全文获取类型
收费全文 | 1964篇 |
免费 | 215篇 |
国内免费 | 3篇 |
出版年
2023年 | 19篇 |
2022年 | 35篇 |
2021年 | 64篇 |
2020年 | 25篇 |
2019年 | 32篇 |
2018年 | 47篇 |
2017年 | 32篇 |
2016年 | 60篇 |
2015年 | 100篇 |
2014年 | 129篇 |
2013年 | 143篇 |
2012年 | 165篇 |
2011年 | 151篇 |
2010年 | 108篇 |
2009年 | 93篇 |
2008年 | 126篇 |
2007年 | 112篇 |
2006年 | 108篇 |
2005年 | 74篇 |
2004年 | 80篇 |
2003年 | 84篇 |
2002年 | 68篇 |
2001年 | 20篇 |
2000年 | 10篇 |
1999年 | 19篇 |
1998年 | 20篇 |
1997年 | 11篇 |
1996年 | 13篇 |
1994年 | 14篇 |
1993年 | 8篇 |
1992年 | 10篇 |
1991年 | 13篇 |
1989年 | 7篇 |
1987年 | 8篇 |
1986年 | 8篇 |
1984年 | 9篇 |
1983年 | 9篇 |
1982年 | 8篇 |
1981年 | 9篇 |
1980年 | 9篇 |
1979年 | 8篇 |
1978年 | 5篇 |
1977年 | 5篇 |
1976年 | 8篇 |
1975年 | 11篇 |
1974年 | 8篇 |
1973年 | 7篇 |
1972年 | 6篇 |
1971年 | 6篇 |
1969年 | 6篇 |
排序方式: 共有2182条查询结果,搜索用时 46 毫秒
81.
Torres-Padilla ME Richardson L Kolasinska P Meilhac SM Luetke-Eversloh MV Zernicka-Goetz M 《Developmental biology》2007,309(1):97-112
Initiation of the development of the anterior-posterior axis in the mouse embryo has been thought to take place only when the anterior visceral endoderm (AVE) emerges and starts its asymmetric migration. However, expression of Lefty1, a marker of the AVE, was recently found to initiate before embryo implantation. This finding has raised two important questions: are the cells that show such early, preimplantation expression of this AVE marker the real precursors of the AVE and, if so, how does this contribute to the establishment of the AVE? Here, we address both of these questions. First, we show that the expression of another AVE marker, Cer1, also commences before implantation and its expression becomes consolidated in the subset of ICM cells that comprise the primitive endoderm. Second, to determine whether the cells showing this early Cer1 expression are true precursors of the AVE, we set up conditions to trace these cells in time-lapse studies from early periimplantation stages until the AVE emerges and becomes asymmetrically displaced. We found that Cer1-expressing cells are asymmetrically located after implantation and, as the embryo grows, they become dispersed into two or three clusters. The expression of Cer1 in the proximal domain is progressively diminished, whilst it is reinforced in the distal-lateral domain. Our time-lapse studies demonstrate that this distal-lateral domain is incorporated into the AVE together with cells in which Cer1 expression begins only after implantation. Thus, the AVE is formed from both part of an ancestral population of Cerl-expressing cells and cells that acquire Cer1 expression later. Finally, we demonstrate that when the AVE shifts asymmetrically to establish the anterior pole, this occurs towards the region where the earlier postimplantation expression of Cer1 was strongest. Together, these results suggest that the orientation of the anterior-posterior axis is already anticipated before AVE migration. 相似文献
82.
Are we underestimating the diversity and incidence of insect bacterial symbionts? A case study in ladybird beetles 总被引:1,自引:0,他引:1
Vertically transmitted bacterial symbionts are common in arthropods. However, estimates of their incidence and diversity are based on studies that test for a single bacterial genus and often only include small samples of each host species. Focussing on ladybird beetles, we collected large samples from 21 species and tested them for four different bacterial symbionts. Over half the species were infected, and there were often multiple symbionts in the same population. In most cases, more females than males were infected, suggesting that the symbionts may be sex ratio distorters. Many of these infections would have been missed in previous studies as they only infect a small proportion of the population. Furthermore, 11 out of the 17 symbionts discovered by us were either in the genus Rickettsia or Spiroplasma, which are rarely sampled. Our results suggest that the true incidence and diversity of bacterial symbionts in insects may be far greater than previously thought. 相似文献
83.
