scholarly journals Sex-Specific Differences in Fat Storage, Development of Non-Alcoholic Fatty Liver Disease and Brain Structure in Juvenile HFD-Induced Obese Ldlr-/-.Leiden Mice

Nutrients ◽  
2019 ◽  
Vol 11 (8) ◽  
pp. 1861 ◽  
Author(s):  
Sophie A.H. Jacobs ◽  
Eveline Gart ◽  
Debby Vreeken ◽  
Bart A.A. Franx ◽  
Lotte Wekking ◽  
...  
Keyword(s):  
Adipose Tissue ◽  
Hepatic Steatosis ◽  
Fat Mass ◽  
Brain Structure ◽  
Brain Function ◽  
Liver Pathology ◽  
Male Mice ◽  
Male And Female ◽  
Female Mice ◽  
Juvenile Obesity

Background: Sex-specific differences play a role in metabolism, fat storage in adipose tissue, and brain structure. At juvenile age, brain function is susceptible to the effects of obesity; little is known about sex-specific differences in juvenile obesity. Therefore, this study examined sex-specific differences in adipose tissue and liver of high-fat diet (HFD)-induced obese mice, and putative alterations between male and female mice in brain structure in relation to behavioral changes during the development of juvenile obesity. Methods: In six-week-old male and female Ldlr-/-.Leiden mice (n = 48), the impact of 18 weeks of HFD-feeding was examined. Fat distribution, liver pathology and brain structure and function were analyzed imunohisto- and biochemically, in cognitive tasks and with MRI. Results: HFD-fed female mice were characterized by an increased perigonadal fat mass, pronounced macrovesicular hepatic steatosis and liver inflammation. Male mice on HFD displayed an increased mesenteric fat mass, pronounced adipose tissue inflammation and microvesicular hepatic steatosis. Only male HFD-fed mice showed decreased cerebral blood flow and reduced white matter integrity. Conclusions: At young age, male mice are more susceptible to the detrimental effects of HFD than female mice. This study emphasizes the importance of sex-specific differences in obesity, liver pathology, and brain function.

scholarly journals NPY Deficiency Prevents Postmenopausal Adiposity by Augmenting Estradiol-Mediated Browning

2018 ◽  
Vol 75 (6) ◽  
pp. 1042-1049
Author(s):  
Seongjoon Park ◽  
Erkhembayar Nayantai ◽  
Toshimitsu Komatsu ◽  
Hiroko Hayashi ◽  
Ryoichi Mori ◽  
...  
Keyword(s):  
Body Weight ◽  
Adipose Tissue ◽  
Fat Metabolism ◽  
Male Mice ◽  
Wild Type ◽  
Male And Female ◽  
Female Mice ◽  
Age Related ◽  
Promising Target ◽  
Intracellular Mechanism

Abstract The orexigenic hormone neuropeptide Y (NPY) plays a pivotal role in the peripheral regulation of fat metabolism. However, the mechanisms underlying the effects of sex on NPY function have not been extensively analyzed. In this study, we examined the effects of NPY deficiency on fat metabolism in male and female mice. Body weight was slightly decreased, whereas white adipose tissue (WAT) mass was significantly decreased as the thermogenic program was upregulated in NPY-/- female mice compared with that in wild-type mice; these factors were not altered in response to NPY deficiency in male mice. Moreover, lack of NPY resulted in an increase in luteinizing hormone (LH) expression in the pituitary gland, with concomitant activation of the estradiol-mediated thermogenic program in inguinal WAT, and alleviated age-related modification of adiposity in female mice. Taken together, these data revealed a novel intracellular mechanism of NPY in the regulation of fat metabolism and highlighted the sexual dimorphism of NPY as a promising target for drug development to reduce postmenopausal adiposity.

scholarly journals Carbonyl Reductase 1 Overexpression in Adipose Amplifies Local Glucocorticoid Action and Impairs Glucose Tolerance in Lean Mice

2021 ◽  
Vol 5 (Supplement_1) ◽  
pp. A806-A806
Author(s):  
Rachel Bell ◽  
Elisa Villalobos ◽  
Mark Nixon ◽  
Allende Miguelez-Crespo ◽  
Matthew Sharp ◽  
...  
Keyword(s):  
Adipose Tissue ◽  
Glucose Tolerance ◽  
Fat Mass ◽  
High Fat Diet ◽  
Fasting Glucose ◽  
High Fat ◽  
Male And Female ◽  
Over Expression ◽  
Female Mice ◽  
Diet Induced Obesity

