全文获取类型
收费全文 | 5294篇 |
免费 | 38篇 |
国内免费 | 42篇 |
专业分类
系统科学 | 640篇 |
丛书文集 | 388篇 |
教育与普及 | 173篇 |
理论与方法论 | 10篇 |
现状及发展 | 417篇 |
研究方法 | 679篇 |
综合类 | 3065篇 |
自然研究 | 2篇 |
出版年
2021年 | 7篇 |
2016年 | 7篇 |
2015年 | 8篇 |
2014年 | 16篇 |
2013年 | 10篇 |
2012年 | 290篇 |
2011年 | 357篇 |
2010年 | 91篇 |
2009年 | 21篇 |
2008年 | 304篇 |
2007年 | 327篇 |
2006年 | 468篇 |
2005年 | 522篇 |
2004年 | 379篇 |
2003年 | 339篇 |
2002年 | 299篇 |
2001年 | 249篇 |
2000年 | 401篇 |
1999年 | 114篇 |
1998年 | 51篇 |
1997年 | 24篇 |
1996年 | 15篇 |
1995年 | 28篇 |
1994年 | 37篇 |
1993年 | 52篇 |
1992年 | 39篇 |
1991年 | 31篇 |
1990年 | 23篇 |
1989年 | 25篇 |
1988年 | 16篇 |
1987年 | 16篇 |
1986年 | 28篇 |
1985年 | 19篇 |
1984年 | 13篇 |
1983年 | 23篇 |
1982年 | 20篇 |
1981年 | 15篇 |
1980年 | 12篇 |
1979年 | 10篇 |
1971年 | 9篇 |
1970年 | 19篇 |
1966年 | 7篇 |
1959年 | 90篇 |
1958年 | 124篇 |
1957年 | 101篇 |
1956年 | 99篇 |
1955年 | 75篇 |
1954年 | 78篇 |
1948年 | 16篇 |
1946年 | 6篇 |
排序方式: 共有5374条查询结果,搜索用时 453 毫秒
121.
Miwi catalysis is required for piRNA amplification-independent LINE1 transposon silencing 总被引:1,自引:0,他引:1
Reuter M Berninger P Chuma S Shah H Hosokawa M Funaya C Antony C Sachidanandam R Pillai RS 《Nature》2011,480(7376):264-267
122.
Learning causes a change in how information is processed by neuronal circuits. Whereas synaptic plasticity, an important cellular mechanism, has been studied in great detail, we know much less about how learning is implemented at the level of neuronal circuits and, in particular, how interactions between distinct types of neurons within local networks contribute to the process of learning. Here we show that acquisition of associative fear memories depends on the recruitment of a disinhibitory microcircuit in the mouse auditory cortex. Fear-conditioning-associated disinhibition in auditory cortex is driven by foot-shock-mediated cholinergic activation of layer 1 interneurons, in turn generating inhibition of layer 2/3 parvalbumin-positive interneurons. Importantly, pharmacological or optogenetic block of pyramidal neuron disinhibition abolishes fear learning. Together, these data demonstrate that stimulus convergence in the auditory cortex is necessary for associative fear learning to complex tones, define the circuit elements mediating this convergence and suggest that layer-1-mediated disinhibition is an important mechanism underlying learning and information processing in neocortical circuits. 相似文献
123.
A vascular niche and a VEGF-Nrp1 loop regulate the initiation and stemness of skin tumours 总被引:1,自引:0,他引:1
Beck B Driessens G Goossens S Youssef KK Kuchnio A Caauwe A Sotiropoulou PA Loges S Lapouge G Candi A Mascre G Drogat B Dekoninck S Haigh JJ Carmeliet P Blanpain C 《Nature》2011,478(7369):399-403
Angiogenesis is critical during tumour initiation and malignant progression. Different strategies aimed at blocking vascular endothelial growth factor (VEGF) and its receptors have been developed to inhibit angiogenesis in cancer patients. It has become increasingly clear that in addition to its effect on angiogenesis, other mechanisms including a direct effect of VEGF on tumour cells may account for the efficiency of VEGF-blockade therapies. Cancer stem cells (CSCs) have been described in various cancers including squamous tumours of the skin. Here we use a mouse model of skin tumours to investigate the impact of the vascular niche and VEGF signalling on controlling the stemness (the ability to self renew and differentiate) of squamous skin tumours during the early stages of tumour progression. We show that CSCs of skin papillomas are localized in a perivascular niche, in the immediate vicinity of endothelial cells. Furthermore, blocking VEGFR2 caused tumour regression not only by decreasing the microvascular density, but also by reducing CSC pool size and impairing CSC renewal properties. Conditional deletion of Vegfa in tumour epithelial cells caused tumours to regress, whereas VEGF overexpression by tumour epithelial cells accelerated tumour growth. In addition to its well-known effect on angiogenesis, VEGF affected skin tumour growth by promoting cancer stemness and symmetric CSC division, leading to CSC expansion. Moreover, deletion of neuropilin-1 (Nrp1), a VEGF co-receptor expressed in cutaneous CSCs, blocked VEGF's ability to promote cancer stemness and renewal. Our results identify a dual role for tumour-cell-derived VEGF in promoting cancer stemness: by stimulating angiogenesis in a paracrine manner, VEGF creates a perivascular niche for CSCs, and by directly affecting CSCs through Nrp1 in an autocrine loop, VEGF stimulates cancer stemness and renewal. Finally, deletion of Nrp1 in normal epidermis prevents skin tumour initiation. These results may have important implications for the prevention and treatment of skin cancers. 相似文献
124.
125.
Don't judge species on their origins 总被引:1,自引:0,他引:1
126.
127.
Melt-induced speed-up of Greenland ice sheet offset by efficient subglacial drainage 总被引:1,自引:0,他引:1
Fluctuations in surface melting are known to affect the speed of glaciers and ice sheets, but their impact on the Greenland ice sheet in a warming climate remains uncertain. Although some studies suggest that greater melting produces greater ice-sheet acceleration, others have identified a long-term decrease in Greenland's flow despite increased melting. Here we use satellite observations of ice motion recorded in a land-terminating sector of southwest Greenland to investigate the manner in which ice flow develops during years of markedly different melting. Although peak rates of ice speed-up are positively correlated with the degree of melting, mean summer flow rates are not, because glacier slowdown occurs, on average, when a critical run-off threshold of about 1.4?centimetres a day is exceeded. In contrast to the first half of summer, when flow is similar in all years, speed-up during the latter half is 62?±?16 per cent less in warmer years. Consequently, in warmer years, the period of fast ice flow is three times shorter and, overall, summer ice flow is slower. This behaviour is at odds with that expected from basal lubrication alone. Instead, it mirrors that of mountain glaciers, where melt-induced acceleration of flow ceases during years of high melting once subglacial drainage becomes efficient. A model of ice-sheet flow that captures switching between cavity and channel drainage modes is consistent with the run-off threshold, fast-flow periods, and later-summer speeds we have observed. Simulations of the Greenland ice-sheet flow under climate warming scenarios should account for the dynamic evolution of subglacial drainage; a simple model of basal lubrication alone misses key aspects of the ice sheet's response to climate warming. 相似文献
128.
129.
130.