Günther S Wallace L Patzewitz EM McMillan PJ Storm J Wrenger C Bissett R Smith TK Müller S 《PLoS pathogens》2007,3(12):e189
Lipoic acid (LA) is an essential cofactor of alpha-keto acid dehydrogenase complexes (KADHs) and the glycine cleavage system. In Plasmodium, LA is attached to the KADHs by organelle-specific lipoylation pathways. Biosynthesis of LA exclusively occurs in the apicoplast, comprising octanoyl-[acyl carrier protein]: protein N-octanoyltransferase (LipB) and LA synthase. Salvage of LA is mitochondrial and scavenged LA is ligated to the KADHs by LA protein ligase 1 (LplA1). Both pathways are entirely independent, suggesting that both are likely to be essential for parasite survival. However, disruption of the LipB gene did not negatively affect parasite growth despite a drastic loss of LA (>90%). Surprisingly, the sole, apicoplast-located pyruvate dehydrogenase still showed lipoylation, suggesting that an alternative lipoylation pathway exists in this organelle. We provide evidence that this residual lipoylation is attributable to the dual targeted, functional lipoate protein ligase 2 (LplA2). Localisation studies show that LplA2 is present in both mitochondrion and apicoplast suggesting redundancy between the lipoic acid protein ligases in the erythrocytic stages of P. falciparum. 相似文献
84.
Swan L. S. Sow Mark V. Brown Laurence J. Clarke Andrew Bissett Jodie van de Kamp Thomas W. Trull Eric J. Raes Justin R. Seymour Anna R. Bramucci Martin Ostrowski Philip W. Boyd Bruce E. Deagle Paula C. Pardo Bernadette M. Sloyan Levente Bodrossy 《Environmental microbiology》2022,24(5):2449-2466
We investigated the Southern Ocean (SO) prokaryote community structure via zero-radius operational taxonomic unit (zOTU) libraries generated from 16S rRNA gene sequencing of 223 full water column profiles. Samples reveal the prokaryote diversity trend between discrete water masses across multiple depths and latitudes in Indian (71–99°E, summer) and Pacific (170–174°W, autumn-winter) sectors of the SO. At higher taxonomic levels (phylum-family) we observed water masses to harbour distinct communities across both sectors, but observed sectorial variations at lower taxonomic levels (genus-zOTU) and relative abundance shifts for key taxa such as Flavobacteria, SAR324/Marinimicrobia, Nitrosopumilus and Nitrosopelagicus at both epi- and bathy-abyssopelagic water masses. Common surface bacteria were abundant in several deep-water masses and vice-versa suggesting connectivity between surface and deep-water microbial assemblages. Bacteria from same-sector Antarctic Bottom Water samples showed patchy, high beta-diversity which did not correlate well with measured environmental parameters or geographical distance. Unconventional depth distribution patterns were observed for key archaeal groups: Crenarchaeota was found across all depths in the water column and persistent high relative abundances of common epipelagic archaeon Nitrosopelagicus was observed in deep-water masses. Our findings reveal substantial regional variability of SO prokaryote assemblages that we argue should be considered in wide-scale SO ecosystem microbial modelling. 相似文献
85.
Biological Invasions - Pest carp species are a problem around the world, particularly in Australia where European carp (Cyprinus carpio) account for up to 90% of fish biomass in several major river... 相似文献
86.
Ralf Buettner Le Xuan Truong Nguyen Bijender Kumar Corey Morales Chao Liu Lisa S. Chen Tea Pemovska Timothy W. Synold Joycelynne Palmer Ryan Thompson Ling Li Dinh Hoa Hoang Bin Zhang Lucy Ghoda Claudia Kowolik Mika Kontro Calum Leitch Krister Wennerberg Xiaochun Yu Ching-Cheng Chen David Horne Varsha Gandhi Vinod Pullarkat Guido Marcucci Steven T. Rosen 《Journal of cellular physiology》2019,234(9):16295-16303
87.
