全文获取类型
收费全文 | 8510篇 |
免费 | 928篇 |
出版年
2023年 | 39篇 |
2022年 | 88篇 |
2021年 | 195篇 |
2020年 | 99篇 |
2019年 | 124篇 |
2018年 | 142篇 |
2017年 | 125篇 |
2016年 | 241篇 |
2015年 | 399篇 |
2014年 | 405篇 |
2013年 | 470篇 |
2012年 | 567篇 |
2011年 | 613篇 |
2010年 | 380篇 |
2009年 | 345篇 |
2008年 | 469篇 |
2007年 | 442篇 |
2006年 | 438篇 |
2005年 | 417篇 |
2004年 | 369篇 |
2003年 | 326篇 |
2002年 | 279篇 |
2001年 | 172篇 |
2000年 | 155篇 |
1999年 | 135篇 |
1998年 | 89篇 |
1997年 | 73篇 |
1996年 | 62篇 |
1995年 | 65篇 |
1994年 | 64篇 |
1993年 | 72篇 |
1992年 | 124篇 |
1991年 | 102篇 |
1990年 | 112篇 |
1989年 | 92篇 |
1988年 | 91篇 |
1987年 | 79篇 |
1986年 | 61篇 |
1985年 | 72篇 |
1984年 | 62篇 |
1983年 | 61篇 |
1982年 | 44篇 |
1981年 | 33篇 |
1979年 | 54篇 |
1978年 | 48篇 |
1977年 | 47篇 |
1976年 | 46篇 |
1974年 | 36篇 |
1973年 | 38篇 |
1969年 | 40篇 |
排序方式: 共有9438条查询结果,搜索用时 15 毫秒
991.
992.
Brendan P. Kelly Oriana H. Badajos Mervi Kunnasranta John R. Moran Micaela Martinez-Bakker Douglas Wartzok Peter Boveng 《Polar Biology》2010,33(8):1095-1109
Population structure and patterns of habitat use among ringed seals (Phoca hispida) are poorly known, in part because seasonal movements have not been adequately documented. We monitored the movements of
98 ringed seals in the Beaufort and Chukchi seas between 1990 and 2006 using three forms of telemetry. In the winter—spring
period (when the seals were occupying shorefast ice), we used radio and ultra-sonic tags to track movements above and below
the ice, respectively. We used satellite-linked transmitters in summer and fall (when the seals ranged away from their winter
sites) to track at-sea movements. In the shorefast ice habitat, the home ranges of 27 adult males ranged from <1 to 13.9 km2 (median = 0.628) while the home ranges of 28 adult females ranged from <1 to 27.9 km2 (median = 0.652). The 3-dimensional volumes used by 9 seals tracked acoustically under the ice averaged 0.07 (SD = 0.04) km3 for subadults and adult males and 0.13 (SD = 0.04) km3 for adult females. Three of the radio-tracked seals and 9 tracked by satellite ranged up to 1,800 km from their winter/spring
home ranges in summer but returned to the same small (1–2 km2) sites during the ice-bound months in the following year. The restricted movements of ringed seals during the ice-bound season—including
the breeding season—limits their foraging activities for most of the year and may minimize gene flow within the species. 相似文献
993.
