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   3141篇
  免费   213篇
  2024年   3篇
  2023年   25篇
  2022年   13篇
  2021年   94篇
  2020年   59篇
  2019年   73篇
  2018年   98篇
  2017年   90篇
  2016年   121篇
  2015年   213篇
  2014年   207篇
  2013年   267篇
  2012年   310篇
  2011年   295篇
  2010年   188篇
  2009年   159篇
  2008年   196篇
  2007年   169篇
  2006年   147篇
  2005年   136篇
  2004年   123篇
  2003年   88篇
  2002年   57篇
  2001年   13篇
  2000年   14篇
  1999年   21篇
  1998年   20篇
  1997年   17篇
  1996年   8篇
  1995年   4篇
  1994年   13篇
  1993年   10篇
  1992年   13篇
  1991年   8篇
  1990年   6篇
  1989年   7篇
  1988年   5篇
  1987年   6篇
  1986年   6篇
  1985年   8篇
  1984年   9篇
  1982年   4篇
  1981年   3篇
  1979年   3篇
  1978年   6篇
  1977年   3篇
  1975年   2篇
  1973年   2篇
  1969年   2篇
  1968年   2篇
排序方式: 共有3354条查询结果,搜索用时 218 毫秒
11.
The movement of Ca2+ across the basolateral plasma membrane was determined in purified preparations of this membrane isolated from rabbit proximal and distal convoluted tubules. The ATP-dependent Ca2+ uptake was present in basolateral membranes from both these tubular segments, but the activity was higher in the distal tubules. A very active Na+/Ca2+ exchange system was also demonstrated in the distal-tubular membranes, but in proximal-tubular membranes this exchange system was not demonstrable. The presence of Na+ outside the vesicles gradually inhibited the ATP-dependent Ca2+ uptake in the distal-tubular-membrane preparations, but remained without effect in those from the proximal tubules. The activity of the Na+/Ca2+ exchange system in the distal-tubular membranes was a function of the imposed Na+ gradient. These results suggest that the major differences in the characteristics of Ca2+ transport in the proximal and in the distal tubules are due to the high activity of a Na+/Ca2+ exchange system in the distal tubule and its virtual absence in the proximal tubule.  相似文献   
12.
A seven year follow-up of immune parameters is reported for a patient with chronic immune thrombocytopenic purpura (ITP) pre and post human immunodeficiency virus (HIV) seroconversion. Therapies such as intravenous IgG, prednisone, vincristine, or Ciclosporin A had no clear-cut beneficial effect on platelet counts. A long-term normalization of platelet counts was achieved by splenectomy. At splenectomy the patient was seropositive for HIV, most likely transmitted by blood products received half a year prior to laparatomy. Mean plasma levels of the second component of complement, C2, were half of the normal values prior to and within the lowest normal range post HIV seroconversion. Nevertheless, the T cell-dependent B cell response to HIV, which is dependent on the activation of C3 via the classical pathway of complement, was normal: Western blot analysis of total IgG and of IgG subclass responses to individual HIV antigens proved to be unimpaired.  相似文献   
13.
Escherichia coli strains causing urinary tract infections in dogs produce fimbriae composed of fimbrial subunits closely related to the F12 and F13 fimbriae of human uropathogenic strains [4]. The adhesins carried by the fimbriae of human and canine isolates differ, however, as concluded from a different hemagglutination pattern and from the fact that the dog strains do not agglutinate latex beads coated with P-fimbriae receptor. This possible difference in adhesive specificity was confirmed by experiments in which the adhesion of human and dog isolates to dog kidney epithelial cells (MDCK cells) and human bladder epithelial cells (T24 cells) was compared. Dog uropathogenic strains, in contrast to human uropathogenicE. coli strains, adhere to MDCK cells but hardly to T24 cells. Adhesion to MDCK cells correlates with the presence of F12 or F13 fimbriae on the dog strains. These results suggest that homologous fimbrial subunits can carry different adhesin molecules and that these adhesin molecules can be responsible for species-specific adherence. On the contrary, adhesion of a number of dog uropathogenicProteus mirabilis strains to MDCK and T24 cells was not species specific; it depended on the mere presence of fimbriae.  相似文献   
14.
Spreading and orientation of epithelial cells on grooved substrata   总被引:4,自引:1,他引:3  
The spreading and orientation of epithelial (E) cells was studied on titanium-coated grooved substrata by light, transmission (TEM) and scanning electron microscopy (SEM). Vertical-walled grooves and V-shaped grooves, 3-60 microns deep, were produced in silicon wafers by micromachining, a process which was developed for the fabrication of micro-electronic components, and the grooved substrata were replicated in Epon. Photolithography was used to prepare photoresist-based and silicon dioxide-silicon substrata with grooves of approximately 2 and approximately 0.5 micron deep, respectively. Cell clusters were markedly oriented by all the grooved substrata examined, with the orientation index being highest for substrata with grooves of the smallest repeat spacing. Time-lapse cinemicrography showed that the grooves directed the migration of E cells, but the control was not absolute, as some cells crossed over the ridges and descended into the grooves. The 0.5 micron grooves appeared less effective than the deeper grooves in directing cell locomotion. SEM and TEM of E cells spreading on the grooved substrata demonstrated that cell processes, including lamellae and filopodia, were capable of bending around and closely adapting to groove edges. E cells did not flatten as extensively on a substratum with 22 microns deep V-shaped grooves as on a smooth surface, although some cells were markedly elongated. One mechanism proposed to explain contact guidance of fibroblasts is that linear elements of the locomotory system, such as microfilament bundles, are unable to operate when bent. The observed flexibility of epithelial cell processes and the ability of substrata with shallow grooves to orient E cells indicate that contact guidance of E cells on micromachined substrata cannot be explained by the mechanical stiffness of long linear cytoskeletal elements.  相似文献   
15.
