首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   427篇
  免费   62篇
  489篇
  2021年   4篇
  2017年   4篇
  2016年   7篇
  2015年   15篇
  2014年   14篇
  2013年   12篇
  2012年   13篇
  2011年   17篇
  2010年   6篇
  2009年   10篇
  2008年   17篇
  2007年   20篇
  2006年   11篇
  2005年   11篇
  2004年   17篇
  2003年   13篇
  2002年   12篇
  2001年   13篇
  2000年   14篇
  1999年   11篇
  1998年   10篇
  1997年   5篇
  1996年   9篇
  1995年   8篇
  1994年   4篇
  1993年   3篇
  1992年   8篇
  1991年   13篇
  1990年   15篇
  1989年   15篇
  1988年   13篇
  1987年   10篇
  1986年   4篇
  1985年   8篇
  1984年   3篇
  1983年   3篇
  1982年   7篇
  1981年   4篇
  1980年   6篇
  1979年   8篇
  1978年   7篇
  1977年   5篇
  1976年   5篇
  1975年   3篇
  1974年   6篇
  1973年   8篇
  1972年   5篇
  1969年   3篇
  1968年   6篇
  1931年   3篇
排序方式: 共有489条查询结果,搜索用时 0 毫秒
101.
102.
103.
BACKGROUND: There is a paucity of information about risk factors for the human eye anomalies anophthalmia and microphthalmia. In this population-based case-control study we investigated whether periconceptional intakes of supplemental folic acid, dietary folate, vitamin A, and several other nutrients were associated with these eye defects. METHODS: This study included data on deliveries that had estimated due dates from 1997-2002 and were part of the National Birth Defects Prevention Study (the National Birth Defects Prevention Study is a population-based case-control study of a wide spectrum of birth defects, incorporating data from 10 birth defects surveillance systems in the United States [Arkansas, California, Georgia/Centers for Disease Control and Prevention, Iowa, Massachusetts, New Jersey, New York, North Carolina, Texas, and Utah]). Cases were those infants or fetuses born with either anophthalmia or microphthalmia. Liveborn infants without major malformations were eligible as controls. Maternal interviews were conducted, primarily by telephone, in English or Spanish. Participation in the interview was 71% among case mothers and 68% among control mothers. Interviews were completed with 89 case mothers and 4,143 control mothers. A shortened version of the food frequency questionnaire from the Nurse's Health Study was used to assess frequency of intake of 58 food items during the year before pregnancy. RESULTS: Our results did not indicate reduced risks for these eye malformations associated with maternal intake of vitamin supplements containing folic acid. The data did not show an association between malformation risk and higher or lower intakes of vitamin A. We also did not observe strong evidence that an abundance or a lack of dietary intake of any other nutrient was associated with increased risk of the studied eye malformations. CONCLUSIONS: Our observations contribute to a limited body of findings on these rare eye defects.  相似文献   
104.
In order to identify pathogenic correlates of refractory rheumatoid arthritis (RA), antibodies against anti-cyclic citrullinated protein (ACPAs) were investigated in RA patients in whom the dysregulated immune system had been ablated by high-dose chemotherapy (HDC) and autologous haematopoietic stem cell transplantation (HSCT). Six patients with refractory RA were extensively characterized in terms of levels of total immunoglobulins, RA-specific autoantibodies (ACPAs and rheumatoid factor) and antibodies against rubella, tetanus toxoid (TT) and phosphorylcholine before and after HDC plus HSCT. Additionally, the avidity of ACPAs was measured before and after treatment and compared with the avidity of TT antibodies following repeated immunizations. Synovial biopsies were obtained by arthroscopy before HDC plus HSCT, and analyzed by immunohistochemistry. In the three patients with clinically long-lasting responses to HDC plus HSCT (median 423 days), significant reductions in ACPA-IgG levels after therapy were observed (median level dropped from 215 to 34 arbitrary units/ml; P = 0.05). In contrast, stable ACPA-IgG levels were observed in three patients who relapsed shortly after HDC plus HSCT (median of 67 days). Clinical responders had ACPA-IgG of lower avidity (r = 0.75; P = 0.08) and higher degree of inflammation histologically (r = 0.73; P = 0.09). Relapse (after 38 to 530 days) in all patients was preceded by rising levels of low avidity ACPA-IgG (after 30 to 388 days), in contrast to the stable titres of high avidity TT antibodies. In conclusion, humoral autoimmune responses were differentially modulated by immunoablative therapy in patients with synovial inflammation and low avidity ACPA-IgG autoantibodies as compared with patients with high levels of high avidity ACPA-IgG. The distinct clinical disease course after immunoablative therapy based on levels and avidity of ACPA-IgG indicates that refractory RA is not a single disease entity.  相似文献   
105.
Arctic foxes are highly mobile arctic predators with a very weak population genetic structure over large parts of their range. Less is, however, known about the more local genetic structure within regions. Here, we analyze genotypes at 12 microsatellite loci for 561 arctic foxes trapped in the high-arctic archipelago Svalbard and investigate the genetic structure in three different age classes. Significant linkage disequilibrium, deficit of heterozygotes, genetic differentiation, and a decrease in relatedness with distance among animals trapped in their first winter suggested that some litter mates remain in proximity of each other during the first winter. This pattern was stronger for females than for males, indicating male-biased juvenile dispersal, and weaker for older animals. There was no genetic differentiation among adult foxes harvested in different hunting areas. The foxes from the protected area around Hornsund were however more differentiated than expected based on geographic distance alone, suggesting a possible disrupting effect of harvest on the spatial genetic structure in the rest of Svalbard. Our results also indicated a possible kin structure among adult females, suggesting natal philopatry, but further investigations will be needed to reach firm conclusions concerning kin structure in arctic foxes.  相似文献   
106.
Methyl α-cellobioside (methyl β-d-glucopyranosyl-(1→4)-α-d-glucopyranoside) was labeled with 13C at C4′ for use in NMR studies in DMSO-d6 solvent to attempt the detection of a trans-H-bond J-coupling (3hJCCOH) between C4′ and OH3. Analysis of the OH3 signal at 600 MHz revealed only the presence of two homonuclear J-couplings: 3JH3,OH3 and a smaller, longer range JHH. No evidence for 3hJC4′,OH3 was found. The longer range JHH was traced to 4JH4,OH3 based on 2D 1H–1H COSY data and inspection of the H2 and H4 signal lineshapes. A limited set of DFT calculations was performed on a methyl cellobioside mimic to evaluate the structural dependencies of 4JH2,O3H and 4JH4,O3H on the H3–C3–O3–H torsion angle. Computed couplings range from about −0.7 to about +1.1 Hz, with maximal values observed when the C–H and O–H bonds are roughly diaxial.  相似文献   
107.
108.
High-throughput approaches are beginning to have an impact on many areas of yeast biology. Two recent studies, using different experimental platforms, provide insight into new pathways involved in the response of yeast to DNA damage.  相似文献   
109.