Brittany M. Vester Alison N. Beloshapka Ingmar S. Middelbos Sarah L. Burke Cheryl L. Dikeman Lee G. Simmons Kelly S. Swanson 《Zoo biology》2010,29(4):432-448
The objective of this study was to determine the effects of feeding commercially available beef‐ and horse‐based diets on nutrient digestibility and fecal characteristics of large captive exotic felids and domestic cats. Four species of large exotic felids including cheetahs, Malayan tigers, jaguars, and Amur tigers, and domestic cats were utilized in a crossover design. Raw meat diets included a beef‐based diet (57% protein; 28% fat) and a horse‐based diet (51% protein; 30% fat). All cats were acclimated to the diet for 16 days followed by a 4 day collection period, where total feces, including one fresh sample, were collected. All feces were scored on collection. Intake did not differ due to diet, but fecal output was greater when cats consumed the horse‐based diet. Total tract apparent dry matter (DM) digestibility was higher (P<0.05) and organic matter (OM) and crude protein (CP) digestibilities were lower (P<0.05) when cats were fed the beef‐based diet compared with the horse‐based diet. CP digestibility was similar in domestic cats and cheetahs, and greater (P<0.05) than Amur tigers. Fecal scores were lower and fecal DM was greater (P<0.05) when cats consumed the horse‐based diet compared with the beef‐based diet. Domestic cats had lower (P<0.05) fecal ammonia concentrations compared with all other species. Fecal ammonia concentrations were lowest (P<0.05) when cats were fed the horse‐based diet. Fecal total short‐chain fatty acid (SCFA), branched‐chain fatty acid (BCFA), and butyrate concentrations were higher (P<0.05) when cats consumed the beef‐based diet. Our results suggest that the domestic cat serves as an appropriate model for large exotic felid species, but differences among the species exist. Decreased nutrient digestibility by tigers and jaguars should be considered when developing feeding recommendations for these species based on domestic cat data. Zoo Biol 29:432–448, 2010. © 2009 Wiley‐Liss, Inc. 相似文献
994.
Kelly Kyro Surya P. Manandhar Daniel Mullen Walter K. Schmidt Mark D. Distefano 《Bioorganic & medicinal chemistry》2010,18(15):5675-5684
Isoprenylation is a post-translational modification that increases protein hydrophobicity and helps target certain proteins to membranes. Ras converting enzyme 1 (Rce1p) is an endoprotease that catalyzes the removal of a three residue fragment from the C-terminus of isoprenylated proteins. To obtain structural information about this membrane protein, photoaffinity labeling agents are being prepared and employed. Here, we describe the synthesis of a benzophenone-containing peptide substrate analogue for Rce1p. Using a continuous spectrofluorometric assay, this peptide was shown to be a substrate for Rce1p. Mass spectrometry was performed to confirm the site of cleavage and structure of the processed probe. Photolysis of the biotinylated compound in the presence of membranes containing Rce1p followed by streptavidin pull-down and Western blot analysis indicated that Rce1p had been labeled by the probe. Photolysis in the presence of both the biotinylated, benzophenone-containing probe and a farnesylated peptide competitor reduced the extent of labeling, suggesting that labeling is occurring in the active site. 相似文献
995.
Cadmium stress: an oxidative challenge 总被引:5,自引:0,他引:5
Ann Cuypers Michelle Plusquin Tony Remans Marijke Jozefczak Els Keunen Heidi Gielen Kelly Opdenakker Ambily Ravindran Nair Elke Munters Tom J. Artois Tim Nawrot Jaco Vangronsveld Karen Smeets 《Biometals》2010,23(5):927-940
At the cellular level, cadmium (Cd) induces both damaging and repair processes in which the cellular redox status plays a crucial role. Being not redox-active, Cd is unable to generate reactive oxygen species (ROS) directly, but Cd-induced oxidative stress is a common phenomenon observed in multiple studies. The current review gives an overview on Cd-induced ROS production and anti-oxidative defense in organisms under different Cd regimes. Moreover, the Cd-induced oxidative challenge is discussed with a focus on damage and signaling as downstream responses. Gathering these data, it was clear that oxidative stress related responses are affected during Cd stress, but the apparent discrepancies observed in between the different studies points towards the necessity to increase our knowledge on the spatial and temporal ROS signature under Cd stress. This information is essential in order to reveal the exact role of Cd-induced oxidative stress in the modulation of downstream responses under a diverse array of conditions. 相似文献
996.