Abstract Glucocorticoids play a critical role in metabolic homeostasis. Chronic or excessive activation of the glucocorticoid receptor (GR) in adipose tissue contributes to metabolic disorders such as glucose intolerance and insulin resistance. Steroid-metabolising enzymes in adipose, such as 11β-HSD1 or 5α-reductase, modulate the activation of GR by converting primary glucocorticoids into more or less potent ligands. Carbonyl reductase 1 (CBR1) is a novel regulator of glucocorticoid metabolism, converting corticosterone/cortisol to 20β-dihydrocorticosterone/cortisol (20β-DHB/F); a metabolite which retains GR activity. CBR1 is abundant in adipose tissue and increased in obese adipose of mice and humans1 and increased Cbr1 expression is associated with increased fasting glucose1. We hypothesised that increased Cbr1/20β-DHB in obese adipose contributes to excessive GR activation and worsens glucose tolerance. We generated a novel murine model of adipose-specific Cbr1 over-expression (R26-Cbr1Adpq) by crossing conditional knock-in mice with Adiponectin-Cre mice. CBR1 protein and activity were doubled in subcutaneous adipose tissue of male and female R26-Cbr1Adpq mice compared with floxed controls; corresponding to a two-fold increase 20β-DHB (1.6 vs. 4.2ng/g adipose; P=0.0003; n=5-7/group). There were no differences in plasma 20β-DHB or corticosterone. Bodyweight, lean or fat mass, did not differ between male or female R26-Cbr1Adpq mice and floxed controls. Lean male R26-Cbr1Adpq mice had higher fasting glucose (9.5±0.3 vs. 8.4±0.3mmol/L; P=0.04) and worsened glucose tolerance (AUC 1819±66 vs. 1392±14; P=0.03). Female R26-Cbr1Adpq mice also had a worsened glucose tolerance but fasting glucose was not altered with genotype. There were no differences in fasting insulin or non-esterified fatty acid between genotypes in either sex. Expression of GR-induced genes Pnpla2, Gilz and Per1, were increased in adipose of R26-Cbr1Adpq mice. Following high-fat diet induced obesity, no differences in bodyweight, lean or fat mass, with genotype were observed in male and female mice, and genotype differences in fasting glucose and glucose tolerance were abolished. In conclusion, adipose-specific over-expression of Cbr1 in lean male and female mice led to increased levels of 20β-DHB in adipose but not plasma, and both sexes having worsened glucose tolerance. The influence of adipose CBR1/20β-DHB on glucose tolerance was not associated with altered fat mass or bodyweight and was attenuated by high-fat diet-induced obesity. These metabolic consequences of Cbr1 manipulation require careful consideration given the wide variation in CBR1 expression in the human population, the presence of inhibitors and enhancers in many foodstuffs and the proposed use of inhibitors as an adjunct for cancer treatment regimens. Reference: Morgan et al., Scientific Reports. 2017; 7.

scholarly journals Sex-Dependent Effects of Eicosapentaenoic Acid and UCP1 Deficiency on Hepatic Steatosis in Diet-Induced Obese Mice

2021 ◽  
Vol 5 (Supplement_2) ◽  
pp. 1193-1193
Author(s):  
Kembra Albracht-Schulte ◽  
Salvador Galindo ◽  
Sarah Anjum ◽  
Mandana Pahlavani ◽  
Latha Ramalingam ◽  
...  
Keyword(s):  
Adipose Tissue ◽  
Eicosapentaenoic Acid ◽  
Ambient Temperature ◽  
Hepatic Steatosis ◽  
Uncoupling Protein ◽  
Expression Profiles ◽  
Mitochondrial Protein ◽  
Male And Female ◽  
Female Mice ◽  
Brown Adipose

