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Biomedical subjects

M D Bennett

Publications and source records attributed to M D Bennett.

At least 55 records · Page 3Linked to original sources
PubMed1984

Premeiotic events and meiotic chromosome pairing.

There is practical difficulty in identifying when meiosis begins. Moreover, because of contradictory definitions there is ambiguity and some confusion as to when, in terms of the cell cycle, premeiosis ends and meiosis begins. Nevertheless, results for several organisms show clearly that meiotic chromosome behaviour is affected by premeiotic events and especially by events during the final premeiotic mitosis and/or premeiotic interphase. This review considers only premeiotic events which do (or might) affect meiotic chromosome pairing by their effect on genomic characters, such as: chromosome number, homology, condition and position, with particular emphasis on the last. Interpreted in its widest sense 'premeiotic events affecting meiotic chromosome pairing' must include karyogamy. Moreover, while karyogamy is the normal means of achieving the diploid chromosome number and pairs of homologues essential for normal chromosome pairing, it is not the only way, as illustrated by the remarkable premeiotic adaptations seen in the apogamous ferns and the frog Rana esculenta. Little is known about the condition (including the molecular organization) of chromosomes during their approach and switch to meiosis. However, completion during premeiosis of some DNA synthesis may be essential for normal meiotic chromosome pairing. Various results (including different effects of colchicine given first at different premeiotic stages) have been claimed as evidence of one or other type of premeiotic spatial ordering of chromosomes which might favour, or be essential for, meiotic chromosome pairing. Chromosome placement has been studied recently using the electron microscope, serial thin-section, reconstruction technique. This has revealed clear evidence of non-random spatial placement of chromosomes in non-meiotic and premeiotic cells. For example, in root-tip cells of barley, Hordeum vulgare L. cv. Tuleen 346 (2n = 2x = 14), it showed: a significant spatial separation of two haploid chromosome sets; no somatic association of homologues; but a significant expression of a predicted mean order of heterologues (namely, linkage groups -4-1-2-3-5-6-7-) within haploid sets. This spatial arrangement of chromosomes might have important implications for the switch from premeiotic to meiotic development, if it occurred in germ line cells. Consequently, metaphase cells of Tuleen 346 at the final premeiotic mitosis were reconstructed next. Analysis of centromere positions showed a significantly ordered arrangement of haploid sets, and of heterologues within haploid sets, essentially the same as that previously noted for root-tip cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1984

Ordering decision and clinic cost variation among resident physicians.

Physicians vary considerably in the services they order for their patients. We examined ordering variation among 47 resident physicians during 4,991 continuity clinic visits with patients who had specific, chronic medical problems. We ranked the physicians by their average charge per visit and grouped them into three equal categories. High-charge physicians averaged $164 per visit, medium-charge $124 and low-charge $97. In comparing the frequencies with which physicians in each group ordered a wide array of specific laboratory tests, x-ray studies, medications and miscellaneous items, we found that ordering variation among the physician groups was not confined to certain decisions or categories of services. High-charge physicians ordered a little more of nearly every item or service. Although the magnitude of ordering variation for each item was small, the sum over many items was great, suggesting that cost-containment efforts may have to focus on the "little ticket" decision-making style of expensive physicians.

Cost Control↗
PubMed1984

Chromosome order--possible implications for development.

Chromosomes are arranged in ordered haploid sets around the centre of the metaphase plate at mitosis in several grass species and hybrids. Each chromosome is in a fixed mean position relative to other, heterologous chromosomes, this order can be predicted using Bennett's model, and is clearly demonstrated from reconstructions of electron micrographs of serial sections (see Heslop-Harrison & Bennett, 1983a,b,c). The nucleus contains spatial domains of genes with similar functions. Chromosomes with major effects on nuclear behaviour--division or meiotic pairing--may be at special positions in the order. Changing spatial relationships of chromosomes with respect both to each other and the nuclear envelope (during the cell cycle and during development) may affect cell differentiation and gene activity. Chromosome order may have implications for the control of development within the nucleus and the organism. Order may constrain karyotype and hence species evolution.

Cell Differentiation↗
PubMed1983

Impact of a cost-containment educational program on housestaff ambulatory clinic charges.