We used PCRs with omp A primers to determine if spotted fever group rickettsiae occurred in Amblyomma variegatum from 6 Caribbean islands. Positive amplicons were obtained from ticks from the U.S. Virgin Islands (9/18; 50%), Dominica (39/171; 30%), Montserrat (2/5; 40%), Nevis (17/34; 50%), St. Kitts (46/227; 20%), and St. Lucia (1/14; 7%). Sequences for a convenience sample of reaction products obtained from A. variegatum on St. Kitts (7), American Virgin Islands (4), Montserrat (2), and St. Lucia (1) were 100% homologous with that of Rickettsia africae , the agent of African tick-bite fever. To determine if transmission of R. africae occurred, we used Rickettsia rickettsii antigen in IFA tests and found positive titers (≥ 1/80) with sera from cattle, goats, and sheep from Dominica (24/95 [25%], 2/136 [2%], 0/58 [0%]), Nevis (12/45 [27%], 5/157 [3%], 0/90 [0%]), St. Kitts (2/43 [5%], 1/25 [4%), 1/35 [3%]), and St. Lucia (6/184 [3%] cattle), respectively. No seropositive animals were found in Grenada (0/4, 0/98/, 0/86), Montserrat (0/12, 0/26, 0/52), or Puerto Rico (0/80 cattle). Our study indicates that R. africae and African tick-bite fever are widespread in the Caribbean. 相似文献
997.
Elizabeth J. Kelly Rebecca Nace Glen N. Barber Stephen J. Russell 《Journal of virology》2010,84(3):1550-1562
Vesicular stomatitis virus (VSV) has long been regarded as a promising recombinant vaccine platform and oncolytic agent but has not yet been tested in humans because it causes encephalomyelitis in rodents and primates. Recent studies have shown that specific tropisms of several viruses could be eliminated by engineering microRNA target sequences into their genomes, thereby inhibiting spread in tissues expressing cognate microRNAs. We therefore sought to determine whether microRNA targets could be engineered into VSV to ameliorate its neuropathogenicity. Using a panel of recombinant VSVs incorporating microRNA target sequences corresponding to neuron-specific or control microRNAs (in forward and reverse orientations), we tested viral replication kinetics in cell lines treated with microRNA mimics, neurotoxicity after direct intracerebral inoculation in mice, and antitumor efficacy. Compared to picornaviruses and adenoviruses, the engineered VSVs were relatively resistant to microRNA-mediated inhibition, but neurotoxicity could nevertheless be ameliorated significantly using this approach, without compromise to antitumor efficacy. Neurotoxicity was most profoundly reduced in a virus carrying four tandem copies of a neuronal mir125 target sequence inserted in the 3′-untranslated region of the viral polymerase (L) gene.Vesicular stomatitis virus (VSV) is a nonsegmented, negative-strand rhabdovirus widely used as a vaccine platform as well as an anticancer therapeutic. While VSV is predominantly a pathogen of livestock (34), it has a very broad species tropism. The cellular tropism of VSV is determined predominantly at postentry steps, since the G glycoprotein of the virus mediates entry into most tissues in nearly all animal species (10).Though viral entry can take place in nearly all cell types, in vivo models of VSV infection have revealed that the virus is highly sensitive to the innate immune response, limiting its pathogenesis (4). VSV is intensively responsive to type I interferon (IFN), as the double-stranded RNA (dsRNA)-dependent PKR (2), the downstream effector of pattern recognition receptors MyD88 (32), and other molecules mediate shutdown of viral translation and allow the adaptive immune response to clear the virus. The vulnerability of the virus to the type I IFN response, typically defective in many cancers, has been exploited to generate tumor-selective replication (49), such that the virus is now poised to enter phase I trials. However, the virus remains potently neurotoxic, causing lethal encephalitis not only in rodent models (7, 22, 53) but also in nonhuman primates (25).VSV very often infiltrates the central nervous system (CNS) through infection of the olfactory nerves (41). When administered intranasally, the virus replicates rapidly in the nasal epithelium and is transmitted to olfactory neurons, from which it then moves retrograde axonally to the brain and replicates robustly, causing neuropathogenesis. While intranasal inoculation does cause neuropathy in mice, neurotoxicity following viral administration also occurs when the virus is delivered intravascularly (47), intraperitoneally (42), and (not surprisingly) intracranially (13). Previously, other groups have modified the VSV genome to be more sensitive to cellular IFNs (49) and have actually encoded IFN in the virus (36). However, the former can result in attenuation of the virus, such that it has reduced anticancer potential, while the latter still results in lethal encephalitis (unpublished results). In order to mitigate the effects of VSV infection on the brain without perturbing the potent oncolytic activity of the virus, we utilized a microRNA (miRNA) targeting paradigm, whereby viral replication is restricted