Abstract Objectives Nonalcoholic fatty liver disease (NAFLD), characterized by hepatic triglyceride (TG) accumulation, is associated with expansion of white adipose tissue (WAT). By contrast, brown adipose tissue (BAT) may prevent obesity and NAFLD through activity of mitochondrial uncoupling protein 1 (UCP1), which is involved in energy dissipation. Previous studies in our lab have shown that eicosapentaenoic acid (EPA) ameliorates obesity and hepatic steatosis in high-fat (HF) fed male, B6 mice at thermoneutral conditions, independent of UCP1. However, it is unknown whether similar effects of EPA and UCP1 deficiency will be observed at ambient temperature, and whether they differ by sex. Thus, the goal of this project was to investigate sex-dependent mechanisms of EPA in the livers of diet-induced obese, wild type (WT) and UCP1 knockout (KO) mice housed at ambient temperature. Methods WT and UCP1 KO B6 male and female mice were fed a HF diet (45% kcal fat; WT-HF, KO-HF) or HF diet supplemented with 36g/kg EPA (WT-EPA, KO-EPA) for 14 weeks. Body weight (BW), liver histology, and specific metabolic gene expression profiles were assessed. Results Although BW significantly varied by sex, diet, and genotype, UCP1 inactivation did not significantly increase BW compared to WT in either sex. Hepatic TG accumulation varied significantly by genotype with no significant differences seen with EPA supplementation in either sex. However, markers of lipogenesis were sex-dependently impacted by genotype and diet: there were no significant differences in markers of lipogenesis (Fasn and Acaca) with UCP1 KO or EPA supplementation in males; while these markers were reduced in female KO mice compared to female WT with no response to EPA. By contrast, lipogenic markers were reduced with EPA in female WT mice. Conclusions Our findings reveal an association between NAFLD and UCP1 deficiency, indicating the importance of this mitochondrial protein in limiting hepatic lipid accumulation, particularly in females. These findings also suggest a genotypic difference in response to dietary EPA supplementation on the livers of male and female mice, with beneficial effects reported in the female WT group. Funding Sources Funded by NIH/NCCIH grant #R15AT008879-01A1.

Sexual dimorphism in obesity-mediated left ventricular hypertrophy

2013 ◽  
Vol 305 (2) ◽  
pp. H211-H218 ◽  
Author(s):  
Christian Böhm ◽  
Verena Benz ◽  
Markus Clemenz ◽  
Christiane Sprang ◽  
Beata Höft ◽  
...  
Keyword(s):  
Adipose Tissue ◽  
Left Ventricular Hypertrophy ◽  
Fat Mass ◽  
Ventricular Hypertrophy ◽  
Epicardial Adipose Tissue ◽  
Control Diet ◽  
Left Ventricular ◽  
Male Mice ◽  
Female Mice ◽  
Fold Induction

In the present study we investigated the influence of sex difference on the development of left ventricular hypertrophy (LVH) during obesity. Male and female C57BL/6J mice were fed for 15 and 25 wk with a high-fat diet (HFD) or low-fat control diet (LFD). Analysis of body composition, monitoring of body weight (BW), and echocardiographic analysis were performed, as well as analysis of expression of different adipocytokines in epicardial adipose tissue. The increment in left ventricular mass (LVM) after HFD (25 wk) was significantly stronger in male mice compared with female mice [LVM: male, 116.9 ± 2.9 (LFD) vs. 142.2 ± 9.3 mg (HFD); female, 84.3 ± 3.3 (LFD) vs. 93.9 ± 1.7 mg (HFD), Psex < 0.01]. In parallel, males developed a higher BW and fat mass after 25 wk HFD than female mice [BW: male, 33 ± 0.9 (LFD) vs. 53 ± 0.8 g (HFD); fat mass: male, 8.8 ± 0.9 (LFD) vs. 22.8 ± 0.7 g (HFD); BW: female, 22.5 ± 0.4 (LFD) vs. 33.7 ± 1.3 g (HFD); fat mass: female, 4.0 ± 0.2 (LFD) vs. 13.2 ± 1.2 g (HFD)] ( P < 0.01 for BW+ fat mass female vs. male). The mRNA expression of adipocytokines in epicardial fat after 25 wk of diet showed higher levels of adiponectin (2.8-fold), leptin (4.2-fold), and vaspin (11.9-fold) in male mice compared with female mice ( P < 0.05). To identify new adipose-derived molecular mediators of LVH, we further elucidated the cardiac impact of vaspin. Murine primary cardiac fibroblast proliferation was significantly induced by vaspin (1.8-fold, vaspin 1 μg/l, P < 0.05 vs. control) compared with 1.9-fold induction by angiotensin II (10 μM). The present study demonstrates a sex-dependent regulation of diet-induced LVH associated with sexual dimorphic expression of adipocytokines in epicardial adipose tissue.