Background

The risk of nodal metastasis is higher in women with bigger breast. It is not clear if this increase is due to the size of the breast (largely related to obesity) or is the result of larger tumour size at presentation (due to delayed diagnosis). It is hypothesised that women with large breasts are more likely to have node positive disease mainly attributable to their breast size.

Patients and methods

One hundred and twenty consecutive patients who underwent mastectomy during the year 2004 and 2005 for primary breast cancers in a large Teaching Hospital were included in the study. Patient's variable and tumour variable were collected and analysed by SPSS® computer programme.

Results

It was found that big breasted women (those patients with mastectomy weight greater than 800 g) had a significantly greater tumour size than those with smaller breasts (p = 0.019, Mann-Whitney test) but there was no significant difference in grade (Kendall's tau-b = 0.055, p = 0.57) or lymph node positivity (Kendall's tau-b = 0.011, p = 0.93) between the two groups. Although, the tumour size was significantly greater in those with lymph node metastases (p < 0.001) but mastectomy weight was not found to be significantly greater in those with lymph node metastases (p = 0.11). For patients with similar tumour sizes mastectomy weight was not significantly greater in those patients with lymph node metastases (p = 0.28).

Conclusion

It is concluded that increased incidence of lymph node positivity at presentation big-breasted women is because of larger size of the primary tumour and not due to the size of the breast alone.
  相似文献   
110.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号