in the brain without altering the tropism of the virus for other tissues.To redirect the tissue tropism of anticancer therapeutics, we (26) and others (11, 14, 55) have previously exploited the tissue-specific expression of cellular miRNAs. miRNAs are ∼22-nucleotide (nt) regulatory RNAs that regulate a diverse and expansive array of cellular activities. Through recognition of sequence-complementary target elements, miRNAs can either translationally suppress or catalytically degrade both cellular (6) and viral (50) RNAs. We have determined that cellular miRNAs can potentially regulate numerous steps of a virus life cycle and that this regulation of the virus by endogenous miRNAs can then abrogate toxicities of replication-competent viruses (27; E. J. Kelly et al., unpublished data).miRNAs are known to be highly upregulated in many different tissues, including (but not limited to) muscle (40), lung (44), liver (15, 44), spleen (44, 46), and kidney (51). In addition, the brain has a number of upregulated miRNAs, with each different subtype of cell having a unique miRNA profile. miR-125 is highly upregulated in all cells in the brain (neurons, astrocytes, and glia cells), while miR-124 is found predominantly in neuronal cells (48). Glial cells and glioblastomas are thought to have decreased expression of miR-128 compared to neurons (17), while miR-134 is particularly abundant in dendrites of neurons in the hippocampus (43). In addition to these miRNAs, the tumor suppressor miRNA let-7 and miRs 9, 26, and 29 (51) are also found to be enriched in the brain, with expression varying not only between different cell types and regions of the brain but also temporally (48).MicroRNAs have previously been exploited to modulate the tissue tropism of nonreplicating lentiviral vectors (8, 9), as well as curbing known toxicities of replication-competent picornaviruses (5, 26), adenoviruses (11), herpes simplex virus 1 (33), and influenza A virus (39). In addition, a recombinant VSV encoding a tumor suppressor target was found to be responsive to sequence-complementary miRNAs in vitro, possibly by affecting expression of the matrix (M) protein (14), and evidence from Dicer-deficient mice suggests that endogenously expressed microRNA targets within the P and L genes of VSV could restrict enhanced pathogenicity of the virus (37). However, in vivo protection from neuropathogenesis by this means has not been demonstrated for VSV.Here we evaluate the efficiencies of different brain-specific miRNAs for shutting down gene expression and extensively characterize the ability of miRNA targeting to attenuate the neurotoxicity of vesicular stomatitis virus in vivo. We constructed and evaluated recombinant VSVs with miRNA target (miRT) insertions at different regions of the viral genome, with special focus upon those affecting viral L expression. In addition, we looked at the regulatory efficiency of different brain-specific miRNAs and the impact of miRT orientation on VSV replication and determined the impact of the virus on oncolytic activity in vivo. 相似文献
998.
Ryan K. Swenerton Giselle M. Knudsen Mohammed Sajid Ben L. Kelly James H. McKerrow 《The Journal of biological chemistry》2010,285(41):31120-31129
Proteases are a ubiquitous group of enzymes that play key roles in the life cycle of parasites, in the host-parasite relationship, and in the pathogenesis of parasitic diseases. Furthermore, proteases are druggable targets for the development of new anti-parasitic therapy. The subtilisin protease (SUB; Clan SB, family S8) of Leishmania donovani was cloned and found to possess a unique catalytic triad. This gene was then deleted by gene knock-out, which resulted in reduced ability by the parasite to undergo promastigote to amastigote differentiation in vitro. Electron microscopy of SUB knock-out amastigotes revealed abnormal membrane structures, retained flagella, and increased binucleation. SUB-deficient Leishmania displayed reduced virulence in both hamster and murine infection models. Histology of spleens from SUB knock-out-infected hamsters revealed the absence of psammoma body calcifications indicative of the granulomatous lesions that occur during Leishmania infection. To delineate the specific role of SUB in parasite physiology, two-dimensional gel electrophoresis was carried out on SUB−/− versus wild-type parasites. SUB knock-out parasites showed altered regulation of the terminal peroxidases of the trypanothione reductase system. Leishmania and other trypanosomatids lack glutathione reductase, and therefore rely on the novel trypanothione reductase system to detoxify reactive oxygen intermediates and to maintain redox homeostasis. The predominant tryparedoxin peroxidases were decreased in SUB−/− parasites, and higher molecular weight isoforms were present, indicating altered processing. In addition, knock-out parasites showed increased sensitivity to hydroperoxide. These data suggest that subtilisin is the maturase for tryparedoxin peroxidases and is necessary for full virulence. 相似文献
999.