scholarly journals Gender differences in the response of CD-1 mouse bone to parathyroid hormone: potential role of IGF-I

2006 ◽  
Vol 189 (2) ◽  
pp. 279-287 ◽  
Author(s):  
Yongmei Wang ◽  
Takeshi Sakata ◽  
Hashem Z Elalieh ◽  
Scott J Munson ◽  
Andrew Burghardt ◽  
...  
Keyword(s):  
Gender Differences ◽  
Parathyroid Hormone ◽  
Cortical Bone ◽  
Mineral Apposition Rate ◽  
Mrna Levels ◽  
Male Mice ◽  
Bone Formation Rate ◽  
Male And Female ◽  
Female Mice ◽  
Igf I

Parathyroid hormone (PTH) exerts both catabolic and anabolic actions on bone. Studies on the skeletal effects of PTH have seldom considered the effects of gender. Our study was designed to determine whether the response of mouse bone to PTH differed according to sex. As a first step, we analyzed gender differences with respect to bone mass and structural properties of 4 month old PTH treated (80 μg/kg per day for 2 weeks) male and female CD-1 mice. PTH significantly increased fat free weight/body weight, periosteal bone formation rate, mineral apposition rate, and endosteal single labeling surface, while significantly decreasing medullary area in male mice compared with vehicle treated controls, but induced no significant changes in female mice. We then analyzed the gender differences in bone marrow stromal cells (BMSC) isolated from 4 month old male and female CD-1 mice following treatment with PTH (80 μg/kg per day for 2 weeks). PTH significantly increased the osteogenic colony number and the alkaline phosphatase (ALP) activity (ALP/cell) by day 14 in cultures of BMSCs from male and female mice. PTH also increased the mRNA level of receptor activator of nuclear factor κB ligand in the bone tissue (marrow removed) of both females and males. However, PTH increased the mRNA levels of IGF-I and IGF-IR only in the bones of male mice. Our results indicate that on balance a 2-weeks course of PTH is anabolic on cortical bone in this mouse strain. These effects are more evident in the male mouse. These differences between male and female mice may reflect the greater response to PTH of IGF-I and IGF-IR gene expression in males enhancing the anabolic effect on cortical bone.

scholarly journals Reevaluation of Hepatocellular Neoplasms in CD-1 Mice from a 2-year Oral Carcinogenicity Study with Permethrin

2018 ◽  
Vol 47 (1) ◽  
pp. 11-17 ◽  
Author(s):  
Erin M. Quist ◽  
Gary A. Boorman ◽  
John M. Cullen ◽  
Robert R. Maronpot ◽  
Amera K. Remick ◽  
...  
Keyword(s):  
Chronic Toxicity ◽  
Expert Panel ◽  
Biological Significance ◽  
Male Mice ◽  
Male And Female ◽  
Female Mice ◽  
Carcinogenicity Study ◽  
Dose Dependent Increase ◽  
Dose Dependent ◽  
The Continuum

A 24-month oral carcinogenicity study of permethrin was conducted by feeding male and female CD-1 mice diets containing concentrations of 0, 20, 500, and 2,000 ppm of permethrin (males) or 0, 20, 2,500, and 5,000 ppm of permethrin (females). After approximately two years on study, surviving mice were sacrificed for the evaluation of chronic toxicity and/or carcinogenicity. An expert panel of pathologists was convened as a Pathology Working Group (PWG) to review coded liver histology sections from male and female mice and to classify all liver neoplasms according to current nomenclature and diagnostic criteria guidelines. The PWG results indicate that permethrin induced a significant dose-dependent increase in the incidence of hepatocellular neoplasms in treated female mice ( p < .01) as well as a nonstatistically significant increase in the incidence of hepatocellular tumors in treated male mice. Given the continuum of the diagnoses of adenoma and carcinoma, and the difficulty in distinguishing some of the lesions, it is appropriate to consider only the combined incidences of hepatocellular tumors (adenoma and/or carcinoma) for biological significance and risk assessment.

scholarly journals Sex differences in cardio-pulmonary pathology of SARS-CoV2 infected and Trypanosoma cruzi co-infected mice

