress to areas of microvilli depletion. This interaction is mediated by the lateral crest, a specialized contractile structure in the ventral disc of the parasite. We have analyzed the plasma membrane of the ventral disc using freeze fracture electron microscopy to study the distribution of large integral proteins, and freeze fracture cytochemistry with filipin to reveal sterol-containing sites. A previously undetected structural specialization was found at the lateral crest as a horseshoe-shaped membrane domain characterized by a near absence of intramembrane particles, in sharp contrast to the remainder of the ventral disc plasma membrane. In addition, the lateral crest showed a striking paucity of cholesterol-containing complexes in replicas of trophozoites treated with filipin. These observations demonstrate that the plasma membrane of G. lamblia displays a microheterogeneity in the planar distribution of cholesterol and intramembrane particles. Both membrane components are noticeably less abundant at the outer rim of the ventral disc where contraction takes place.

Animals↗