Silverman JM Clos J Horakova E Wang AY Wiesgigl M Kelly I Lynn MA McMaster WR Foster LJ Levings MK Reiner NE 《Journal of immunology (Baltimore, Md. : 1950)》2010,185(9):5011-5022
We investigated the properties of leishmania exosomes with respect to influencing innate and adaptive immune responses. Exosomes from Leishmania donovani modulated human monocyte cytokine responses to IFN-γ in a bimodal fashion by promoting IL-10 production and inhibiting that of TNF-α. Moreover, these vesicles were inhibitory with respect to cytokine responses (IL-12p70, TNF-α, and IL-10) by human monocyte-derived dendritic cells. Exosomes from wild-type (WT) L. donovani failed to prime monocyte-derived dendritic cells to drive the differentiation of naive CD4 T cells into IFN-γ-producing Th1 cells. In contrast, vesicles from heat shock protein (HSP)100(-/-) L. donovani showed a gain-of-function and proinflammatory phenotype and promoted the differentiation of naive CD4 lymphocytes into Th1 cells. Proteomic analysis showed that exosomes from WT and HSP100(-/-) leishmania had distinct protein cargo, suggesting that packaging of proteins into exosomes is dependent in part on HSP100. Treatment of C57BL/6 mice with WT L. donovani exosomes prior to challenge with WT organisms exacerbated infection and promoted IL-10 production in the spleen. In contrast, HSP100(-/-) exosomes promoted spleen cell production of IFN-γ and did not adversely affect hepatic parasite burdens. Furthermore, the proparasitic properties of WT exosomes were not species specific because BALB/c mice exposed to Leishmania major exosomes showed increased Th2 polarization and exacerbation of disease in response to infection with L. major. These findings demonstrate that leishmania exosomes are predominantly immunosuppressive. Moreover, to our knowledge, this is the first evidence to suggest that changes in the protein cargo of exosomes may influence the impact of these vesicles on myeloid cell function. 相似文献
1000.
Podocopid ostracods have a calcified carapace encasing their uncalcified body parts like an envelope. A marginal infold (calcified
inner lamella) develops along the free margin of both valves, notably in the adult stage. Radial pore canals, which often
exhibit a distinct branched shape, can be seen in the free margin and they connect to the space between the outer and inner
calcified cuticles called “vestibule.” These characters associated with the marginal infold have been recognized as important
criteria both taxonomically and anatomically, but the calcification process of the marginal infold has never been investigated.
In this study, we observed the calcification process of the anterior free margin in Leptocythere species. The free margin of the adult specimen starts its calcification just after ecdysis, but the degree of calcification
remains the same as in the juvenile until approximately 35 h after ecdysis. The marginal infold of the adult specimen then
begins to calcify from its distal part around 40 h postecdysis, and short simple pore canals can still be observed in the
free margin. Marginal pore canals become more branched and narrower as calcification proceeds beyond 100 h postecdysis. These
results indicate that the calcification of the free margin in a podocopid carapace occurs in two steps, and needs much time
to complete the process even in small species of Leptocythere. In addition, these observations provide a basis for discussion on the correlation of the carapace size, environmental factors
of the habitat, and the development of the vestibule in some Krithe species, “Krithe problem.” 相似文献