A cost-containment project was established to determine if a broad educational program that coupled clinical logic with feedback of charges to internal medicine residents in an ambulatory clinic setting could alter behavior. The project was evaluated by comparing charge data from a preintervention comparison year with charge data from the intervention year. Decreases in charges for laboratory procedures occurred in six of the seven diagnostic categories during intervention and were significant (t-test) for four categories. Regression analysis evaluated the influence of the intervention and 12 other variables on charges and demonstrated that the intervention was associated with a mean reduction of lab charges of $ 6.30 (p less than 0.0001) and a mean reduction of total encounter charges of $ 10.36 (p less than 0.01). We conclude that our educational program resulted in a decrease in mean laboratory changes and total charges for ambulatory encounters.

Awareness↗
PubMed1983

Comparison of family medicine and internal medicine: charges for continuing ambulatory care.

The authors compared charges for ambulatory and continuing patient care prescribed by residents in internal medicine and family medicine. An analysis of covariance showed that the charges per encounter in internal medicine were $38.83 greater than charges in family medicine after accounting for differences such as patient age, diagnosis, and severity of condition. However, family practitioners scheduled patient encounters more frequently. The median number of days between encounters was 30 for family medicine and 60 for internal medicine. The duration of medical care for each patient (up to 18 months was possible) and the summation of encounter charges over that duration of medical care were evaluated. Because only two out of eight duration-of-medical-care categories were significantly less in family medicine, the differences in charges between the specialties essentially disappeared. Comparative studies which consider only charges per encounter potentially have a large bias.

Analysis of Variance↗
PubMed1983

DNA density in mitotic and meiotic metaphase chromosomes of plants and animals.

Studies of chromosome disposition at metaphase using serial thin-sectioning and three-dimensional reconstruction techniques have produced accurate estimates of the total volume of chromosomes per cell in 15 plant and two animal species. Comparing this character with the 4C DNA amount showed no indiction of systemic differences in DNA density between either organisms with widely different (greater than 200-fold) C values or different groups or organisms. For example, there was no significant difference between the density of DNA in somatic metaphase chromosomes of man (0.141 pg/micrometers3) and its mean in 14 angiosperm plant species (0.182 pg/micrometers3), or between four dicotyledons (0.180 pg/miocrometers3) and 10 monocotyledons (0.182 pg/micrometers3). However, evidence was found showing that DNA density can vary significantly within a species. Thus, although the total chromosome volume per cell was closely correlated (r greater than 0.97) with 4C DNA amount in somatic and meiotic cells, the density of DNA in metaphase chromosomes was significantly lower in meiocytes (0.131 pg/micrometers3) than in somatic metaphase cells (0.19 pg/micrometers3).

Animals↗
PubMed1983

The positions of centromeres on the somatic metaphase plate of grasses.

Centromere positions on the metaphase plate of 48 root-tip cells of four grass species were analysed using metaphases reconstructed from electron micrographs of serial thin sections. Centromere alignment was almost perfect on a plane in 15 untreated metaphases of cereals where 2n = 14; only 2% of the total variance in centromere position was out of the plane of the metaphase plate. In 23 similar cells pretreated with ice-water, the mean out-of-plate variance was 9%, compared to an expectation of 18% if centromeres were positioned randomly in space. In cold-treated cells of Zea mays (2n = 20), the out-of-plate variance (14%) was significantly less than a random expectation of 20%. The distances of centromeres from the mean centromere position (MCP) were also analysed. They showed that the centromeres tended to be normally distributed about a circle around the MCP in all the species. Thus centromeres tend towards a ring arrangement in metaphase cells. Analysis of separation distances of all pairs of centromeres in each reconstructed cell showed that the distribution of distances between centromeres is not random: there are significantly fewer centromeres that are close together and more that are at medium separation distances than is expected on a random basis. This is different from previously published assumptions about centromere disposition in squashed metaphases.

Centromere↗
PubMed1982

Evaluation of educational methods in a comprehensive cost-containment project in ambulatory care.

The cost of medical care in the United States is a matter of great concern to many health policy makers and physicians as well as to the seekers of care. At the University of New Mexico, a comprehensive cost-containment education program was devised and put into effect with residents from three primary care disciplines. In addition to documenting the effectiveness of the educational package in reducing patients charges, we asked house staff to state which parts of the package were of most use. Wall posters displaying charges for commonly ordered tests, procedures, and prescription items and the distribution of sample bills for patients seen by the individual resident were rated most highly. A test of knowledge of charges to patients did not show resident improvement after education. Cost-containment education can be effective in reducing patient charges, even if physicians do not internalize the actual charge data. We recommend the inclusion of cost-containment education in all primary care training programs, as these physicians play such an important role in the generation of patient charges.