Polyamines are short-chain aliphatic amines required for normal cellular growth that are ubiquitously found in all living tissues. Polyamine content has been shown to correlate with cellular proliferation. Quantitation of polyamines may thus provide a biochemical measure of proliferation in the colorectal mucosa where dysregulated epithelial proliferation is associated with colorectal cancer risk. A case-control study was conducted to validate the hypothesized association between mucosal polyamine measurements and colorectal cancer risk. Polyamines were measured in 4–6 multiple rectal mucosal biopsies from 11 normal control subjects and seven case patients with colon cancer. Compared with the controls, mean polyamine measurements, after adjustment for age and sex, were significantly increased for spermidine (P < 0.003) and spermine (P < 0.017). Subsequent analyses indicated that in controls 1–4 biopsies appeared adequate to characterize an individual. However, mucosal polyamines in the cases exhibited more sampling variability, requiring 4–8 biopsies to achieve an acceptable level of reliability. After adjustment for age and sex, the odds ratios for spermidine and spermine levels, compared to the controls, were 4.8 (95% confidence interval: 1.6–33.7) and 2.3 (1.2–6.3), respectively. The results of this study indicate that increases of mucosal polyamine measurements, after taking the sampling and methodological variability into account, are significantly associated with colorectal cancer risk, and suggest that polyamine measurements in rectal mucosa may play an important role as biomarkers for identifying high-risk individuals and/or for using as intermediate endpoints in prevention trials. © 1996 Wiley-Liss, Inc. 相似文献
88.
89.
Lina Gonzlez Gordon Paul R. Bessell Egbe F. Nkongho Victor N. Ngwa Vincent N. Tanya Melissa Sander Lucy Ndip Kenton L. Morgan Ian G. Handel Stella Mazeri Barend MdeC Bronsvoort Robert F. Kelly 《PLoS neglected tropical diseases》2022,16(3)
BackgroundCrimean-Congo Haemorrhagic Fever (CCHF) is a tick-borne viral zoonotic disease distributed across several continents and recognized as an ongoing health threat. In humans, the infection can progress to a severe disease with high fatality, raising public health concerns due to the limited prophylactic and therapeutic options available. Animal species, clinically unaffected by the virus, serve as viral reservoirs and amplifier hosts, and can be a valuable tool for surveillance. Little is known about the occurrence and prevalence of Crimean-Congo Haemorrhagic Fever Virus (CCHFV) in Cameroon. Knowledge on CCHFV exposure and the factors associated with its presence in sentinel species are a valuable resource to better understand transmission dynamics and assess local risks for zoonotic disease emergence.Methods and findingsWe conducted a CCHFV serological survey and risk factor analysis for animal level seropositivity in pastoral and dairy cattle in the North West Region (NWR) and the Vina Division (VD) of the Adamawa Region in Cameroon. Seroprevalence estimates were adjusted for sampling design-effects and test performance. In addition, explanatory multivariable logistic regression mixed-effects models were fit to estimate the effect of animal characteristics, husbandry practices, risk contacts and ecological features on the serological status of pastoral cattle. The overall seroprevalence was 56.0% (95% CI 53.5–58.6) and 6.7% (95% CI 2.6–16.1) among pastoral and dairy cattle, respectively. Animals going on transhumance had twice the odds of being seropositive (OR 2.0, 95% CI 1.1–3.8), indicating that animal movements could be implicated in disease expansion. From an ecological perspective, absolute humidity (OR 0.6, 95% CI 0.4–0.9) and shrub density (OR 2.1, 95% CI 1.4–3.2) were associated with seropositivity, which suggests an underlying viral dynamic connecting vertebrate host and ticks in a complex transmission network.ConclusionsThis study demonstrated high seroprevalence levels of CCHFV antibodies in cattle in Cameroon indicating a potential risk to human populations. However, current understanding of the underlying dynamics of CCHFV locally and the real risk for human populations is incomplete. Further studies designed using a One Health approach are required to improve local knowledge of the disease, host interactions and environmental risk factors. This information is crucial to better project the risks for human populations located in CCHFV-suitable ecological niches. 相似文献
90.