2021 ◽  
Author(s):  
Dhanya Dhanyalayam ◽  
Kezia Lizardo ◽  
Neelam Oswal ◽  
Hariprasad Thangavel ◽  
Enriko Dolgov ◽  
...  
Keyword(s):  
Adipose Tissue ◽  
Viral Load ◽  
Trypanosoma Cruzi ◽  
Cardiac Damage ◽  
Male And Female ◽  
Female Mice ◽  
Chagas Cardiomyopathy ◽  
Ppar Signaling ◽  
Pulmonary Damage ◽  
Heart Pathology

Coronavirus disease-2019 (COVID-19) caused by Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2; CoV2) is a deadly contagious infectious disease. For those who survived COVID-19, post-COVID cardiac damage poses a major threat for the progression of cardiomyopathy and heart failure. Currently, the number of COVID-related cases and deaths is increasing in Latin America, where a major COVID comorbidity is Chagas heart disease (caused by the parasite Trypanosoma cruzi). Here, we investigated the effect of T. cruzi infection on the pathogenesis and severity of CoV2 infection and, conversely, the effect of CoV2 infection on heart pathology during coinfection. We used transgenic human angiotensin-converting enzyme 2 (huACE2) mice infected with CoV2, T. cruzi, or coinfected with both in this study. Our study shows for the first time that white adipose tissue (WAT) serves as a reservoir for CoV2 and the persistence of CoV2 in WAT alters adipose tissue morphology and adipocyte physiology. Our data demonstrate a correlation between the loss of fat cells and the pulmonary adipogenic signaling and pathology in CoV2 infection. The viral load in the lungs is inversely proportional to the viral load in WAT, which differs between male and female mice. Our findings also suggest that adiponectin-PPAR signaling may differently regulate Chagas cardiomyopathy in coinfected males and females. We conclude that adipogenic signaling may play important roles in cardio-pulmonary pathogenesis during CoV2 infection and T. cruzi coinfection. The levels of adiponectin isomers differ between male and female mice during CoV2 infection and coinfection with T. cruzi, which may differently regulate inflammation, viral load, and pathology in the lungs of both the sexes. Our findings are in line with other clinical observations that reported that males are more susceptible to COVID-19 than females and suffer greater pulmonary damage.

scholarly journals Ovarian hormones mediate the prophylactic efficacy of (R,S)-ketamine and (2R,6R)-hydroxynorketamine in female mice

2019 ◽  
Author(s):  
Briana K. Chen ◽  
Christina T. LaGamma ◽  
Xiaoming Xu ◽  
Shi-Xian Deng ◽  
Rebecca A. Brachman ◽  
...  
Keyword(s):  
Learned Helplessness ◽  
Hormone Replacement ◽  
Ovarian Hormones ◽  
Contextual Fear Conditioning ◽  
Male Mice ◽  
Prophylactic Efficacy ◽  
Male And Female ◽  
Female Mice ◽  
Learned Fear ◽  
Necessary And Sufficient

ABSTRACTBACKGROUNDFemales are more likely than males to develop major depressive disorder (MDD) after exposure to stress. We previously reported that the administration of (R,S)-ketamine before stress can prevent stress-induced depressive-like behavior in male mice but have yet to assess efficacy in female mice or for other compounds, such as the metabolites of (R,S)-ketamine.METHODSWe administered (R,S)-ketamine or its metabolites (2R,6R)-hydroxynorketamine ((2R,6R)-HNK) and (2S,6S)-HNK at various doses 1 week before one of a number of stressors, including contextual fear conditioning (CFC), learned helplessness (LH), and chronic immobilization stress (CIS), in male and female 129S6/SvEv mice. To examine the interaction between ovarian hormones and stress resilience, female mice also underwent ovariectomy surgery (OVX) and a hormone replacement protocol prior to drug administration.RESULTS(R,S)-ketamine and (2S,6S)-HNK, but not (2R,6R)-HNK, attenuated learned fear in male mice. (R,S)-ketamine and (2R,6R)-HNK, but not (2S,6S)-HNK, significantly reduced stress-induced depressive-like behavior in male and female mice. (R,S)-ketamine and (2R,6R)-HNK) were prophylactically effective at a lower dose (10 mg/kg and 0.025 mg/kg, respectively) in female mice than in male mice (30 mg/kg and 0.075 mg/kg, respectively). Moreover, ovarian-derived hormones were necessary and sufficient for prophylaxis in female mice.CONCLUSIONSOur results suggest that prophylactics against stress-induced depressive-like behavior can be developed in a sex-specific manner and that ovarian hormones mediate prophylactic efficacy in females. To our knowledge, this is the first demonstration of the prophylactic efficacy of the metabolites of (R,S)-ketamine in male and female mice.