Ambulatory Care↗
PubMed1981

The intranuclear relationship between centromere volume and chromosome size in Festuca scariosa X drymeja.

In the hybrid Festuca scarisoa X drymeja where pairing is incomplete at pachytene, there is preferential pairing between the longer chromosomes of the complement. EM serial-section reconstruction of nuclei at zygotene and pachytene reveals that there is equally pronounced preferential pairing between larger centromeres. This evidence suggests that the longer chromosomes have large centromeres and that centromere volume is correlated with chromosome length. Confirmation of this comes from the comparison of the frequency distributions of observed centromere volumes and those predicted on the basis of chromosome length. Although there is a positive correlation between centromere volume and chromosome length, it is not possible to identify the centromeres of each individual chromosome within the complement because (a) the differences between the lengths of each chromosome are small and (b) the estimates of relative centromere volumes vary significantly between cells.

Cell Nucleus↗
PubMed1981

The relationship between nuclear DNA content and centromere volume in higher plants.

The total volume of centromeres per nucleus varies widely within Triticum aestivum cv. Chinese Spring (7-fold) and between 11 higher plant species (24-fold). Such variation is closely correlated with nuclear DNA content, nuclear volume and, to a lesser extent, the total volume of nucleoli per nucleus. Centromere volume reflects minor intraspecific developmental fluctuations in nuclear size independent of variation in nuclear DNA content, but variation in nuclear DNA plays the major role in determining centromere volume. Thus, in general a given total volume of centromeric material is apparently characteristic of an approximately constant nuclear volume and mass of nuclear DNA, but largely independent of chromosome number. The range of volume of single centromeres in 4 taxa corresponds with the ranges of their single chromosome lengths or chromosome DNA contents. The centromere is, therefore, not a unit structure of constant size and mass but a chromosome segment whose highly variable volume closely reflects the volume and mass of the chromosome to which it belongs. The correlation between centromere size and chromosome size and DNA content is potentially useful for identifying single centromeres in unsquashed interphase and dividing nuclei; thereby facilitating studies of the intranuclear disposition of chromosomes. The present results for centromeres provide probably the first example to indicate that variation in the total DNA content of small segments present on each chromosome sometimes varies directly in proportion to large interspecific variation in nuclear DNA C-value. The close correlation between centromere volume, and nuclear DNA content is probably nucleotypic in origin. The functional significance of the variation in centromere volume is unknown, as is the nature of the mechanism which determines that centromere volume closely reflects nuclear and chromosome size and mass.

Cell Cycle↗
PubMed1981

Hordeum and Secale mitotic genomes lie apart in a hybrid.

In both unpretreated root tip metaphases and pretreated mitoses of Hordeum vulgare L. cv. Sultan x Secale africanum Stapf F1 hybrids, Hordeum chromosomes tended to be nearer the centre of the mitosis than Secale chromosomes. This was clear in 4 serially sectioned cells examined in the electron microscope. In Feulgen squashes of 38 of 40 cells studied in the light microscope, the mean distances in each cell from the mean centromere position for the cell was less for Hordeum centromeres than for Scale centromeres. Such spatial separation of parent genomes might prevent pairing of homoeologues in hybrids.

Centromere↗
PubMed1979

Colchicine-induced paracrystals in the tapetum of wheat anthers.

Prolonged colchicine treatment induced the formation of numerous cytoplasmic para-crystals in tapetal cells of wheat anthers containing germ line cells at pre-meiotic mitosis or early pre-meiotic interphase. Such paracrystals were up to 8 microns long and 0.7 microns in diameter, and filled up to about 8% of the volume of the cytoplasm. Paracrystals were not bounded by a membrane. A paracrystal consisted of tubules running in parallel with its long axis. Paracrystals cut at right angles to their long axes revealed hexagonal arrays of circles with a maximum diameter of about 35 nm and electron-dense walls 6--7 nm in thickness. Similar paracrystals were not seen in any cell type of untreated anthers, or in germ line cells of treated anthers, containing stages between pre-meiotic mitosis and first metaphase of meiosis inclusive. The possible significance of the paracrystals in relation to the function of the tapetum is discussed.

Cell Cycle↗
PubMed1979

The effect of colchicine on fibrillar material in wheat meiocytes.