The contribution deals with the phytocenological analysis of theRondeletio correifoliae-Andropogonetum multinervosi, an endemic savanna association occuring on the Siguanea Hills (Sierra de la Siguanea) near the Colony Hotel.  相似文献   
16.
17.
Acridine orange, an intercalating dye usually employed in the curing of bacterial plasmids, was tested for its ability to cure K1 and K2 killer strains (laboratory and wine strains). The results showed a high curing percentage of the killer character. This was demonstrated by the loss of M1 or M2 dsRNAs (responsible for toxin production and resistance to it) and because the meiotic products exhibited non-Mendelian segregation. The curing percentages varied, depending on the strain but not on the killer type, and showed similar efficiency as compared with other known curing agents.  相似文献   
18.
Summary The adult mouse submandibular salivary gland provides a good model system to study gene regulation during normal and abnormal cell behavior because it synthesizes functionally distinct products ranging from growth factors and digestive enzymes to factors of relevance to homeostatic mechanisms. The present study describes the long-term growth and differentiation of submandibular salivary epithelial cells from adult male mice as a function of the culture substratum. Using a two-step partial dissociation procedure, it was possible to enrich for ductal cells of the granular convoluted tubules, the site of epidermal growth factor synthesis. Long-term cell growth over a period of 2 to 3 mo. with at least 3 serial passages was obtained only within three-dimensional collagen gels. Cells grew as ductal-type structures, many of which generated lumens with time in culture. Electron microscopic analysis in reference to the submandibular gland in vivo revealed enrichment for and maintenance of morphologic features of granular convoluted tubule cells. Reactivity with a keratin-specific monoclonal antibody established the epithelial nature of the cells that grew within collagen. Maintenance of cell differentiation, using immunoreactivity for epidermal growth factor as criterion, was determined by both cytochemical and biochemical approaches and was found to be dependent on the collagen matrix and hormones. Greater than 50% of the cells in primary collagen cultures contained epidermal growth factor only in the presence of testosterone and triiodothyronine. In contrast, cells initially seeded on plastic or cycled to plastic from collagen gels were virtually negative for epidermal growth factor. Biochemical analysis confirmed the presence of a protein with an apparent molecular weight of 6000 which comigrated with purified mouse epidermal growth factor. Epidermal growth factor was also present in detectable levels in Passage 1 cells. This culture system should permit assessment of whether modulation of submandibular gland ductal cell growth can be exerted via a mechanism that in itself includes epidermal growth factor and its receptor and signal transduction pathway. This work was supported by Public Health Service grant DE07766 from the National Institute of Dental Research, National Institutes of Health, Bethesda, MD.  相似文献   
19.
Chromosome 14 and late-onset familial Alzheimer disease (FAD)   总被引:7,自引:5,他引:2       下载免费PDF全文
Familial Alzheimer disease (FAD) is genetically heterogeneous. Two loci responsible for early-onset FAD have been identified: the amyloid precursor protein gene on chromosome 21 and the as-yet-unidentified locus on chromosome 14. The genetics of late-onset FAD is unresolved. Maximum-likelihood, affected-pedigree-member (APM), and sib-pair analyses were used, in 49 families with a mean age at onset ≥60 years, to determine whether the chromosome 14 locus is responsible for late-onset FAD. The markers used were D14S53, D14S43, and D14S52. The LOD score method was used to test for linkage of late-onset FAD to the chromosome 14 markers, under three different models: age-dependent penetrance, an affected-only analysis, and age-dependent penetrance with allowance for possible age-dependent sporadic cases. No evidence for linkage was obtained under any of these conditions for the late-onset kindreds, and strong evidence against linkage (LOD score ≤ –2.0) to this region was obtained. Heterogeneity tests of the LOD score results for the combined group of families (early onset, Volga Germans, and late onset) favored the hypothesis of linkage to chromosome 14 with genetic heterogeneity. The positive results are primarily from early-onset families. APM analysis gave significant evidence for linkage of D14S43 and D14S52 to FAD in early-onset kindreds (P < .02). No evidence for linkage was found for the entire late-onset family group. Significant evidence for linkage to D14S52, however, was found for a subgroup of families of intermediate age at onset (mean age at onset ≥60 years and <70 years). These results indicate that the chromosome 14 locus is not responsible for Alzheimer disease in most late-onset FAD kindreds but could play a role in a subset of these kindreds.  相似文献   
20.
D. G. Müller  Elisa Parodi 《Protoplasma》1993,175(3-4):121-125
Summary The marine brown algaEctocarpus siliculosus is invaded by a polyhedric virus, whose genome consists of circular, double-stranded DNA. In laboratory experiments this virus can infect a different host species,Feldmannia simplex. InfectedFeldmannia plants show severe somatic malformations. However, no functional virus particles are formed. SuchFeldmannia plants recover to resume a normal, symptom-free appearance. This result raises the possibility of intergeneric gene transfer in the natural habitat.  相似文献   
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