scholarly journals Adolescent intermittent ethanol exposure induces sex-dependent divergent changes in ethanol drinking and motor activity in adulthood in C57BL/6J mice

2020 ◽  
Author(s):  
Antoniette M. Maldonado-Devincci ◽  
Joseph G. Makdisi ◽  
Andrea M. Hill ◽  
Renee C. Waters ◽  
Nzia I. Hall ◽  
...  
Keyword(s):  
Open Field ◽  
Ethanol Consumption ◽  
Ethanol Exposure ◽  
Male Mice ◽  
Open Field Behavior ◽  
Male And Female ◽  
Female Mice ◽  
Binge Ethanol Exposure ◽  
Binge Ethanol

AbstractWith alcohol readily accessible to adolescents, its consumption leads to many adverse effects, including impaired learning, attention, and behavior. Adolescents report higher rates of binge drinking compared to adults. Adolescents are also more prone to substance use disorder during adulthood due to physiological changes during the adolescent developmental period. We used C57BL/6J male and female mice to investigate the long-lasting impact of binge ethanol exposure during adolescence on voluntary ethanol intake and open field behavior during later adolescence and in young adulthood. The present set of experiments were divided into four stages: (1) chronic intermittent vapor inhalation exposure, (2) abstinence, (3) voluntary ethanol intake, and (4) open field behavioral testing. During adolescence, male and female mice were exposed to air or ethanol using an intermittent vapor inhalation with repeated binge pattern ethanol exposure from postnatal day (PND) 28–42. Following this, mice underwent abstinence during late adolescence from PND 43–49 (Experiment 1) or PND 43–69 (Experiment 2). Beginning on PND 49–76 (Experiment 1) or PND 70–97 (Experiment 2), mice were assessed for intermittent voluntary ethanol consumption using a two-bottle drinking procedure over 28 days. Male mice that were exposed to ethanol during adolescence showed increased ethanol consumption during later adolescence (Experiment 1) and in emerging adulthood (Experiment 2), while the female mice showed decreased ethanol consumption. These data demonstrate a sexually divergent shift in ethanol consumption following binge ethanol exposure during adolescence and differences in open field behavior. These data highlight sex-dependent vulnerability to developing substance use disorders in adulthood.Significance StatementCurrently, it is vital to determine the sex-dependent impact of binge alcohol exposure during adolescence, given that until recently females have largely been ignored. Here we show that adolescent male mice that are exposed to binge ethanol during adolescence show long-term changes in behavior in adulthood. In contrast, female mice show a transient decrease in ethanol consumption in adulthood and decreased motor activity spent in the center zone of the open field test. Male mice appear to be more susceptible to the long-term changes in ethanol consumption following binge ethanol exposure during adolescence.

Certain aspects of the host-parasite relationship of Nematospiroides dubius (Baylis). I. Resistance of male and female mice to experimental infections

Parasitology ◽  
1961 ◽  
Vol 51 (1-2) ◽  
pp. 173-179 ◽  
Author(s):  
Colin Dobson
Keyword(s):  
Relative Length ◽  
Industrial Research ◽  
Male Mice ◽  
Male And Female ◽  
Female Mice ◽  
Host Parasite Relationship ◽  
Parasite Relationship ◽  
Nematospiroides Dubius ◽  
Host Parasite ◽  
Relationship Of

1. It has been shown that there is a difference between the resistance of male and female mice to infection with Nematospiroides dubius.2. More parasites were harboured, during both the larval and adult parasitic phases, by male mice.3. These worms were found to occupy a similar relative length of the intestine between the stomach and the caecum in male and female mice infected for either 5 or 10 days.4. The relative length of the intestine infected on the fifth day was significantly greater than that infected on the tenth day.This investigation was carried out during the tenure of a Research Studentship from the Department of Scientific and Industrial Research. I should like to thank Professor I. Chester Jones, in whose department the work was done, for the facilities provided and Dr E. T. B. Francis for his helpful and critical supervision.

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