Wheat meiocytes contain intranuclear bundles of microfibres at stages from pre-meiotic mitosis until zygotene. This includes early pre-meiotic interphase, the stage when colchicine induces univalents at subsequent metaphase I. This paper reports the effect of colchicine on the structure and distribution of such fibrillar material. Many normal-looking bundles of intranuclear fibrillar material formed after prolonged colchicine treatment (up to 5 days) which totally suppressed microtubule assembly in meiocytes. Thus, fibrillar material is unlikely to comprise tubulin. However, the formation of intranuclear fibrillar material was retarded in colchicine-treated meiocytes, and fibrillar material was absent during the colchicine-sensitive stage for univalency. The occurrence of cytoplasmic fibrillar material was delayed and greatly increased in colchicine-treated meiocytes compared with controls. A novel curved form of fibrillar material was seen in colchicine-treated meiocytes. It is concluded that colchicine had significant effects on the assembly and distribution of fibrillar material. These results are, therefore, compatible with the possibility that colchicine causes univalency in wheat via its effects on fibrillar material.

Cell Cycle↗
PubMed1977

The time and duration of meiosis.

Ever since meiosis was recognized as a process there has been a continuing interest in its temporal aspects. Two main types of meiotic timing experiments have been conducted: first, experiments to estimate the duration of meiosis (and sometimes its stages); second, experiments to locate the sensitive stage(s) when exposure of meiocytes to various treatments can affect meiotic chromosome behaviour (e.g. pairing or recombination). Such experiments have played an important role in increasing our understanding of the meiotic process. The duration of meiosis has been estimated in about 70 organisms, including two prokaryotes (yeast and Chlamydomonas) and the following eukaryotes: 1 Basidiomycete (Coprinus lagopus), 2 Gymnosperms (Larix decidua and Thuja plicata gracilis). at least 39 angiosperms, and at least 26 animal species. The duration of female meiosis has been estimated in far fewer species than male meiosis. However, estimates of the duration of female meiosis are available for 6 angiosperms. Drosophila melanogaster, Xenopus laevis, and several mammals. Comparison of these data shows that the duration of meiosis is one of the most variable aspects of the meiotic process, ranging from less than 6 h in yeast to more than 40 years in the human female. Developmental holds at different stages of meiosis are common in plants and animals, and inevitably prolong the meiotic division. However, even among species without developmental holds, the duration of meiosis is very variable. For instance, in animals it ranges from about 1-2 days in male Drosophila melanogaster to more than 24 days in male Homo sapiens and several Orthopterans. Despite the large variation in the duration of meiosis three generalizations can be made: (i) first prophase is always very long compared with the remaining meiotic stages, (ii) the rate of meiotic development is very slow compared with the rate of development in dividing somatic meristem cells of the same organisms under the same conditions, (iii) the duration of meiosis is characteristic of the genotype and species. Four main factors have been recognized which effect or determine the duration of meiosis, namely (1) environmental factors (e.g. temperature); (2) nuclear DNA content; (3) ploidy level of the organism; and, (4) the genotype. Because nuclear DNA content plays a major role in determining the duration of meiosis, it has been suggested that DNA influences the rate of meiotic development in two ways: first through its informational content (the genotype), and second indirectly by the physical and mechanical effects of its mass independently of its informational content (i.e. the nucleotype). Thus, the observed duration of meiosis is the result of a complex genotype-nucleotype-environment interaction. With the obvious exception of variation caused by developmental holds, changes in the duration of meiosis usually involve proportional changes in the durations of all its stages...

Animals↗
PubMed1976

Nuclear dna amounts in angiosperms.

The number of angiosperm species for which nuclear DNA amount estimates have been made has nearly trebled since the last collected lists of such values were published, and therefore, publication of a more comprehensive list is over due. This paper lists absolute nuclear DNA amounts for 753 angiosperm species. The dats were assembled primarily for reference purposes, and so the species are listed in alphabetical order, as this was felt to be more helpful to cyto- and biochemists whom, it is anticipated, will be among its major users. The paper also reviews aspects of the history, nomenclature, methods, accuracy and problems of nuclear DNA estimation in angiosperms. No attempt is made to reconsider those aspects of nuclear DNA estimation which have been fully revised previously, although the bibliography of such aspects is given. Instead, the paper is intended as a source of basic information regarding the terminology, practice and limitations of nuclear DNA estimation, especially by Feulgen microdensitometry, as currently practiced.

Cell Nucleus↗